Australia’s NationalScience Agency Characterising groundwater resources of theMontejinniLimestone andSkull CreekFormationin the Victoriacatchment, Northern Territory A technical report from the CSIROVictoria RiverWater ResourceAssessmentfor theNational Water Grid AndrewR Taylor1, JodieL Pritchard1, Russell S Crosbie1,KarenEBarry1, Anthony Knapton2, GeoffHodgson1,Shane Mule1, Steven Tickell3, Axel Suckow1 1 CSIRO;2CloudGMS;3Northern TerritoryDepartment of Environment, Parks and Water Security A blue and white cloud logo Description automatically generated A logo with black text Description automatically generated ISBN 978-1-4863-2084-4 (online) A logo with black text Description automatically generated ISBN 978-1-4863-2083-7 (print) Citation Taylor AR, Pritchard JL, Crosbie RS, Barry KE, Knapton A, Hodgson G, Mule S, Tickell S, and Suckow A (2024) Characterising groundwater resources of the Montejinni Limestone and Skull Creek Formation in the Victoria catchment, Northern Territory. A technical report from the CSIRO Victoria River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Copyright © Commonwealth Scientific and Industrial Research Organisation 2024. To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO. Important disclaimer CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. CSIRO is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please contact Email CSIRO Enquiries . CSIRO Victoria River Water Resource Assessment acknowledgements This report was funded through the National Water Grid’s Science Program, which sits within the Australian Government’s Department of Climate Change, Energy, the Environment and Water. Aspects of the Assessment have been undertaken in conjunction with the NT Government. The Assessment was guided by two committees: i. The Assessment’s Governance Committee: CRC for Northern Australia/James Cook University; CSIRO; National Water Grid (Department of Climate Change, Energy, the Environment and Water); Northern Land Council; NT Department of Environment, Parks and Water Security; NT Department of Industry, Tourism and Trade; Office of Northern Australia; Queensland Department of Agriculture and Fisheries; Queensland Department of Regional Development, Manufacturing and Water ii. The Assessment’s joint Roper and Victoria River catchments Steering Committee: Amateur Fishermen’s Association of the NT; Austrade; Centrefarm; CSIRO; National Water Grid (Department of Climate Change, Energy, the Environment and Water); Northern Land Council; NT Cattlemen’s Association; NT Department of Environment, Parks and Water Security; NT Department of Industry, Tourism and Trade; NT Farmers; NT Seafood Council; Office of Northern Australia; Parks Australia; Regional Development Australia; Roper Gulf Regional Council Shire; Watertrust Responsibility for the Assessment’s content lies with CSIRO. The Assessment’s committees did not have an opportunity to review the Assessment results or outputs prior to their release. This report was reviewed by Dr Rebecca Doble and Dr Olga Barron of CSIRO who’s review comments greatly improved earlier versions of this report. The authors of this report would like to acknowledge numerous people that provided a great deal of help, support and encouragement throughout the duration of this study. It would not have been possible to complete the groundwater research without their time, engagement, knowledge and contributions. To Diane Brodie from the Northern Land Council, who assisted the authors with the engagement of Traditional Owners across the catchment to seek permission to access different parts of the catchment to undertake field observations and collect water samples. Specifically, the authors would like to thank Shuana and Darcy King for providing cultural heritage clearance to access springs and groundwater bores to undertake groundwater observations and sampling. The authors would also like to thank numerous pastoral station owners and managers who also assisted with granting access to groundwater bores and springs. This included: (i) Sandy Watson of Camfield Station, (ii) Zach Weir of Kidman Springs Research Station, (iii) Callum McLachlan, Micheal Stanley, and Alex Stanely of Killarney Station, (iv) Jack Litter and Adam Ballantine of Montejinni Station, (v) Rusty Richter of Victoria River Downs Station, (vi) Cameron Latter of Pigeon Hole Station, and (vii) Brent Sneezby of Moolooloo Station. The authors are also very grateful for support and advice during field sampling by Professor Steven Gorelick of Stanford University. Punjehl Crane of CSIRO provided support with preparing noble gas samples from groundwater bores and springs for measurement at CSIRO’s Noble Gas Facility, Waite Campus, Adelaide. The authors would also like to acknowledge the graphics expertise of Greg Rinder at Gregs Graphics who assisted with drafting numerous hydrogeological cross sections and conceptual block models to publication standard. Acknowledgement of country CSIRO acknowledges the Traditional Owners of the lands, seas and waters, of the area that we live and work on across Australia. We acknowledge their continuing connection to their culture and pay our respects to their elders past and present. Photo Outcropping siltstone rocks of the Bynoe Formation surrounding Bulls Head Spring. Source: CSIRO Director’s foreword Sustainable development and regional economic prosperity are priorities for the Australian and Northern Territory (NT) governments. However, more comprehensive information on land and water resources across northern Australia is required to complement local information held by Indigenous Peoples and other landholders. Knowledge of the scale, nature, location and distribution of likely environmental, social, cultural and economic opportunities and the risks of any proposed developments is critical to sustainable development. Especially where resource use is contested, this knowledge informs the consultation and planning that underpin the resource security required to unlock investment, while at the same time protecting the environment and cultural values. In 2021, the Australian Government commissioned CSIRO to complete the Victoria River Water Resource Assessment. In response, CSIRO accessed expertise and collaborations from across Australia to generate data and provide insight to support consideration of the use of land and water resources in the Victoria catchment. The Assessment focuses mainly on the potential for agricultural development, and the opportunities and constraints that development could experience. It also considers climate change impacts and a range of future development pathways without being prescriptive of what they might be. The detailed information provided on land and water resources, their potential uses and the consequences of those uses are carefully designed to be relevant to a wide range of regional-scale planning considerations by Indigenous Peoples, landholders, citizens, investors, local government, and the Australian and NT governments. By fostering shared understanding of the opportunities and the risks among this wide array of stakeholders and decision makers, better informed conversations about future options will be possible. Importantly, the Assessment does not recommend one development over another, nor assume any particular development pathway, nor even assume that water resource development will occur. It provides a range of possibilities and the information required to interpret them (including risks that may attend any opportunities), consistent with regional values and aspirations. All data and reports produced by the Assessment will be publicly available. Chris Chilcott Project Director C:\Users\bru119\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Word\C_Chilcott_high.jpg The Victoria River Water Resource Assessment Team Project Director Chris Chilcott Project Leaders Cuan Petheram, Ian Watson Project Support Caroline Bruce, Seonaid Philip Communications Emily Brown, Chanel Koeleman, Jo Ashley, Nathan Dyer Activities Agriculture and socio- economics Tony Webster, Caroline Bruce, Kaylene Camuti1, Matt Curnock, Jenny Hayward, Simon Irvin, Shokhrukh Jalilov, Diane Jarvis1, Adam Liedloff, Stephen McFallan, Yvette Oliver, Di Prestwidge2, Tiemen Rhebergen, Robert Speed3, Chris Stokes, Thomas Vanderbyl3, John Virtue4 Climate David McJannet, Lynn Seo Ecology Danial Stratford, Rik Buckworth, Pascal Castellazzi, Bayley Costin, Roy Aijun Deng, Ruan Gannon, Steve Gao, Sophie Gilbey, Rob Kenyon, Shelly Lachish, Simon Linke, Heather McGinness, Linda Merrin, Katie Motson5, Rocio Ponce Reyes, Nathan Waltham5 Groundwater hydrology Andrew R. Taylor, Karen Barry, Russell Crosbie, Geoff Hodgson, Anthony Knapton6, Shane Mule, Jodie Pritchard, Steven Tickell7, Axel Suckow Indigenous water values, rights, interests and development goals Marcus Barber/Kirsty Wissing, Peta Braedon, Kristina Fisher, Petina Pert Land suitability Ian Watson, Jenet Austin, Bart Edmeades7, Linda Gregory, Jason Hill7, Seonaid Philip, Ross Searle, Uta Stockmann, Mark Thomas, Francis Wait7, Peter L. Wilson, Peter R. Wilson, Peter Zund Surface water hydrology Justin Hughes, Matt Gibbs, Fazlul Karim, Steve Marvanek, Catherine Ticehurst, Biao Wang Surface water storage Cuan Petheram, Giulio Altamura8, Fred Baynes9, Kev Devlin4, Nick Hombsch8, Peter Hyde8, Lee Rogers, Ang Yang Note: Assessment team as at September, 2024. All contributors are affiliated with CSIRO unless indicated otherwise. Activity Leaders are underlined. For the Indigenous water values, rights, interests and development goals activity, Marcus Barber was Activity Leader for the project duration except August 2022 – July 2023 when Kirsty Wissing (a CSIRO employee at the time) undertook this role. 1James Cook University; 2DBP Consulting; 3Badu Advisory Pty Ltd; 4Independent contractor; 5 Centre for Tropical Water and Aquatic Ecosystem Research. James Cook University; 6CloudGMS; 7NT Department of Environment, Parks and Water Security; 8Rider Levett Bucknall; 9Baynes Geologic Shortened forms SHORT FORM FULL FORM 14C Carbon-14 2H deuterium 3H tritium 18O oxygen-18 87Sr/86Sr strontium-87/strontium-86 AEM airborne electromagnetics AET actual evapotranspiration Al aluminium AOI area of interest AMS accelerator mass spectrometry APE areal potential evaporation APV Antrim Plateau Volcanics Ar argon As arsenic B boron BoM Bureau of Meteorology Br bromide Ca calcium Cd cadmium CaCO3 calcium carbonate C carbon CBE charge balance error CFCs chlorofluorocarbons (CFC-11 & CFC-12) CLA Cambrian Limestone Aquifer Cl chloride CMB chloride mass balance CO2 carbon dioxide Cr chromium Co cobalt CSIRO Commonwealth Scientific and Industrial Research Organisation Cu copper D deposition DEA Digital Earth Australia DEM digital elevation model DEPWS Department of Environment, Parks and Water Security DO dissolved oxygen DIWA Directory of Important Wetlands in Australia EC electrical conductivity EMM exponential model ET evapotranspiration F fluoride Fe iron fMol femtomole GA Geoscience Australia GDE groundwater-dependent ecosystem GEMIS Geoscience Exploration and Mining Information Systems GIS geographical information system GNS Institute of Geological and Nuclear Sciences GPS global positioning system GWWAP Georgina Wiso Water Allocation Plan H1301 bromotrifluoromethane H2S hydrogen sulphide HCO3 bicarbonate He helium HNO3 nitric acid ICPOES inductively coupled plasma optical emission spectrometry IRMS isotope ratio mass spectrometer K hydraulic conductivity K+ potassium KIP Kalkarindji Igneous province Kr krypton LMWL local meteoric water line LPM lumped parameter model mAHD metres above Australian Height Datum mBGL metres below ground level mBTOC metres below top of casing Mg magnesium ML Montejinni Limestone mMSL metres relative to mean sea level Mn manganese Mo molybdenum MRT mean residence time N nitrogen Na sodium NASY Northern Australia Sustainable Yields project NAWRA Northern Australia Water Resource Assessment NDVI Normalised Difference Vegetation Index Ne neon NGIS National Groundwater Information System Ni nickel NO3 nitrate NR Maps Natural Resources Maps NT Northern Territory NTGS Northern Territory Geological Survey O oxygen P annual average rainfall P phosphorus P-AET Potential actual evapotranspiration Pb lead PDAs Proterozoic Dolostone Aquifers PE potential evaporation PET potential evapotranspiration PET polyethylene terephthalate PM piston flow model QA/QC quality assurance and quality control QLD Queensland R recharge RC runoff coefficient RSWL reduced standing water level SO4 sulphate SrCO3 strontium carbonate Sr strontium Sb antinomy SF6 sulfur hexafluoride Se selenium SI saturation indices Si silicon SREBA Strategic Regional Environmental and Baseline Assessment SRTM Shuttle Radar Topography Mission STP standard temperature and pressure SWL standing water level Sy specific yield T transmissivity TDIC total dissolved inorganic carbon TDS total dissolved solids VSMOW Vienna standard mean ocean water VRD Victoria River District WA Western Australia WAP water allocation plan WOfS Water Observations from Space Xe xenon Zn zinc Units UNIT DESCRIPTION % percent ‰ per mille °C degrees Celsius cc or cc(STP) cubic centimetre of gas at standard temperature and pressure cm centimetre d day g gram GL gigalitre ha hectare kg kilogram km kilometre L litre m metre mm millimetre meq milliequivalent mg milligrams ML megalitres fmol femtomole mM millimolar pMC percent modern carbon pMol picomole s second TU Tritium units VPDB Vienna Pee Dee Belemnite y year μm micrometre μS microsiemens Preface Sustainable development and regional economic prosperity are priorities for the Australian and NT governments and science can play its role. Acknowledging the need for continued research, the NT Government (2023a) announced a Territory Water Plan priority action to accelerate the existing water science program ‘to support best practice water resource management and sustainable development.’ Governments are actively seeking to diversify regional economies, considering a range of factors. For very remote areas like the Victoria catchment (Preface Figure 1-1), the land, water and other environmental resources or assets will be key in determining how sustainable regional development might occur. Primary questions in any consideration of sustainable regional development relate to the nature and the scale of opportunities, and their risks. Preface Figure 1-1 Map of Australia showing Assessment area (Victoria catchment and other recent CSIRO Assessments FGARA = Flinders and Gilbert Agricultural Resource Assessment; NAWRA = Northern Australia Water Resource Assessment. How people perceive those risks is critical, especially in the context of areas such as the Victoria catchment, where approximately 75% of the population is Indigenous (compared to 3.2% for Australia as a whole) and where many Indigenous Peoples still live on the same lands they have inhabited for tens of thousands of years. About 31% of the Victoria catchment is owned by Indigenous Peoples as inalienable freehold. For more information on this figure please contact CSIRO on enquiries@csiro.au Access to reliable information about resources enables informed discussion and good decision making. Such information includes the amount and type of a resource or asset, where it is found (including in relation to complementary resources), what commercial uses it might have, how the resource changes within a year and across years, the underlying socio-economic context and the possible impacts of development. Most of northern Australia’s land and water resources have not been mapped in sufficient detail to provide the level of information required for reliable resource allocation, to mitigate investment or environmental risks, or to build policy settings that can support good judgments. The Victoria River Water Resource Assessment aims to partly address this gap by providing data to better inform decisions on private investment and government expenditure, to account for intersections between existing and potential resource users, and to ensure that net development benefits are maximised. The Assessment differs somewhat from many resource assessments in that it considers a wide range of resources or assets, rather than being a single mapping exercise of, say, soils. It provides a lot of contextual information about the socio-economic profile of the catchment, and the economic possibilities and environmental impacts of development. Further, it considers many of the different resource and asset types in an integrated way, rather than separately. The Assessment has agricultural developments as its primary focus, but it also considers opportunities for and intersections between other types of water-dependent development. The Assessment was designed to inform consideration of development, not to enable any particular development to occur. The outcome of no change in land use or water resource development is also valid. As such, the Assessment informs – but does not seek to replace – existing planning, regulatory or approval processes. Importantly, the Assessment does not assume a given policy or regulatory environment. Policy and regulations can change, so this flexibility enables the results to be applied to the widest range of uses for the longest possible time frame. It was not the intention of – and nor was it possible for – the Assessment to generate new information on all topics related to water and irrigation development in northern Australia. Topics not directly examined in the Assessment are discussed with reference to and in the context of the existing literature. CSIRO has strong organisational commitments to reconciliation with Australia’s Indigenous Peoples and to conducting ethical research with the free, prior and informed consent of human participants. The Assessment consulted with Indigenous representative organisations and Traditional Owner groups from the catchment to aid their understanding and potential engagement with its fieldwork requirements. The Assessment conducted significant fieldwork in the catchment, including with Traditional Owners through the activity focused on Indigenous values, rights, interests and development goals. CSIRO created new scientific knowledge about the catchment through direct fieldwork, by synthesising new material from existing information, and by remotely sensed data and numerical modelling. Functionally, the Assessment adopted an activities-based approach (reflected in the content and structure of the outputs and products), comprising activity groups, each contributing its part to create a cohesive picture of regional development opportunities, costs and benefits, but also risks. Preface Figure 1-2 illustrates the high-level links between the activities and the general flow of information in the Assessment. Preface Figure 1-2 Schematic of the high-level linkages between the eight activity groups and the general flow of information in the Assessment Assessment reporting structure Development opportunities and their impacts are frequently highly interdependent and, consequently, so is the research undertaken through this Assessment. While each report may be read as a stand-alone document, the suite of reports for each Assessment most reliably informs discussion and decisions concerning regional development when read as a whole. The Assessment has produced a series of cascading reports and information products: • Technical reports present scientific work with sufficient detail for technical and scientific experts to reproduce the work. Each of the activities (Preface Figure 1-2) has one or more corresponding technical reports. • A catchment report, which synthesises key material from the technical reports, providing well- informed (but not necessarily scientifically trained) users with the information required to inform decisions about the opportunities, costs and benefits, but also risks associated with irrigated agriculture and other development options. • A summary report provides a shorter summary and narrative for a general public audience in plain English. • A summary fact sheet provides key findings for a general public audience in the shortest possible format. The Assessment has also developed online information products to enable users to better access information that is not readily available in print format. All of these reports, information tools and data products are available online at https://www.csiro.au/victoriariver. The webpages give users access to a communications suite including fact sheets, multimedia content, FAQs, reports and links to related sites, particularly about other research in northern Australia. For more information on this figure please contact CSIRO on enquiries@csiro.au Summary The Victoria catchment is located in the far northwest of the Northern Territory, adjacent the border with Western Australia, occupying an area of approximately 82,400 km2. The catchment has a hot and arid climate and hosts the NT’s longest river, the Victoria River. The headwaters of the Victoria River originate south of Kalkarindji and flow northward, traversing basalt plains, dissected plateaux, escarpments, hills, and alluvial plains, before discharging into the Joseph Bonaparte Gulf in the Timor Sea. The catchment overlies six major geological basins and provinces comprised of various geological units important for hosting or controlling the occurrence of groundwater resources. These basins and provinces, from oldest to youngest, include: (i) Birrindudu Basin, (ii) Fitzmaurice Basin, (iii) Victoria Basin, (iv) Kalkarindji Igneous Province (KIP), (v) Wiso Basin, and (vi) Bonaparte Basin. Within the Victoria catchment, the availability and quality of the groundwater resources are influenced by the north–south climate gradient and the physical characteristics of the rocks and sediments in the major geological basins and provinces. Various aquifer types are hosted in each of the basins and provinces, including: (i) the productive regional-scale fractured or karstic limestone and dolostone of the Wiso Basin in the east, (ii) the productive intermediate-scale fractured or karstic dolostone and sandstone of the Birrindudu Basin in the centre and south, (iii) the variably productive fractured and weathered basalt of the KIP in the east, south, and north, (iv) the fractured and weathered or porous sandstone of the Victoria and Bonaparte basins in the west and north, and (v) the fractured and weathered shale, siltstone and mudstone aquifers of the Birrindudu and Victoria basins, which host only minor groundwater resources in the centre and north. In addition, alluvial aquifers are found in patches associated with minor parts of rivers, creeks, and their floodplains and channels throughout parts of the catchment. This groundwater study formed the groundwater hydrology component of the Victoria River Water Resource Assessment project. It involved several key components aimed at identifying the most promising opportunities for future groundwater resource development, including: (i) a literature and data review of all previous hydrogeological investigations in the catchment, (ii) regional-scale desktop data collation and analyses, including a review of digital geology datasets to map aquifers, digitising data contained in hand-written and typed drilling records to attribute aquifers to groundwater bore data, and evaluating spatial trends in groundwater levels, groundwater salinity, and bore yields, (iii) a regional-scale recharge modelling assessment of all aquifers in the catchment, (iv) identification and mapping of potential groundwater discharge areas using remote sensing, and (v) targeted investigations of the most promising aquifers to provide new information to underpin the future planning, investment and management of key groundwater resources. The literature and data review offered valuable insights into all aquifers and their existing knowledge gaps. The regional-scale assessment served as an effective screening tool for identifying spatial trends in important groundwater attributes, including: (i) the spatial extent of aquifers, (ii) ranges for groundwater levels, groundwater salinity, bore yields, and aquifer hydraulic properties, and (iii) provided a dataset for input into regional-scale recharge modelling at the catchment scale. Additionally, it helped identify the most promising aquifers for targeted investigations and provided information essential for: (i) strategically selecting groundwater bores suitable for environmental tracer and general chemistry sampling, (ii) identifying key locations for surface water sampling for environmental tracers and general chemistry, and (iii) establishing baseline datasets for use in detailed desktop analyses and modelling. The outcomes of the literature review and regional-scale desktop assessment identified the Cambrian Limestone Aquifer (CLA), hosted in the Montejinni Limestone in the eastern part of the catchment, and the Proterozoic Dolostone Aquifers (PDAs), primarily hosted in the Skull Creek Formation and, to a lesser extent, in the Bynoe and Timber Creek formations in the central part of the catchment, as the most promising aquifers for future groundwater resource development. Subsequently, these aquifers were the focus of more detailed desktop, field and modelling investigations. The outcomes also provided insights into other aquifers with potential for future development, but requiring further investigation, including: (i) PDAs hosted in the Campbell Springs and Pear Tree dolostones in the southern part of the catchment, and (ii) localised alluvial aquifers associated with the Angalarri and West Baines rivers in the northern part of the catchment. Furthermore, the literature review and regional-scale desktop assessment highlighted those aquifers that are most likely limited to development for stock and domestic use only. These include localised aquifers hosted in Proterozoic sandstone and shale, as well as in Cambrian basalt. Aquifers hosted in Devonian to Carboniferous sandstone, located near the coast, are likely vulnerable to saltwater intrusion. Cambrian Limestone Aquifer Targeted investigations of the CLA in the catchment helped refine the existing hydrogeological conceptual model for the aquifer. Recharge to the CLA is temporally and spatially variable and occurs directly in the aquifer outcrop or where it is unconfined beneath overlying Cretaceous and Cenozoic strata. Recharge processes include: (i) localised preferential infiltration of rainfall and streamflow via sinkholes in the aquifer outcrop, and (ii) broad diffuse infiltration of rainfall through overlying strata, which leaks to the underlying CLA. Recharge processes and rates are spatially and temporally variable, depending on interannual variability in rainfall, and the occurrence and nature of overlying strata. Where the aquifer outcrops or the overlying strata is thin (<30 m), groundwater is generally fresh (<500 mg/L total dissolved solids (TDS)), has a depleted isotopic composition, and has higher concentrations of tritium (3H) and dichlorodifluoromethane (CFC-12) indicating lower levels of groundwater recharge. Where the overlying cover is thicker (>30 m), groundwater is generally of a slightly higher salinity (between 500 and 1000 mg/L TDS) and has a lower concentrations of 3H and CFC-12 indicating lower levels of groundwater recharge. Mean annual recharge rates estimated by both upscaled chloride mass balance (CMB) and 3H concentrations in groundwater ranging between 3 and 20 mm/year. Numerical modelling indicated the annual recharge flux for the spatial extent of the CLA in the Victoria catchment occurring within the model domain (approximately 70% of the aquifer within the catchment) was 93 GL/year. Interpretation of the available static groundwater-level data for the CLA found that groundwater flow occurs mostly at a local to intermediate scale (flow paths of up to 25 km) in the CLA beneath the Victoria catchment. Groundwater flow is generally from east to west, following a subdued form of the topographic gradient, with some flow heading east outside the catchment. A combination of slightly higher elevation along the western edge of the Sturt Plateau and the shift from areas with and without Cretaceous and Cenozoic cover of the Carpentaria Basin influences the position of a groundwater flow divide. Westerly flowing groundwater discharges along the western margin of the CLA around Top Springs, while easterly flowing groundwater connects to the Flora flow path, discharging well north of the catchment at the Flora River. Mean residence times (MRTs) for localised groundwater flow range from several years to a few decades for short flow paths (<15 km) discharging at springs, and from a few decades to about a hundred years for longer and deeper flow paths originating near the groundwater flow divide. Discharge from the CLA occurs via a combination of: (i) intermittent lateral outflow to streams (Armstrong River and Bullock, Cattle, and Montejinni creeks), where they are incised into the aquifer outcrop, (ii) perennial localised spring discharge (Old Top, Lonely, Palm, and Horse springs), (iii) evapotranspiration via riparian and spring-fed vegetation, (iv) vertical leakage to the underlying Antrim Plateau Volcanics (APV) and (v) groundwater extraction for stock and domestic use, including community water supply at Top Springs. Discharge at streams is intermittent and dependent on localised recharge in the aquifer outcrop and has short flow paths (<15 km). As wet- season groundwater levels decline below stream level, streams dry up and cease to flow. Perennial localised discharge occurs mainly at contact springs along the western margin of the CLA where the karstic aquifer meets the highly heterogeneous APV. Most spring flow relies on localised recharge in the aquifer outcrop that discharges from short flow paths with short MRTs ranging between a few years to about a decade, as indicated by high concentrations of 3H in springs and nearby groundwater. Some springs, such as Old Top Spring and Palm Spring, receive discharge from longer flow paths (>15 km) with longer MRTs, as shown by lower concentrations of 3H in springs and groundwater. Old Top Spring, for example, is located along the western margin of the CLA, where lateral outflow from the CLA occurs to agglomerate and chert units of the APV, creating greater horizontal interconnectivity between the Montejinni Limestone and the APV. In the case of Palm Spring, its occurrence may be associated with the occurrence of both a structural high point in the underlying APV and a topographic low at the land surface, resulting in the CLA being thin (<20 m) at this location and groundwater being forced upward to discharge at the spring. Changes in the topography of the underlying APV, as shown in the hydrogeological cross-section, significantly influence the saturated thickness of the CLA in the Victoria catchment. Scenario-based numerical groundwater flow modelling of the CLA within the Victoria catchment was conducted to evaluate: (i) the current water balance and (ii) the potential impacts of changes in rainfall and potential evaporation and/or potential increased groundwater resource development on groundwater levels and spring discharge in the vicinity of the western margin of the CLA around the Top Springs region. Mean annual recharge modelling using the historical climate record indicates that recent climate conditions have led to higher recharge than the long-term mean. Using the historical climate to simulate the future climate to 2060, a hypothetical increase in groundwater extraction of between 9 and 15 GL/year will result in a modelled reduction in spring discharge along the western aquifer margin of between 11% and 18%, respectively, compared with the current level of groundwater extraction. The cumulative modelled drawdown in groundwater levels ranges from approximately 1 m (20 km to the south and east of the hypothetical extraction sites near the springs) to approximately 25 m (at the hypothetical extraction sites). The reduction in groundwater levels is proportional to the extraction rate. The reductions in mean modelled groundwater discharge and groundwater levels, under groundwater extraction scenarios B9, B12 and B15 over the 40-year time period are due to the small spatial extent of the CLA in the Victoria catchment, and the short distance (~15 km) to the closest hypothetical groundwater extraction site relative to the spring complexes around Top Springs. This highlights the fact that changes in the aquifer’s water balance depend on a range of factors including the location, magnitude and duration of extraction, as well as the effects the hydrogeological properties (saturated aquifer thickness, aquifer hydraulic properties, hydrogeological conceptual model) have on spatial and temporal changes in groundwater flow in an aquifer. Simulating a future drier climate to 2060, under Scenario C (10% reduction in rainfall) and current groundwater extraction, compared with the historical climate and current groundwater extraction (Scenario A), results in a 34% reduction in modelled spring discharge along the western aquifer margin. Modelled groundwater levels are also lower across most parts of the aquifer under Scenario C compared to under Scenario A. Simulating a future drier climate and hypothetical groundwater extraction (Scenario D) compared to Scenario A (current climate and current groundwater extraction) results in larger reductions in modelled spring discharge and groundwater drawdown, except for the (Dwet) scenarios. When considering the mean modelled groundwater discharge via evapotranspiration (ET) and springs from the CLA at spring complexes along the western margin of the CLA under a projected future climate (scenarios C and D), the results illustrate the effects climate variability may have on the aquifer’s water balance relative to the hypothetical extraction tested in this study. Under Scenario Cdry (projected future dry climate with current groundwater development), the reduction in groundwater recharge to the aquifer will have a larger impact on groundwater discharge via ET and localised spring discharge than current climate and the hypothetical groundwater extraction under Scenario B. This is because the CLA outcrops around and to the north and south of Top Springs, which receives localised recharge, and has relatively short groundwater flow paths and short MRTs for groundwater flow prior to discharge at the springs. Consequently, inter-annual variations in climate are evident in inter- annual variations in discharge. These results are specific to the scenarios tested in this study, but highlights that the potential hydrological impacts of both climate variability and groundwater extraction are equally important considerations for adaptive management of the CLA’s groundwater resources into the future. Proterozoic dolostone aquifers Targeted investigations of the PDAs in the catchment have helped validate and refine the existing hydrogeological conceptual model for the aquifer. Recharge to the PDAs is temporally and spatially variable and occurs via a combination of: (i) localised preferential infiltration of rainfall or streamflow through sinkholes, fractures, and faults where streams traverse the outcrop, and (ii) broad diffuse infiltration of rainfall through the overlying Cenozoic strata, which vertically leaks to the underlying aquifers. Due to the dynamic topography and structural complexity of the PDAs, some recharge occurs in fractures and faults across elevated areas, but most occurs in topographic lows where runoff accumulates. Elsewhere, dolostone aquifers are confined by overlying Proterozoic sandstones and shales or the APV. Both topography and the presence or absence of these overlying units influence the spatial variability in recharge to and discharge from the aquifers. Across the outcropping and subcropping areas of the PDAs, groundwater is quite fresh (<500 mg/L TDS), has a depleted isotopic composition, shows little sign of evaporation prior to recharge, and has reasonable concentrations of 3H and CFC-12. Mean annual recharge rates estimated by both upscaled CMB and 3H and CFC-12 concentrations in groundwater indicate recharge rates for the PDAs ranging from 15 to 50 mm/year, reflective of the higher rainfall zone where the aquifers occur. Information related to groundwater flow is limited for the PDAs due to a lack of data. However, static groundwater-level data and hydrogeological cross-sections suggest that groundwater flow in the outcropping and subcropping areas follows a subdued form of topography. Groundwater flows from higher elevation areas (such as Mount Dempsey and the Fitzgerald Range) to lower elevations around streams and springs. MRTs for local- to intermediate-scale groundwater flow range from several years to a few decades for short flow paths (<15 km) at most springs, and from a few decades to a few hundred years for slightly longer flow paths (~25 km), such as at Bulls Head Spring. There is currently a lack of information on groundwater flow in deeper, confined parts of the PDAs, but MRTs for intermediate- to regional-scale flow are expected to exceed hundreds of years in confined parts of the aquifers. Discharge from the PDAs occurs via a combination of: (i) intermittent lateral outflow to streams (East Baines River and Crawford, Giles, and Middle creeks), where they incise into the aquifer outcrop; (ii) perennial localised spring discharge at Bulls Head, Kidman, and Crawford springs across the centre of the catchment, as well as at Depot, Farquharson, and Wickham springs across the south of the catchment; (iii) evapotranspiration via riparian and spring-fed vegetation; and (iv) groundwater extraction for stock and domestic use, including community water supply at Timber Creek. Stream discharge is intermittent and relies on localised recharge in the aquifer outcrop and short flow paths (<15 km). As wet-season groundwater levels decline below stream level, streams dry up and cease to flow. Perennial localised discharge primarily occurs at contact springs in topographic low points along the margins of the outcropping and subcropping areas of the PDAs, where fractures and karsts in the aquifers intersect with adjacent highly heterogeneous Proterozoic sandstone and shale, as well as the basalt of the APV. However, Bulls Head Spring, receives discharge from longer and deeper flow paths (>15 km) with longer MRTs, as evidenced by the lower concentration of 3H and some dissolved helium above atmospheric equilibrium in the spring and nearby groundwater. Opportunities for future groundwater resource development Cambrian Limestone Aquifer Based on results of scenario-based numerical modelling, with appropriately sited borefields, it is estimated that approximately 10 GL/year could potentially be extracted from the CLA in areas to the south-east of Top Springs (from Top Springs south to Cattle Creek and east to the catchment boundary), depending upon community and government acceptance of potential impacts on GDEs and existing groundwater users. The CLA in areas to the north of Top Springs can have a thin saturated thickness (i.e. <20 m), or be unsaturated, and is less promising for future development. However, in the area from Top Springs south towards Cattle Creek and east to the catchment boundary, the CLA is unconfined, has a saturated thickness in places of >50 m and occurs at relatively shallow depths ~50 m BGL. Due to time lags associated with groundwater flow (tens of years), additional hypothetical extraction in this area may result in between 11% and 14% reduction in modelled groundwater discharge to the spring complexes and groundwater-fed vegetation along the western aquifer margin. It also may result in modelled reductions in groundwater levels of approximately 15 m (at the centre of the location of the hypothetical developments) to 1 m (at locations up to 20 km away). The hypothetical scenario-based modelling in this study has demonstrated that potential impacts from any future development will depend on a range of factors, including the location, magnitude and duration of extraction, as well as the effects the hydrogeological properties (saturated aquifer thickness, aquifer hydraulic properties, hydrogeological conceptual model) have on spatial and temporal changes in groundwater flow in an aquifer. In addition, modelled changes in the water balance under a projected drier future climate are larger than under the modelled future hypothetical groundwater development tested in this study. Furthermore, any proponent seeking a groundwater license will most likely be required to undertake a hydrogeological assessment to assess aquifer properties and bore performance, and to ensure their proposed extraction meets licensing conditions in relation to changes in groundwater storage (groundwater drawdown) and flow. Proterozoic dolostone aquifers Insufficient information exists for developing geological models and water balance models for the PDAs. However, an indicative scale of the resource has been derived by applying the estimated recharge rates for the aquifers to their outcropping and subcropping areas to assess the potential recharge flux to these aquifers. Given the likelihood that the water balance for the PDAs will be sensitive to climate variability similar to that for the CLA, a conservative approach of using the 5th percentile (95th percentile exceedance) of the estimated range in mean annual recharge rates to the outcropping and subcropping areas of the PDAs results in a conservative estimate of the annual recharge flux of 105 GL. Assuming 20% of the conservative recharge flux may potentially be available for future groundwater resource development, an indicative scale of the groundwater resource in the PDAs was estimated to be less than or equal to 20 GL/year. Further hydrogeological investigations (drilling and pump testing) and hydrological risk assessment modelling are required in order to evaluate groundwater extraction and climate variability impacts on existing groundwater users and GDEs. Similarly to the CLA, the actual scale of future development of the PDAs will depend upon community and government acceptance of potential impacts on GDEs and existing groundwater users, and approval of licenses to extract groundwater. Constraints on future groundwater resource development Cambrian Limestone Aquifer Potential constraints for small-to intermediate-scale (1–3 GL/year) future groundwater-based irrigation opportunities from the CLA in the Victoria catchment include: • being constrained to potentially suitable areas across the CLA that occur from Top Springs south towards Cattle Creek and east to the catchment boundary, where the saturated thickness of the aquifer is sufficient (i.e. >20 m) to support the installation and operation of purpose-built production bores • siting and installing successful high-yielding (i.e. bore yields of >10 L/second) production bores may require the drilling of multiple investigation holes to identify productive parts of the aquifer (fractured and karstic limestone and dolostone) as opposed to less productive mudstone parts of the aquifer, which can create uncertainty in the cost of drilling programs • the high hardness of the groundwater has the potential to result in scale build-up on water infrastructure and on the foliage of broad leaf crops • ensuring that cumulative extraction from development does not adversely impact the reliability of access to water for existing groundwater users (for stock and domestic water) across numerous pastoral stations, and on the water supply to the community at Top Springs • ensuring that cumulative extraction does not have an adverse impact the reliability of access to water for numerous culturally and ecologically important GDEs along the western margin of the CLA • maintaining the reliability of access to water for all groundwater users and GDEs as climate variability has a larger influence on the unconfined parts of the aquifers water balance than current groundwater extraction. The latter is reflected by natural variations in recharge to the aquifer under the current and historical climate which is an important consideration for adaptive management of groundwater in the CLA into the future. Proterozoic dolostone aquifers Potential constraints for small- to intermediate-scale (1–3 GL/year) future groundwater-based irrigation opportunities from the PDAs in the Victoria catchment include: • being constrained to lower-elevation areas (~100 mAHD) across the PDAs between Timber Creek and Yarralin, where the topography is relatively flat, but development could occur at sufficient distances away from karstic contact springs along the margin of the outcropping and subcropping areas • the need to better characterise the water balance, saturated thickness, groundwater levels and hydraulic properties of the aquifers, as the PDAs are currently data sparse across large areas. This is particularly relevant to aquifers hosted in the Campbell Springs and Pear Tree dolostones in the south of the catchment which were not the subject of targeted investigations in this study • siting and installing successful high-yielding (i.e. bore yields of >10 L/second) production bores may require the drilling of multiple investigation holes to identify productive parts of the aquifer (fractured and karstic dolostone and sandstone) as opposed to less productive siltstone parts of the aquifer which can create uncertainty in the cost of drilling programs • ensuring that cumulative extraction from development does not have an adverse impact on the reliability of access to water for existing groundwater users (for stock and domestic water) across numerous pastoral stations, or on the water supply to the community at Timber Creek • ensuring that cumulative extraction does not have an adverse impact on the reliability of access to water for numerous culturally and ecologically important GDEs along the margin of the outcropping and subcropping areas • maintaining the reliability of access to water for all groundwater users and GDEs, as climate variability similar to that observed in numerical modelling of the CLA, is likely to have an influence on the unconfined parts of the aquifers’ water balance. Contents Director’s foreword .......................................................................................................................... i The Victoria River Water Resource Assessment Team ................................................................... ii Shortened forms .............................................................................................................................iii Units .............................................................................................................................. vii Preface ............................................................................................................................. viii Summary ............................................................................................................................... xi Part I Introduction and overview 1 1 Introduction ........................................................................................................................ 2 1.1 Overview ................................................................................................................ 2 1.2 Aims of the groundwater hydrology activity ......................................................... 3 1.3 Summary of previous hydrogeological investigations .......................................... 3 1.4 Key aquifers for further detailed investigation ..................................................... 5 1.5 Current water demand .......................................................................................... 6 1.6 Report structure .................................................................................................... 8 2 Study area ........................................................................................................................... 9 2.1 Physiography and demography ............................................................................. 9 2.2 Climate ................................................................................................................. 10 2.3 Geology ................................................................................................................ 14 2.4 Hydrogeology....................................................................................................... 24 2.5 Surface water hydrology ..................................................................................... 32 2.6 Water dependent ecosystems ............................................................................ 34 Part II Methods 37 3 Regional desktop and modelling assessment of the Victoria catchment ........................ 38 3.1 Geology, hydrogeology and aquifer-types .......................................................... 38 3.2 Groundwater levels ............................................................................................. 39 3.3 Groundwater salinity ........................................................................................... 40 3.4 Bore yields ........................................................................................................... 40 3.5 Aquifer hydraulic properties ............................................................................... 41 3.6 Recharge estimation ............................................................................................ 41 3.7 Identifying potential groundwater discharge areas using remote sensing ........ 51 4 Targeted field, desktop and modelling investigations ..................................................... 55 4.1 Hydrogeological framework ................................................................................ 55 4.2 Groundwater recharge and flow ......................................................................... 60 4.3 Numerical flow modelling ................................................................................... 80 Part III Results 87 5 Regional assessment of the Victoria catchment .............................................................. 88 5.1 Groundwater levels ............................................................................................. 88 5.2 Groundwater salinity ........................................................................................... 95 5.3 Bore yields ........................................................................................................... 98 5.4 Recharge estimation .......................................................................................... 101 5.5 Identifying potential groundwater discharge areas using remote sensing ...... 109 6 Targeted field, desktop and modelling investigations ................................................... 115 6.1 Hydrogeological framework .............................................................................. 115 6.2 Groundwater recharge and flow ....................................................................... 128 6.3 Numerical groundwater flow modelling ........................................................... 184 7 Discussion ....................................................................................................................... 192 7.1 Summary ............................................................................................................ 192 7.2 Cambrian Limestone Aquifer ............................................................................. 195 7.3 Proterozoic dolostone aquifers ......................................................................... 200 7.4 Opportunities for future groundwater resource development ........................ 204 7.5 Constraints on future groundwater resource development ............................. 210 8 Summary and conclusions .............................................................................................. 212 8.1 Opportunities and constraints for future groundwater development ............. 212 8.2 Knowledge gaps and uncertainty ...................................................................... 214 References ........................................................................................................................... 216 Part IV Appendices 233 ........................................................................................................................... 234 Figures Preface Figure 1-1 Map of Australia showing Assessment area (Victoria catchment and other recent CSIRO Assessments ............................................................................................................ viii Preface Figure 1-2 Schematic of the high-level linkages between the eight activity groups and the general flow of information in the Assessment ........................................................................ x Figure 1-1 Location, type and volume of annual licenced surface water and groundwater entitlements as well as unlicensed community water supply ........................................................ 7 Figure 2-1 Physiographic units of the Victoria catchment ............................................................ 10 Figure 2-2 Historical (a) median annual rainfall and (b) median annual potential evaporation across the Victoria catchment ...................................................................................................... 11 Figure 2-3 Historical monthly rainfall in the Victoria catchment at Auvergne, Yarralin, Wave Hill and Top Springs ............................................................................................................................. 12 Figure 2-4 Historical potential evaporation in the Victoria catchment at Auvergne, Yarralin, Wave Hill and Top Springs ............................................................................................................ 13 Figure 2-5 Major geological provinces of the Victoria catchment ............................................... 16 Figure 2-6 Surface geology of the Victoria catchment ................................................................. 18 Figure 2-7 Simplified regional geology of the Victoria catchment ............................................... 19 Figure 2-8 Spatial changes in modelled depth to basement beneath the Victoria catchment .... 23 Figure 2-9 Major types of aquifers occurring beneath the Victoria catchment ........................... 25 Figure 2-10 Simplified regional hydrogeology of the Victoria catchment .................................... 28 Figure 2-11 Streamflow observation data availability in the Victoria catchment and median annual streamflow ........................................................................................................................ 34 Figure 2-12 Spatial distribution of known and moderate to high potential of ecological assets related to groundwater including Directory of Important Wetlands, groundwater-dependent ecosystems identified in the GDE atlas and known spring locations ........................................... 36 Figure 3-1. Schematic diagram of the methods used showing the calculation of the point recharge, the upscaling using regression kriging and the reporting of the results at the scale of the region and surface geology group .......................................................................................... 42 Figure 3-2 Region used for estimating recharge using the chloride mass balance (CMB) method ....................................................................................................................................................... 43 Figure 3-3 The (a) mean, (b) standard deviation, and (c) skewness of the chloride deposition across the study region ................................................................................................................. 46 Figure 3-4 Runoff coefficient across the study area. .................................................................... 47 Figure 3-5 Covariates used in upscaling (a) rainfall, (b) clay content of the soil, (c) Normalised Difference Vegetation Index (NDVI) and (d) simplified geology. .................................................. 50 Figure 3-6 Relationship used for determining the threshold October actual evapotranspiration (AET) for estimating potential discharge areas from aridity index ............................................... 53 Figure 4-1 Electrical conductivity (resistivity) of common earth materials .................................. 56 Figure 4-2 Location of existing airborne electromagnetic (AEM) surveys that have been acquired across parts of the Victoria catchment ......................................................................................... 57 Figure 4-3 Bores reviewed as potential candidates for groundwater sampling from Cambrian Limestone Aquifer including those installed in the Montejinni Limestone and Antrim Plateau Volcanics along the eastern edge of the Victoria catchment ....................................................... 63 Figure 4-4 Bores reviewed as potential candidates for groundwater sampling installed in the Proterozoic dolostone aquifers including those installed in the Battle Creek, Bynoe, Skull Creek and Timber Creek formations in the centre of the Victoria catchment ....................................... 65 Figure 4-5 Sampling groundwater from a stock bore with a diesel-powered Mono pump ......... 68 Figure 4-6 Filtering samples for chemistry analysis ...................................................................... 69 Figure 4-7 Collecting a groundwater sample for CFCs .................................................................. 70 Figure 4-8 Collecting a groundwater sample for sulfur hexafluoride (SF6) .................................. 70 Figure 4-9 Collecting a groundwater sample for dissolved noble gases ...................................... 71 Figure 4-10 Schematic cross-section representations of advective flow in the (a) piston flow model and (b) exponential model in idealised unconfined aquifers ............................................ 75 Figure 4-11 Target area for spring and surface water sampling of potential groundwater discharge locations for the Cambrian Limestone Aquifer in the Victoria catchment .................. 77 Figure 4-12 Target area for spring and surface water sampling of potential groundwater discharge locations for the Proterozoic dolostone aquifers in the Victoria catchment ............... 79 Figure 4-13 Spatial locations for hypothetical groundwater extraction sites across the Cambrian Limestone Aquifer (CLA) relative to land that is potentially suitable for agricultural intensification, current and proposed water management zones and groundwater discharge features (springs) for the aquifer .................................................................................................. 84 Figure 5-1 Static groundwater levels for the major aquifers of the Victoria catchment ............. 89 Figure 5-2 Static groundwater levels for the minor aquifers of the Victoria catchment ............. 90 Figure 5-3 Locations of bores with sufficient time series water level data for generating hydrographs .................................................................................................................................. 91 Figure 5-4 Groundwater hydrographs for temporal static standing water level observations from the Proterozoic dolostone aquifers near Timber Creek between 1984 and 1999 .............. 92 Figure 5-5 Static groundwater level hydrographs for the Yarralin area 1994 to 1999 ................ 94 Figure 5-6 Groundwater salinity for the major aquifers of the Victoria catchment .................... 96 Figure 5-7 Groundwater salinity for the minor aquifers of the Victoria catchment .................... 97 Figure 5-8 Groundwater bore yields for the major aquifers of the Victoria catchment .............. 99 Figure 5-9 Groundwater bore yields for the minor aquifers of the Victoria catchment ............ 100 Figure 5-10 (a) The chloride in groundwater observations within the study region and (b) the median of the point scale estimates of recharge derived from them ........................................ 101 Figure 5-11 Point-scale relationships between log recharge and (a) rainfall, (b) clay content of the soil, and (c) Normalised Difference Vegetation Index (NDVI) .............................................. 102 Figure 5-12 Point scale relationships between log rainfall and log recharge by geology class .. 103 Figure 5-13 Coefficients used in the regression equations for upscaling the 1000 replicates (a-f), (g) the R2 for each of the 1000 replicates ................................................................................... 104 Figure 5-14 (a) The median value of the 1000 replicates of upscaled recharge using the regression equation and (b) the median of the residuals kriged to a regular grid also showing the points used............................................................................................................................ 105 Figure 5-15 The 5th, 50th and 95th percentiles of upscaled recharge from the 1000 replicates using regression kriging .............................................................................................................. 106 Figure 5-16 The 5th, 50th and 95th percentiles of constrained recharge for the Victoria catchment ................................................................................................................................... 107 Figure 5-17 (a) A scatterplot of the 50th percentile of the recharge calculated at a point scale versus the 50th percentile of the regression kriging upscaled recharge estimate (red line is 1:1 for reference), and (b) the R2 of all 1000 replicates for the point estimates of recharge versus the upscaled estimates of recharge as a boxplot ....................................................................... 107 Figure 5-18 Water bodies in the Victoria catchment identified from Digital Earth Australia and the proportion of time that water bodies are inundated from Water Observations from Space ..................................................................................................................................................... 110 Figure 5-19 Excess water across the Victoria catchment ........................................................... 111 Figure 5-20 Areas of potential groundwater discharge across the Victoria catchment. ............ 113 Figure 6-1 Bulk conductivity of the subsurface for an elevation slice at 100 mAHD for all three airborne electromagnetic survey areas ...................................................................................... 116 Figure 6-2 Bulk conductivity of the subsurface for an elevation slice at zero mAHD for all three airborne electromagnetic survey areas ...................................................................................... 117 Figure 6-3 Conductivity-depth section from survey AusAEM1, Line 1010004 ........................... 118 Figure 6-4 Conductivity section from survey AusAEM2, Line 5004004 ...................................... 119 Figure 6-5 Conductivity section from survey CR19980231, Line 1251 ....................................... 119 Figure 6-6 Conductivity section from survey CR19980231, Line 1601. ...................................... 120 Figure 6-7 Locations of three regional-scale hydrogeological cross-sections traversing the Cambrian Limestone Aquifer (CLA: A–A’) and the Proterozoic dolostone aquifers (PDAs: B–B’ and C–C’) ..................................................................................................................................... 121 Figure 6-8 Hydrogeological cross-section A–A’ traversing from south-west to north-east through the Cambrian Limestone Aquifer around the eastern margin of the Victoria catchment ......... 122 Figure 6-9 Hydrogeological cross-section B–B’ traversing from north-west around Timber Creek through parts of the Proterozoic dolostone aquifers (PDAs) to the south-east around Top Springs ......................................................................................................................................... 124 Figure 6-10 Hydrogeological cross-section C–C’ traversing from north-west around Whitewater Falls through parts of the Proterozoic dolostone aquifers (PDAs) to the south-east near the western margin of the Cambrian Limestone Aquifer (CLA) ........................................................ 125 Figure 6-11 Interpolated surface of the depth to the top of the Cambrian limestone .............. 127 Figure 6-12 Interpolated reduced standing water level surface for the Cambrian Limestone Aquifer in the Victoria catchment and Wiso Basin showing the (a) prediction (mAHD), and (b) standard deviation around that prediction in (metres) .............................................................. 129 Figure 6-13 Interpolated reduced standing water level surface for the Antrim Plateau Volcanics using only bores screened in the APV showing the (a) prediction, and (b) standard deviation around that prediction ................................................................................................................ 130 Figure 6-14 Difference in groundwater water levels between the Cambrian Limestone Aquifer and Antrim Plateau Volcanics expressed in (a) metres, and (b) proportion of replicates showing CLA greater than APV .................................................................................................................. 131 Figure 6-15 Interpolated reduced standing water level for the Cambrian Limestone Aquifer .. 132 Figure 6-16 Interpolated depth to standing water level (SWL) surface for the Cambrian Limestone Aquifer ....................................................................................................................... 134 Figure 6-17 Classified depth the standing water level map for the Proterozoic dolostone aquifers in the centre and south of the Victoria catchment ...................................................... 135 Figure 6-18 Locations where temperature and pressure data loggers were installed in bores in aquifers hosted in the Cambrian limestone and Cambrian basalt ............................................. 136 Figure 6-19 Groundwater hydrographs for temporal groundwater level observations from the Cambrian basalt and limestone aquifers near Top Springs between August 2022 and June 2024 ..................................................................................................................................................... 138 Figure 6-20 Groundwater and spring sampling locations associated with the Cambrian Limestone Aquifer ....................................................................................................................... 140 Figure 6-21 Groundwater and spring sampling locations in the Proterozoic dolostone aquifers ..................................................................................................................................................... 141 Figure 6-22 Spatial distribution of water quality types for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and Proterozoic dolostone as well as aquifers hosted in adjacent hydrogeological units ..................................................................... 142 Figure 6-23 Piper diagram showing major ion composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent hydrogeological units (Cambrian basalt) ........................................................................................................................ 144 Figure 6-24 Outcropping and subcropping Montejinni Limestone and Antrim Plateau Volcanics along a reach of Townsend Creek ............................................................................................... 146 Figure 6-25 Major ion ratio plots for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units (Cambrian basalt) ........................................................................................................................ 147 Figure 6-26 Piper diagram showing major ion composition for groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent hydrogeological units ..................................................................................................................................................... 149 Figure 6-27 Major ion ratio plots for groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone aquifers and aquifers hosted in adjacent hydrogeological units ............................................................................................................................................. 152 Figure 6-28 Stable hydrogen and oxygen isotope composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent hydrogeological units (Cambrian basalt) compared to rainfall .................................................. 154 Figure 6-29 Spatial distribution of stable hydrogen isotope composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units ................................................................................................... 155 Figure 6-30 Stable hydrogen and oxygen isotope composition for groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent hydrogeological units compared to rainfall ................................................................................ 156 Figure 6-31 Strontium isotope composition relative to strontium concentration for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units ................................................................................... 157 Figure 6-32 Spatial distribution of strontium isotopic composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units ................................................................................................... 158 Figure 6-33 Strontium isotope composition relative to strontium concentration for groundwater and spring samples collected from aquifers hosted in Proterozoic dolostones ........................ 159 Figure 6-34 Spatial distribution of the tritium concentration for groundwater and spring samples from aquifers hosted in the Cambrian limestone and Proterozoic dolostone, as well as aquifers hosted in the adjacent Cambrian basalt ....................................................................... 160 Figure 6-35 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Cambrian limestone and Cambrian basalt ............................................. 161 Figure 6-36 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Proterozoic dolostone and Cambrian basalt .......................................... 162 Figure 6-37 Measured concentrations of (a) xenon versus krypton and (b) xenon versus neon in groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt ........................................................................................................... 164 Figure 6-38 Measured concentrations of (a) xenon versus krypton and (b) xenon versus neon in groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt .................................................................................................... 165 Figure 6-39 Measured concentrations of (a) sulfur hexafluoride (SF6) (b) bromotrifluoromethane (H1301) in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt .......................................................................................................................... 167 Figure 6-40 Measured concentrations of sulfur hexafluoride (SF6) (b) bromotrifluoromethane (H1301) in groundwater and collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt ........................................................................................................... 168 Figure 6-41 Measured concentrations of CFC-11 and CFC-12 in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt .............................. 169 Figure 6-42 Measured concentrations of CFC-11 and CFC-12 in groundwater collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt ........................... 170 Figure 6-43 Measured concentrations of (a) 13C vs 14C, (b) 14C vs depth below the watertable, and (c) 13C vs depth below the watertable in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt ................................................................... 171 Figure 6-44 Measured concentrations of (a) 13C vs 14C, (b) 14C vs depth below the watertable, and (c) 13C vs depth below the watertable in groundwater collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt................................................................ 172 Figure 6-45 Measured (a) helium and (b) helium-3 to helium-4 ratio versus neon to helium ratio in groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt .................................................................................................... 173 Figure 6-466 Measured (a) helium and (b) helium-3 to helium-4 ratio versus neon to helium ratio in groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt ................................................................................... 174 Figure 6-47 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt .............................. 175 Figure 6-48 Concentrations of CFC-12 in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt versus depth below the watertable ......... 176 Figure 6-49 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Proterozoic dolostone and Cambrian basalt .......................................... 178 Figure 6-50 Concentrations of CFC-12 in groundwater collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt versus depth below the watertable ...... 179 Figure 6-51 Surface geology surrounding springs in the east of the Victoria catchment associated with the Cambrian limestone and Cambrian basalt ................................................. 181 Figure 6-522 Surface geology surrounding springs in the centre of the Victoria catchment associated with the Proterozoic dolostone and adjacent hydrogeological units ....................... 183 Figure 6-53 Modelled annual recharge (a) for the spatial extent of the CLA in the Victoria catchment and (b) for the entire spatial extent of the CLA within the DR2 model domain ...... 185 Figure 6-54 Mean modelled drawdown relative to Scenario A for scenarios (a) B12, (b) Ddry12, (c) Dmid12 and (d) Dwet12 representative of climate conditions at the year 2060 .................. 187 Figure 7-1 Simplified conceptual block model of part of the Cambrian Limestone Aquifer near Top Springs along the eastern margin of the Victoria catchment .............................................. 196 Figure 7-2 Pandanus lining the edge of Old Top Spring .............................................................. 198 Figure 7-3 Spring-fed and riparian vegetation lining Kidman Creek immediately downstream of Kidman Springs ............................................................................................................................ 202 Figure 7-4 Simplified conceptual block model of the springs complex at Kidman Springs associated with the Proterozoic dolostone aquifers in the centre of the Victoria catchment .. 203 Figure 7-5 Outcropping siltstone of the Bynoe Formation at Bulls Head Spring ....................... 204 Figure 7-6 Hydrogeological units with potential for future groundwater resource development ..................................................................................................................................................... 206 Tables Table 4-1 Details of existing Airborne Electromagnetic (AEM) surveys that have been acquired across parts of the Victoria catchment ......................................................................................... 58 Table 4-2 Summary of modelling scenarios A, B, C and D for the Cambrian Limestone Aquifer using 109 years of historical climate and combinations of current and hypothetical future groundwater development ........................................................................................................... 82 Table 5-1 Summary of groundwater level data for bores in aquifers hosted in different hydrogeological units of the Victoria catchment .......................................................................... 88 Table 5-2 Summary of groundwater salinity data for bores in aquifers hosted in different hydrogeological units of the Victoria catchment .......................................................................... 95 Table 5-3 Summary of groundwater bore yield data for bores in aquifers hosted in different hydrogeological units of the Victoria catchment .......................................................................... 98 Table 5-4 Average recharge rates over each of the major aquifers. The 50th percentile is outside the brackets and the 5th and 95th percentiles give a range for the uncertainty within the brackets ....................................................................................................................................... 108 Table 5-5 Mean recharge rates over the simplified surface geology classes (Figure 3-5d). The 50th percentile is outside the brackets and the 5th and 95th percentiles give a range for the uncertainty within the brackets .................................................................................................. 108 Table 5-6 Summary of areas identified as potential groundwater discharge areas................... 112 Table 6-1 Summary of pH, salinity (total dissolved solids (TDS)), water-type and carbonate speciation and mineral saturation indices for groundwater samples collected from aquifers and springs hosted in the Cambrian limestone and adjacent hydrogeological units........................ 145 Table 6-2 Summary of pH, salinity (total dissolved solids (TDS)), water-type and carbonate speciation and mineral saturation indices for groundwater samples collected from aquifers hosted in the Cambrian limestone and adjacent hydrogeological units .................................... 150 Table 6-3 Summary of inferred spring water source for key springs sampled in the east of the Victoria catchment associated with the Cambrian limestone and Cambrian basalt ................. 182 Table 6-4 Summary of inferred spring water source for key springs sampled in the centre of the Victoria catchment associated with the Proterozoic dolostone and adjacent hydrogeological units ............................................................................................................................................. 184 Table 6-5 Mean modelled groundwater levels (mAHD) at six locations within the Cambrian Limestone Aquifer (CLA) under scenarios A and B. .................................................................... 189 Table 6-6 Mean annual modelled groundwater discharge at springs and via evapotranspiration for the 2060 representative conditions (10-year period from 2055 to 2065)............................ 191 Table 7-1 Opportunity-level estimates of the potential scale of groundwater resource development opportunities in the Victoria catchment .............................................................. 207 Part IIntroduction and overview 1 Introduction Section 1 introduces the groundwater study including: (i) its key aims, (ii) a brief summary of relevant previous hydrogeological investigations, (iii) current water demand, (iv) key aquifers for detailed investigation, and (v) an overview of the report structure. 1.1 Overview This study was conducted in the Victoria catchment which is in the south-west of the NT, adjacent the NT-WA border, and is remote and sparsely populated. Streams throughout the catchment are mostly intermittent or ephemeral and groundwater is the most widely used water resource. Groundwater is the major water source for towns and communities and the pastoral industry but has only been characterised in detail in discrete locations for community water supplies (Department of Housing and Community Development, 2017). This includes local-scale investigations at locations including Amanbidji, Bulla, Daguragu, Kalkarindji, Nitjpurru (Pigeon Hole), Yarralin, Timber Creek and Top Springs (Britten, 1986; Moser, 1994; Pearson, 1985). Broader scale water resource surveys and desktop studies have also been conducted to map surface water and groundwater resources across parts of the catchment to understand water supply options at different locations (CSIRO, 2009; Karp, 1995; Kirby and Faulks, 2005b; Matthews, 1997; Tickell and Rajaratnam, 1998). However, at a catchment scale, an assessment identifying the most promising groundwater resources with potential for future development and provision of new detailed information to underpin future water resource planning, investment, and management is yet to be undertaken. The Victoria catchment has a prolonged geological history and is geologically complex, hosting several overlapping and stacked geological basins and provinces. Spatial changes in the geology across each basin and province directly influence the type of aquifers hosted in different hydrogeological units. This includes the composition of rocks and sediments, and the degree to which they are weathered and/or altered by tectonic activity (structural changes). Identifying hydrogeological units that host extensive and productive aquifers with good quality groundwater suitable for a variety of uses is considered important in the context of their potential for future development. However, characterising the nature and scale of opportunities, and risks to their development requires a detailed understanding of their key characteristics. This includes spatial variability in: (i) their extent in the subsurface; (ii) nature of groundwater flow processes including inflows, storage, and outflows, (iv) water quality, and (v) their connectivity with other aquifers and aquitards, and aquatic and terrestrial groundwater-dependent ecosystems (GDEs). GDE-types can include groundwater-fed streams, riparian vegetation, wetlands, waterholes, springs, spring-fed vegetation, and the coastal and marine environment. Consequently, providing new information to guide future groundwater planning, investment, and management of the most promising aquifers involves: (i) characterising their depth, extent, geometry, and water quality (ii) conceptualising the nature of their flow systems, (iii) estimating water balances and (iv) providing estimates of potential future changes to groundwater balances and water availability to existing water users and the environment. This level of information will facilitate groundwater resource planning by understanding current baseline conditions and predicted future changes to groundwater levels and flow in aquifers at particular locations. Such planning will require value judgments of what is deemed as an acceptable impact (change to the natural flow regime) to receptors such as environmental assets or existing water users at a given location. These decisions can be complex and require considerable input from a wide range of stakeholders, particularly government regulators and communities. In this study, information on the spatial changes to groundwater levels or flow for variations in specified extraction rates has been provided for target aquifers, but no expert judgment has been provided on what changes in groundwater drawdown or flow is acceptable at a specified location. Nevertheless, the information provided is suitable for use by various stakeholders to inform discussions on future groundwater resource planning, investment and management. 1.2 Aims of the groundwater hydrology activity The purpose of the groundwater hydrology activity in the Victoria River Water Resource Assessment, is to provide an overview of groundwater resources in the Victoria catchment, within the context of identifying and characterising potential opportunities for and risks associated with future groundwater resource development in the most promising aquifers at an intermediate to regional scale. As part of this study, CSIRO partnered with CloudGMS and the Northern Territory Department of Environment, Parks and Water Security. Key questions the study sought to address included, but was not limited to: • What types of aquifers exist and what is the nature of the flow systems they host? • What are the important attributes that help identify aquifers in the catchment deemed promising for future groundwater resource development and how do they vary spatially? • Can a range in recharge for aquifers be estimated and are these ranges reasonable when considering rainfall, runoff, evapotranspiration, hydraulic properties and groundwater levels? • Where, and how is groundwater discharge occurring and what is its source? • For the most promising aquifers identified, what component of their water balance can be estimated with some level of confidence and what components remain uncertain? • What are the ranges in potential extractable volumes for these most promising aquifers? • What are the main risks to future groundwater development across the catchment? 1.3 Summary of previous hydrogeological investigations Prior to the 1990s, water resources of the Victoria catchment were poorly understood in comparison to the regional geology, with the exception of known locations of streams, springs and seepages exploited for stock and domestic water (Chewings, 1930; Traves, 1955). Some of the first recorded investigations of water resources involved field surveys of Bullo, Coolibah and Fitzroy stations to survey waterholes, springs and creeks, and their potential for use as water supplies for irrigation and stock water (Kneebone, 1964a; 1964b; 1964c). One of the first recorded groundwater investigations was undertaken on Legune Station. A drilling program successfully encountered good supplies of fresh groundwater in both Quaternary alluvium and much older Proterozoic sandstones and shales across different parts of the station (Kemezys, 1966). In the early 1970s, the Bureau of Mineral Resources published several maps of the geology across the Victoria catchment providing detailed information of the outcropping and subcropping extent and composition of different geological units across the region (Bultitude, 1973; Mendum, 1972; Pontifex and Sweet, 1972; Sweet, 1972; 1973a; 1973b). In 1973, the Bureau of Mineral Resources, conducted a regional-scale groundwater investigation of the Wiso Basin. This included some of the first published hydrogeological information on the Montejinni Limestone in the east of the catchment including its thickness, extent, composition, water quality and physical properties, as well as a regional-scale potentiometric surface (Randal, 1973). In the mid to late 1980s, geological mapping from the early 1970s was being used to undertake numerous local-scale hydrogeological investigations (near-surface geophysics, drilling, pump and water quality testing) in search of groundwater resources for community water supplies. Investigations targeted a variety of hydrogeological units hosting aquifers of varying extent, quality, and productivity. This included: (i) localised aquifers in Quaternary alluvium around Yarralin, and Cambrian basalt around Daguragu; and (ii) higher yielding intermediate-scale aquifers hosted in Proterozoic dolostones and sandstones around Timber Creek (Britten, 1986; Karp, 1987; Pearson, 1985). Throughout the 1990s, hydrogeological investigations were being conducted at various scales securing both community water supplies as well as domestic water supplies across pastoral stations (Karp, 1996; Sanders, 1990a; 1990b). Drilling investigations also targeted stock water supplies across pastoral stations, but many encountered localised low yielding aquifers of variable water quality hosted in Proterozoic siltstones and shales, and Cambrian basalt (Sanders and Rajaratnam, 1995c; 1995d; Tickell and Rajaratnam, 1995b). Numerous surface water surveys were also undertaken for stock water supplies as groundwater prospects proved poor in many places (Rajaratnam, 1994a; 1994b; Tickell and Rajaratnam, 1995a). New information from these groundwater and surface water surveys were subsequently used for the first time to describe and map water resources at a much larger scale (1:250,000 scale map sheets) across parts of the catchment (Karp, 1995; Matthews, 1997; Tickell and Rajaratnam, 1998). In the early 2000s, there was a reasonably sound understanding of surface and groundwater resources in the catchment but little information on how they could be managed. In 2004, Jolly and Jackson (2004) investigated the contribution of spring discharge from an aquifer hosted in Proterozoic dolostone to dry-season baseflow of the Wickham River. In 2005, Kirby and Faulks (2005b) undertook a study to assess, describe and report on the land and water resources of the Victoria catchment as part of the Top End Waterways project. This information was the first catchment scale assessment to provide information that could potentially be used for future decision making related to surface water planning and management. At the same time, development of groundwater resources to the north and east of the catchment hosted in the Cambrian limestone which is also present along the eastern margin of the Victoria catchment and its importance for baseflow to numerous streams (Katherine, Roper, Flora and Douglas rivers) was growing (Jolly et al., 2004; Tickell, 2005). In 2006, a regional groundwater model (FEFLOW model) was developed for the Cambrian Limestone Aquifer (CLA) encompassing large parts of the Wiso, Georgina and Daly basins to better understand how development of the aquifer may impact dry- season baseflows to surface water resources (Knapton, 2006). In 2008, Tickell (2008) published the first groundwater map of the NT highlighting the spatial extent and productivity of different aquifer types providing an indication that karstic aquifers hosted in the Cambrian limestones and Proterozoic dolostone are the largest and most productive aquifers in the Victoria catchment. In 2009, the initial FEFLOW model of the CLA was further refined into an integrated surface – groundwater model as part of the Gulf Water Study (Knapton, 2009). In 2009, CSIRO undertook the Northern Australia Sustainable Yields (NASY) project. The project provided information on historical, recent and likely future water availability in northern Australia, including across the Victoria catchment (CSIRO, 2009). The aim of the NASY project was to provide new information to governments, industry and communities for considering the future environmental, social and economic aspects of the sustainable use and management of the water assets of northern Australia. The work by CSIRO (2009) again highlighted the potential productivity and fresh water quality hosted in the CLA and aquifers hosted in the Proterozoic dolostones in the Victoria catchment. Post 2010, the main focus of hydrogeological investigations has been further characterisation of groundwater resources hosted in the CLA (Amery and Tickell, 2022; Bruwer and Tickell, 2015; Deslandes et al., 2019; ELA, 2022; Evans et al., 2020; Fulton and Knapton, 2015; Knapton et al., 2023; Taylor et al., 2023; Tickell and Bruwer, 2018). Collectively, findings from these investigations have been used to develop water allocation plans and water management zones for most parts of the CLA (Department of Environment and Natural Resources, 2019a; 2019b). To better manage groundwater resources in the CLA, in 2020, further refinement was made to the integrated surface – groundwater model currently used to manage the groundwater resource (Knapton, 2020). Management of groundwater from the CLA falls under the Daly Roper Beetaloo water control district which intersects the eastern part of the Victoria catchment (see Figure 1-1). And in 2023, the most recent water allocation plan (Georgina Wiso Water Allocation Plan) for the CLA now includes the Wiso water management zone, which also intersects the eastern part of the Victoria catchment (Northern Territory Government, 2023). 1.4 Key aquifers for further detailed investigation The review of previous hydrogeological investigations across the Victoria catchment summarised in Section 1.3 has provided an indication that aquifers hosted in the Cambrian limestone and Proterozoic dolostone, are likely to offer the most promising opportunities for potential future groundwater resource development at an intermediate- to regional-scale. Hereafter these aquifers are referred to as the Cambrian Limestone Aquifer (CLA) and the Proterozoic dolostone aquifers (PDAs). While many investigations have occurred at a local-scale in search of groundwater for stock or domestic use in the catchment, data and information from these investigations have indicated that these aquifers: • host fresh groundwater in places suitable for a variety of uses • have potential across the outcropping and subcropping areas of the aquifers to be intersected at economically viable depths by drilling to access groundwater • have potential to yield sufficient water for irrigation • are moderately spatially extensive (individual outcropping or subcropping areas of aquifers are >5,000 km2) • their spatial extents may potentially provide opportunities for development where they coincide with soils suitable for agricultural intensification away from existing water users and GDEs. In terms of potential economic constraints on future groundwater resource development, a maximum drilling depth of 300 m below ground level (mBGL) and a maximum depth to groundwater of 100 mBGL was adopted where development may involve individual proponents growing low-value crops. These depth thresholds were determined on the basis that the drilling, construction and installation of production bores and the associated pumping costs for low value crops would likely be prohibitively expensive. However, in cases where potential future groundwater resource development involves growing higher value crops, and/or development involves a consortium of investors these depth thresholds may not be as applicable. In this study, the primary focus was on further characterising the intermediate to regional-scale aquifers hosted in the Proterozoic dolostone and Cambrian limestone, respectively (see Section 6). This involved targeted field, desktop and modelling investigations which were conducted to further characterise the potential nature and scale of opportunities and risks to future groundwater resource development of these aquifers. However, a summary and evaluation of available groundwater data for all aquifers in the Victoria catchment has been provided as local- scale aquifers hosting groundwater resources may present equally attractive future water supply options in discrete parts of the catchment (see Section 5). 1.5 Current water demand Most communities in the Victoria catchment source their stock, domestic and community water supplies from groundwater (Figure 1-1). Surface water is also pumped from streams for stock and domestic use, and also from a few dams for use in agriculture and aquaculture. There are no major water transmission pipelines in the catchment and only a few small dams, except for Forsyth Creek Dam which holds up to 35 GL. Almost all water use in the catchment occurs outside water control districts or water allocation plan areas. The Victoria catchment mostly lies to the west of the Daly Roper Beetaloo Water Control District (RBWCD), though a small portion of the district occupies the eastern margin of the catchment to the north and south of Top Springs (Figure 1-1). The only water allocation plan currently applicable to the Victoria catchment is the Georgina Wiso Water Allocation Plan (GWWAP), which coincides with a small portion of the eastern margin of the catchment to the east of Top Springs (Figure 1-1). 1.5.1 Surface water entitlements Licensed surface water entitlements are sparse across the Victoria catchment. Four surface water licences have been granted for a combination of use for agriculture and aquaculture, all occurring in the northern parts of the catchment (Figure 1-1). The largest entitlement (of 100 GL/year) is for use in aquaculture with the water sourced from Forsyth Creek near the mouth of the Victoria River (Figure 1-1). The second-largest entitlement is 50 GL/year for use in agriculture with the water sourced from Forsyth Creek Dam in the upper reaches of Forsyth Creek in the north-western part of the catchment (NT Department of Environment, Parks and Water Security, 2018). Two smaller surface water entitlements exist for agricultural use: one sourced from Weaner Dam (1.2 GL/year) in the north-western Victoria catchment and the other from the Victoria River (0.7 GL/year) in the northern Victoria catchment (Figure 1-1). Figure 1-1 Location, type and volume of annual licenced surface water and groundwater entitlements as well as unlicensed community water supply Data sources: Licenced entitlement data sourced from Department of Environment Parks and Water Security (2018); Water Allocation Plan areas sourced from Department of Environment, Parks and Water Security (2019c); community water supply data from (Department of Housing and Community Development (2017) For more information on this figure please contact CSIRO on enquiries@csiro.au 1.5.2 Groundwater entitlements There are currently no licensed groundwater entitlements in the Victoria catchment. However, there are three licensed entitlements totalling 7.4 GL/year for use in agriculture to the north-east of the Victoria catchment (Figure 1-1), occurring in the proposed Flora Tindall Water Allocation Plan area (NT Department of Environment, Parks and Water Security, 2018). The groundwater is sourced from the Tindall Limestone Aquifer, which is connected to the limestone aquifer hosted in the Montejinni Limestone along the eastern margin of the Victoria catchment. However, the closest of the three licensed bores occur far outside of the Victoria catchment, approximately 110 km to the north-east of the Victoria River Roadhouse, and approximately 150 km to the north-east of Top Springs. The Montejinni Limestone hosts the largest and most productive regional-scale groundwater system in the catchment. Groundwater resources from a variety of local- to intermediate-scale groundwater systems hosted mostly in fractured and weathered rock aquifers provide important sources of community water supplies. The annual volume of groundwater extracted for community water supplies is only small (i.e. <0.2 GL/year), so a groundwater licence is not required (Figure 1-1). Groundwater is also widely used across the catchment in small quantities for stock and domestic water supplies for which a groundwater licence is also not needed. 1.6 Report structure This report describes the methods, data and outputs used to document this study of hydrogeological systems of the Victoria catchment, as well as targeted field, desktop and modelling investigations of the CLA and PDAs. The report is structured as follows: Section 2 outlines the geography, demography, climate, geology and hydrogeology of the study area. Sections 3 and 4 detail the data sources and methods used to conduct the regional assessment (water levels, bore yields, water quality, recharge estimation and discharge mapping) and targeted field, desktop and modelling investigations, respectively. Sections 5 and 6 present and discuss the results of the regional assessment and targeted field, desktop and modelling investigations, respectively. Section 7 discusses the opportunities and constraints associated with further potential development of groundwater resources from the CLA and PDAs, as well as other aquifers in the catchment. Section 8 states the conclusions of this study and potential options for future work. 2 Study area Section 2 provides information about the study area including: (i) physiography and demography, (ii) climate, (iii) geology and hydrogeology, (iv) surface water hydrology and (v) water dependent ecosystems. Given the importance of geological controls on groundwater resources, descriptions of geological basins and provinces, as well as surface and regional geology are provided. 2.1 Physiography and demography The Victoria catchment occupies an area of approximately 82,400 km2 and defines the boundary of the study area shown in Figure 2-1. The catchment is located in the far northwest of the Northern Territory alongside the border with WA. The catchment hosts the NT’s longest river, the Victoria River (Geoscience Australia, 2023), and its tributaries, the largest being the Wickham, Armstrong, Camfield and Angalarri rivers (Figure 2-1). The headwaters of the Victoria River originate south of Kalkarindji and flow in a northerly direction into the Joseph Bonaparte Gulf in the Timor Sea (Figure 2-1). The catchment exhibits eight different physiographic units previously described by Sweet (1977), of which the Victoria River and its major tributaries mostly traverse: (i) basalt hills and plains in the south; (ii) sandstone and limestone hills in the centre; (iii) alluvial plains in the north, particularly along the lower reaches around the confluences with the Angalarri and West Baines rivers; and (iv) finally the marine plains at the mouth of the river. The population of the Victoria catchment was approximately 2000 in 2021 (Australian Bureau of Statistics, 2021). Indigenous Peoples represent a substantial and growing proportion of the population across the Victoria catchment. Multiple Indigenous communities, organisations and entities control significant natural and cultural resource assets in the catchment. Small population centres occur at Timber Creek, Yarralin, Kalkarindji and Nitjpurru (Pigeon Hole). The main land use in the study area is grazing native vegetation (62%). Other notable land uses are nature conservation (9%) and other protected areas including Indigenous use (15%). In the north of the study area lies the Bradshaw Field Training Area, an Australian Government defence facility, with its southern boundary following the Victoria River. Cropping (both dryland and irrigated) are very sparsely practised (<0.02%) (Department of Environment Parks and Water Security, 2017). Figure 2-1 Physiographic units of the Victoria catchment Data source: Physiographic units adapted from Sweet (1977) 2.2 Climate The following climate summary, unless stated otherwise, comes from the companion technical report on climate data characterisation of the Victoria catchment by McJannet et al. (2023). The Victoria catchment has a hot and arid climate which is characterised by distinctive wet and dry- seasons due to its location in the Australian summer monsoon belt. The median annual rainfall, averaged over the Victoria catchment for the 132-year historical period (1st September 1890 to 31st August 2022) is 661 mm. There is a clear north-south gradient in annual rainfall, which is highest in the most northerly part of the catchment and lowest in the most southerly part (Figure 2-2). This is because the more northerly regions of the catchment receive more wet-season rainfall as a result of active monsoon episodes. The monsoon trough (a zone of low pressure and rising air), a primary trigger for diurnal thunderstorm activity over the catchment, separates moist maritime winds to its north and much drier continental air to its south, hence lower rainfall in southern parts of the catchment. Figure 2-2 Historical (a) median annual rainfall and (b) median annual potential evaporation across the Victoria catchment Data source: gridded annual rainfall and annual potential evaporation from McJannet et al. (2023) The median annual areal potential evaporation (areal PE – as calculated using Morton’s areal wet potential) averaged over the Victoria catchment for the 109-year historical period is 1935 mm. Unlike the clear north-south rainfall gradient, the areal PE gradient across the catchment is less pronounced and more complex. The highest areal PE (~1980 mm) occurs in the west of the catchment, and the lowest areal PE (~1880) in the north-east of the catchment. Approximately 95% of annual rain falls during the wet-season months (1 November to 30 April). The highest monthly rainfall totals typically occur during January, February and March (Figure 2-3). Tropical cyclones do not affect the Victoria catchment every year and so their contribution to total annual rainfall is highly variable from one year to the next. Areal potential evaporation in the Victoria catchment exceeds 1900 mm in most years (Figure 2-4). Evaporation is high all year round, but exhibits a strong seasonal pattern, ranging from over 200 mm per month during the build-up (October through December), which is typically the hottest time of year, to about 100 mm per month during the middle of the dry-season (June) (Figure 2-4). The high average median areal PE and moderate average median annual rainfall result in a large average annual rainfall deficit across most of the catchment (Figure 2-2). Consequently, a large proportion of the catchment is semi-arid. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-3 Historical monthly rainfall in the Victoria catchment at Auvergne, Yarralin, Wave Hill and Top Springs Left column shows monthly rainfall, right column shows time series of annual rainfall. ‘A range’ is the 10th to 90th percentile monthly rainfall. The solid blue line in the right column is the 10-year running mean. "\\fs1-cbr\{lw-rowra}\work\1_Climate\2_Victoria\2_Reporting\plots\climate_report\annual_monthly_rainfall_range_4_station.png" For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-4 Historical potential evaporation in the Victoria catchment at Auvergne, Yarralin, Wave Hill and Top Springs ‘A range’ is the 10th to 90th percentile monthly rainfall. Note: The ‘A mean’ line is directly under the ‘A median’ line in these figures. The solid blue line in the right column is the 10-year running mean. "\\fs1-cbr\{lw-rowra}\work\1_Climate\2_Victoria\2_Reporting\plots\climate_report\annual_monthly_pet_range_4_station.png" For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 2.3 Geology 2.3.1 Geological basins and provinces There are six major geological provinces across the Victoria catchment generally occurring in a north-east to south-west orientation (Figure 2-5). From oldest to youngest these are the: (i) Birrindudu Basin, underlying a large portion of the centre of the catchment and outcropping in the centre and south-west; (ii) Fitzmaurice Basin, which underlies and outcrops across a small portion of the north-west of the catchment and is bound to the south-east by the Victoria River Fault Zone; (iii) Victoria Basin, which overlies the Birrindudu Basin and underlies the central and northern parts of the catchment outcropping mostly across the north; (iv) Kalkarindji Igneous Province, which overlies the Wiso, Victoria and Birrindudu basins occurring mostly across the east and south of the catchment; (v) Wiso Basin, which underlies the eastern part of the catchment where it outcrops; and (vi) Bonaparte Basin, which outcrops in the north-west-most peninsula of the catchment. The Palaeo-Mesoproterozoic Birrindudu Basin is a sedimentary intracratonic basin mostly comprised of sequences of sandstone, dolostone and siltstone (Dunster and Ahmad, 2013a). The Birrindudu Basin unconformably overlies a series of deformed and metamorphosed rocks of the Halls Creek and Pine Creek orogens beneath the Victoria catchment (Dunster and Ahmad, 2013a). The basin extends in the subsurface to the south and west of the Victoria catchment over an area of about 82,000 km2 across the NT, extending beyond the catchment boundary beneath the cover of overlying basins and provinces (Dunster and Ahmad, 2013a). Sedimentary sequences of the Birrindudu Basin can be more than 10 km thick with major outcrops occurring in the centre of the catchment (Figure 2-5). Topographic features associated with the basin include the Fitzgerald Range in the centre of the catchment which in places is dissected by the Victoria, Wickham and East Baines rivers (Cutovinos et al., 2013). The Palaeo-Mesoproterozoic Fitzmaurice Basin has an outcropping area of approximately 2,000 km2 in the Victoria catchment and extends to the north and west of the catchment. It unconformably overlies granites and granodiorite of the Pine Creek and Hall Creek orogens (Dunster, 2013). The Basin hosts a series of stacked sandstone sequences with interlayered siltstone, shale, conglomerate and quartzite with a collective thickness of greater than 3.5 km (Dunster, 2013). It is bounded to the southeast by the Victoria and Birrindudu basins along the Victoria Fault Zone and to the northwest by the younger Bonaparte Basin (Figure 2-5). Outcropping rocks of the basin are heavily faulted and gently folded in places forming the Pinkerton, Spencer and Yambarren ranges, which are dissected by the lower reaches of the Victoria River (Dunster, 2013). The Victoria Basin is a Neoproterozoic sedimentary basin hosting the Auvergne Group which is largely comprised of interlayered sandstone, siltstone and dolostone units with a conglomerate basal unit (Dunster and Ahmad, 2013b). In the Victoria catchment, the Victoria Basin overlies the Pine Creek Orogen in the northwest and unconformably overlies the Birrindudu Basin in the southeast. The Victoria Basin outcrops mostly across the north of the catchment and has a subsurface extent beneath the catchment of approximately 26,000 km2. Its entire subsurface extent in the NT is approximately 36,000 km2 under the cover of the overlying Kalkarindji Igneous Province (KIP) (Dunster and Ahmad, 2013b). The Victoria Basin is bordered by the Victoria River Fault Zone to the northwest and by the Wiso Basin to the southeast (Figure 2-5). Outcropping rocks of the Victoria Basin are faulted in places and form the Newcastle and Stokes ranges in the north of the catchment, which are in places dissected by the East Baines and Victoria rivers, and Timber Creek (Cutovinos et al., 2014). The Neoproterozoic Wolfe Basin is comprised of glacial and fluvio-glacial sediments. It overlies the Birrindudu Basin and only occurs as a minor outcrop in the west of the Victoria catchment with the remainder obscured by the overlying KIP (Glass et al., 2013) (Figure 2-5). As the Wolfe Basin only intersects a minor part of the Victoria catchment is not described in detail here. The KIP was produced by widespread basaltic lava flows deposited over about 2,000,000 km2 during the early Cambrian (Glass et al., 2013). In the Victoria catchment, the KIP has a subsurface extent of approximately 40,000 km2 and is mostly comprised of basalt and basalt breccia with minor sandstone and chert interbeds that can collectively be more than 300 m thick. The KIP unconformably overlies the Birrindudu and Victoria basins in the east and south of the catchment and underlies the Wiso Basin along the eastern margin of the catchment (Figure 2-5). Outcropping rocks of the KIP form gentle basalt hills such as the Tent Hills in the east of the catchment, which are dissected in places by the Armstrong River and its tributaries (Glass et al., 2013). The middle Cambrian Wiso Basin is an intracratonic sedimentary basin that occupies approximately 160,000 km2 of the NT and is mostly comprised of sandstone, siltstone, limestone and dolostone. The Wiso Basin is interconnected laterally with the Daly and Georgina basins to the north-east and south-east of the Victoria catchment, respectively (Kruse and Munson, 2013). Collectively these basins have a combined total area of about 460,000 km2 of which only a small portion in the north-west coincides with the Victoria catchment (~12,000 km2). In the Victoria catchment, the Wiso Basin unconformably overlies and is bounded to the northeast by the KIP (Glass et al., 2013) (Figure 2-5). The sandstone, siltstone, limestone and dolostone sequences of the basin are typically less than 300 m thick and are unconformably overlain by Cretaceous siltstone and claystone of the Mesozoic Carpentaria Basin (Kruse and Munson, 2013; Munson et al., 2013). Outcropping limestone rocks of the basin often form gentle undulating plains to the east of Top Springs, which are incised by tributaries of the Armstrong River (Cutovinos et al., 2013). The onshore and offshore parts of the Bonaparte Basin occupy a total area of 270,000 km2, with the onshore component only occupying an area of approximately 20,000 km2. The late Palaeozoic to Cenozoic Bonaparte Basin is mostly comprised of siliciclastic and carbonate sedimentary rocks deposited in marine and fluvial environments that vary from about 5 km in thickness onshore to a maximum thickness of about 15 km offshore (Ahmad and Munson, 2013). The onshore component of the basin in the Victoria catchment is mostly obscured by overlying Cenozoic sediments such as estuarine and delta deposits, black soil plains, sand plains and alluvium. In the Victoria catchment, the Bonaparte Basin unconformably overlies the Halls Creek Orogen and is bounded to the east by the Pine Creek Orogen and the south by the Fitzmaurice Basin (Figure 2-5). Cenozoic sediments overlying the Bonaparte Basin form marine plains that host mangroves. Figure 2-5 Major geological provinces of the Victoria catchment Data source: Geological provinces from Raymond (2018) 2.3.2 Surface geology The surface geology of the Victoria catchment is shown in Figure 2-6. The surface geology reflects the geological history of the catchment including the major periods of deposition and tectonics (i.e. major structural changes), as well as weathering and erosion. These processes are closely linked to the physical environment that influences the evolution and formation of resources such as valuable minerals, coal, groundwater and soil. Geology also determines topography, which in turn is a key factor influencing hydrological processes such as runoff, streamflow, flooding and groundwater recharge, as well as deposition of soil and sediments. The oldest rocks in the catchment are of Proterozoic age and consist of repeated thick sequences, which include some geological units containing significant amounts of dolostone in the centre of the catchment (Figure 2-6). These units were deposited in a series of basins extending across the area and then gently folded, faulted and uplifted to form highlands (Dunster and Ahmad, 2013a). By the end of the Proterozoic, the highlands had been eroded down to a level not far above that of the current topography. During the Cambrian, there was widespread extrusion of basalt lava across large areas onto the eroded surface of the Proterozoic sediments in the east and south of the catchment (Figure 2-6) (Glass et al., 2013). This event was followed by deposition of a sequence of limestones and dolostones in the east of the catchment (Figure 2-6). Further gentle folding, faulting and uplift then occurred followed by another cycle of erosion, which started after the Cambrian and continued to the mid-Cretaceous and again resulted in erosion down to a level not far above that of the current topography. During the remainder of the Cretaceous subsidence and high global sea levels resulted in deposition of a thin succession of Cretaceous shallow marine sandstone, conglomerate and mudstone. These layers, which were probably deposited across most of the NT are now only preserved in minor parts of the east of the catchment (Figure 2-6). The present landscape has been produced by warping and dissection of a series of erosion surfaces formed during several cycles of erosion that started in the late Cretaceous and ended in the mid-Cenozoic. During this time, stable crustal conditions, subaerial exposure and prolonged subaerial-aerial weathering of the remaining Proterozoic, Cambrian and Cretaceous rocks resulted in the formation of deep weathering profiles on those rocks. Between the middle Cenozoic time and the present day, there has been gentle uplift and warping of the various rocks and their weathered surfaces. Continued erosion led to the emergence of the present-day landscape, which involved the development of incised valley systems that have been superimposed on the underlying Proterozoic rocks, with erosion producing broader valleys where the dolostone rocks have been weathered by dissolution. Minor floodplains and coastal deposits were built up on the margins of modern drainage systems and the coastline, respectively. Figure 2-6 Surface geology of the Victoria catchment Data source: Surface geology sourced from Raymond et al. (2012) 2.3.3 Regional geology A simplified regional geology of the Victoria catchment is presented in Figure 2-7. To highlight the spatial extent of key regional geological units in the subsurface, the blanket of surficial Cenozoic sediments presented in Figure 2-6 in Section 2.3.2 has been removed. In the absence of the Cenozoic cover, the location and extent of geological units beneath the catchment important for either hosting or controlling the occurrence of groundwater resources is visible. The regional geology of the catchment is complex hosting a multitude of different geological groups and units comprised of different rocks and sediments aligned with the major geological provinces of the catchment (Figure 2-5). Figure 2-7 Simplified regional geology of the Victoria catchment To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cenozoic sediments has been removed. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008);g Geological faults data from Department of Industry, Tourism and Trade (2010) A map of a large area Description automatically generated The most important geological basins or provinces in relation to groundwater resources beneath the catchment are the: (i) stacked Birrindudu and Victoria basins, (ii) KIP, and (iii) Wiso Basin. The rocks and sediments of the Birrindudu and Victoria basins are the oldest and most dominant rock types across the catchment. These Proterozoic fractured and weathered sandstones, siltstones, shales and dolostones outcrop and subcrop in the central and northern parts of the catchment (Carson, 2013) (Figure 2-7). The harder more resistive sandstones form ranges across these parts of the catchment and the softer dolostones (Proterozoic carbonate rocks) form valleys. The weathered and karstic parts of the dolostones host productive local to intermediate scale aquifers (CSIRO, 2009; Tickell and Rajaratnam, 1998). Overlying parts of the Proterozoic sandstones, siltstones and dolostones in the eastern, southern and western portions of the catchment is the Antrim Plateau Volcanics (APV) of the KIP, which form gentle hills and plains hosting localised aquifers across a large portion of the catchment (Geoscience Australia and Australian Stratigraphy Commission, 2021; Tickell and Rajaratnam, 1998). In the east of the catchment, the Montejinni Limestone of the Wiso Basin hosts one of the most productive aquifers in the catchment (Randal, 1973) (Figure 2-7). Cretaceous sandstone and claystone overlie a small portion of the Montejinni Limestone in this eastern part of the catchment but are mostly unsaturated. The Auvergne Group of the Victoria Basin occupies the largest portion of the northern and central parts of the subsurface beneath the catchment where it overlies parts of the Wattie, Bullita and Tijunna groups of the Birrindudu Basin (Figure 2-7). Where the Auvergne Group does not outcrop, it occurs beneath a patchy veneer of surficial Cenozoic sediments or is overlain in places by the APV. It hosts a series of stacked sandstone and siltstone units that are flat lying to gently dipping and faulted in places, particularly around the Victoria River Fault Zone (Dunster and Ahmad, 2013b). Regional faulting also occurs around the Coolibah Fault Zone in the north of the catchment (Figure 2-7). The most prominent geological units of the Auvergne Group that either outcrop or occur beneath a patchy veneer of overlying Cenozoic cover in the catchment include the: (i) Jasper Gorge Sandstone, and (ii) Angalarri Siltstone. The Jasper Gorge Sandstone is mostly comprised of silica-cemented quartz sandstone, with minor siltstone, and a basal conglomerate. The Angalarri Siltstone is mostly comprised of interbeds of fine sandstone, shale, minor dolostone and limestone (Cutovinos et al., 2014). Where these sandstone and siltstone units have been sufficiently fractured and or weathered, they host localised low-yielding aquifers such as the one that supplies water to the community of Amanbidji (Sanders and Rajaratnam, 1995a; 1995b). The APV of the Kalkarindji Suite in the KIP occupies a large portion of the eastern, southern and western parts of the catchment where it overlies parts of the Auvergne, Bullita and Tijunna groups (Figure 2-7). Where the APV does not outcrop, it occurs beneath a patchy veneer of surficial Cenozoic sediments or is overlain in the east by the Montejinni Limestone. The APV is generally comprised of a succession of basaltic lava flows and flow breccia and agglomerate with minor sandstone and chert interbeds (Glass et al., 2013). These basaltic rocks are almost entirely flat lying and are faulted, fractured and weathered across large areas in the east, south and west of the catchment (Figure 2-7). Basaltic rocks are the most dominant rock type of the APV across the catchment, with minor occurrences of tuffaceous sandstone and chert occurring in the east and south, and minor occurrences of agglomerate more common in the west of the catchment (Cutovinos et al., 2013; Cutovinos et al., 2002; Dunster et al., 2015). Where the basalt rocks have been faulted, fractured and weathered or co-occur with sandstone and chert interbeds or a basal agglomerate, they host localised low-yielding aquifers that supply an important source of stock and domestic water across large parts of the catchment (Tickell and Rajaratnam, 1998). The Montejinni Limestone of the Wiso Basin is generally flat lying to gently dipping and occupies a large part of the eastern margin of the Victoria catchment (Figure 2-7). Large parts of the limestone are overlain by a surficial veneer of Cenozoic sediments to the south of Top Springs, and Cretaceous rocks and sediments to the east and north-east of Top Springs. The Montejinni Limestone is mostly comprised of limestone, dolostone, siltstone and mudstone with minor sandstone and a basal breccia (Randal, 1973). The Montejinni Limestone is an equivalent geological unit to the Gum Ridge Formation of the Georgina Basin east of the Victoria catchment (Kruse et al., 2013; Randal, 1973). Parts of the limestone and dolostone rocks have been eroded by dissolution forming karsts including sinkholes, caves, caverns and springs. These karstic features host one of the most productive groundwater systems beneath the catchment, the Cambrian Limestone Aquifer (CLA). This aquifer is an important source of stock and domestic water across the eastern part of the catchment including the water supply at Top Springs (Randal, 1973). The Bullita, Wattie and Limbunya groups of the Birrindudu Basin host several stacked geological units that are flat lying to gently dipping and are occasionally faulted and gently folded in places (Cutovinos et al., 2013; Dunster et al., 2015). They occupy the central to southern parts of the catchment with the Bullita Group most prominent in the centre around Timber Creek and Yarralin where it overlies parts of the Wattie Group (Figure 2-7). Where the Bullita Group does not outcrop, it is overlain in places by a patchy veneer of Cenozoic sediments or parts of the Auvergne and Tijunna groups. The Bullita Group is mostly comprised of dolostone, dolarenite, siltstone, sandstone, chert and shale (Geoscience Australia and Australian Stratigraphy Commission, 2021). In the Victoria catchment, the dolostone, sandstone and chert rocks of the Skull Creek and Timber Creek formations of the Bullita Group are most prominent. These carbonate rocks are fractured and fissured and weathered by dissolution in places and host productive karstic aquifers that supply an important source of water for stock and domestic use including Timber Creek’s water supply (Pearson, 1985). They also host Bullita Cave, which is the largest of a series of karstic caves found in Judbarra Gregory National Park (Grimes and Martini, 2016). The Wattie Group overlies the Limbunya Group and is most prominent adjacent the Bullita Group in the central part of the Victoria catchment and the Limbunya Group in the south of the catchment (Figure 2-7). Where the Wattie Group does not outcrop, it is overlain in places by a patchy veneer of Cenozoic sediments or parts of the Auvergne and Bullita groups. The Wattie Group is mostly comprised of sandstone, siltstone, claystone, dolostone, chert and conglomerate (Geoscience Australia and Australian Stratigraphy Commission, 2021). The sandstone and siltstone rocks of the Seale and Wickham formations, respectively, are the most prominent units of the Wattie Group in the Victoria catchment (Dunster et al., 2013). Where the sandstone rocks of the Seale Formation have been weathered and fractured they host localised aquifers that provide an important source of water for stock and domestic use including Daguragu’s water supply (Moser, 1994). The Limbunya Group overlies Proterozoic metamorphic rocks and is most prominent in the south of the catchment (Figure 2-7). The rocks of the Limbunya Group only exhibit small outcrops in the catchment as most of the group is overlain by a surficial veneer of Cenozoic sediments. The Limbunya Group is mostly comprised of dolostone, siltstone, sandstone and mudstone (Geoscience Australia and Australian Stratigraphy Commission, 2021). The dolostone and sandstone rocks of the Campbell Springs Dolostone and Stirling Sandstone are the most prominent units of the Wattie Group in the Victoria catchment (Cutovinos et al., 2002). The Campbell Springs Dolostone is fractured and fissured and weathered by dissolution in places and hosts a productive karstic aquifer in the south of the catchment (Jolly and Jackson, 2004). The sandstone and siltstone rocks of the Weaber and Duerdin groups occupy only minor parts of the north and west of the catchment, respectively, and are therefore not described here in detail (Figure 2-7). 2.3.4 Depth to basement The Northern Territory Seebase (NT Seebase) is a depth‐to‐basement structural model providing a modelled depth to basement based on the shape and depth of major basement structures and the base of sedimentary sections. Depth to basement is modelled using a variety of data sources including magnetic and gravity data, digital elevation model (DEM) data and existing geological maps and publications (Northern Territory Geological Survey and Geognostics Australia Pty Ltd, 2021). Spatial changes in depth to basement have been shown to highlight the major structural features beneath the Victoria catchment including major areas of deposition, basement highs and major faults. The major deposition features beneath the catchment are almost entirely associated with the Birrindudu and Victoria basins in the central and northern parts of the catchment. Beneath the entire catchment there is significant variability in depth influenced by some major structural controls near the western margin of the Victoria Basin, and southern margin of the Birrindudu basin (Figure 2-8). In the west of the catchment, the depth to basement can be less than 800 metres relative to mean sea level (mMSL) in places along the western margin of the Victoria Basin where the Fitzmaurice and Bonaparte basins overlie the Halls Creek Orogen around the Victoria River Fault Zone (Figure 2-8). In the south of the catchment, the depth to basement can be much shallower (<200 mMSL) in places where the southern margin of the Victoria Basin and southern part of the Birrindudu basin overlies the Tanami Region (Figure 2-8). Much greater depths to basement (>6500 mMSL) occur in the Birrindudu Basin and western margin of the Wiso Basin (>5500 mMSL) (Figure 2-8). These depocenter’s host significantly thick sequences of Proterozoic to Cenozoic sedimentary rocks some of which host productive aquifers. In addition, some geological units also host mineral resources including base metals, and energy resources such as petroleum, but to date have been sparsely characterised (Dunster and Ahmad, 2013a). Figure 2-8 Spatial changes in modelled depth to basement beneath the Victoria catchment Depth is in metres relative to Mean Sea Level. Data source: NT Seebase and GIS Digital Information Package – Northern Territory Geological Survey and Geognostics Australia Pty Ltd (2021) For more information on this figure please contact CSIRO on enquiries@csiro.au 2.4 Hydrogeology 2.4.1 Aquifer types Within the Victoria catchment the distribution, availability and quality of groundwater resources are heavily influenced by the physical characteristics of the sediments and rocks of the major geological basins and provinces shown in Figure 2-5. There are three main types of aquifers across the Victoria catchment: (i) fractured and weathered shale, siltstone and mudstone aquifers of the Birrindudu and Victoria basins hosting only minor groundwater resources; (ii) fractured and/or karstic dolostones and limestone aquifers of the Birrindudu and Wiso basins hosting intermediate- scale to regional-scale groundwater systems; and (iii) fractured and weathered basalt aquifers of the KIP, fractured and weathered or porous sandstone aquifers of the Victoria Basin, and fractured and weathered or porous sandstone aquifers of the Bonaparte Basin hosting local-scale groundwater systems. Figure 2-9 summarises the spatial distribution of the main types of aquifers occurring beneath the Victoria catchment. In addition to the main types of aquifers, minor aquifers occur in: (i) surficial sediments predominantly include sandstone, siltstone and claystone in the east of the catchment; and (ii) alluvium (clay, silt, sand and gravel) associated with the major rivers and their tributaries. These surficial aquifers host variably productive and highly localised groundwater systems across minor parts of the catchment but are not shown in Figure 2-9. Their occurrence and distribution are shown in Figure 2-6 in Section 2.3.2. As shown in Figure 2-9, aquifer types regardless of their spatial extent host different groundwater flow systems ranging from minor and local, up to intermediate and regional scale. In this study, major aquifer systems are considered those that contain regional and intermediate-scale groundwater systems, with adequate storage volumes (i.e. gigalitres to a few teralitres) that could potentially yield water at a sufficient rate (i.e. >10 L/second) and be of a sufficient water quality (i.e. <1000 mg/L total dissolved solids (TDS)) for a range of irrigated cropping. Minor aquifers are considered those that contain local-scale groundwater systems with lower storage (i.e. megalitres to a few gigalitres). The bore yields from minor aquifers are variable but are often low yielding (i.e. <5 L/second) and have variable water quality ranging from fresh (~500 mg/L TDS) to saline (~20,000 mg/L TDS) and are mostly only suitable for stock and domestic use where appropriate. Aquifer systems can be unconfined, semi-confined or confined and can be interconnected across multiple hydrogeological units, which are summarised in the sections below. Figure 2-9 Major types of aquifers occurring beneath the Victoria catchment Note: Localised surficial aquifers hosted in sediments and rocks including unconsolidated Quaternary-age regolith and alluvium, and consolidated Cretaceous-age sediments are not shown. Aquifer type data source: Department of Environment Parks and Water Security (2008) A map of a large area Description automatically generated 2.4.2 Hydrogeological units A simplified spatial distribution of hydrogeological units of the Victoria catchment is shown in Figure 2-10. As described in Section 2.4.1, the rocks and sediments of these hydrogeological units host a diverse range of aquifers that vary in extent, storage and productivity. Major aquifers in the Victoria catchment host intermediate to regional-scale groundwater systems and are found in the: (i) fractured and karstic Cambrian limestone of the Wiso Basin in the east of the catchment, and (ii) fractured and karstic Proterozoic dolostone of the Birrindudu Basin in the centre and south of the catchment. Minor aquifers in the catchment host local-scale groundwater systems that occur in the: (i) Cambrian basalt of the KIP in the east of the catchment, (ii) fractured and weathered or porous Proterozoic sandstone of the Victoria Basin in the north and west of the catchment, (iii) fractured and weathered or Devonian–Carboniferous porous sandstone aquifers of the Bonaparte Basin in the far north of the catchment, and (iv) Cretaceous sandstone in the east of the catchment and Cenozoic alluvium that occurs in patches associated with the floodplains and channels of rivers and creeks and their tributaries. Limestone aquifers The Cambrian limestone, more specifically, the Montejinni Limestone occurs along the eastern margin of the catchment (Figure 2-10). The limestone hosts the CLA, which is interconnected across the adjoining Wiso, Daly and Georgina basins extending far to the north-east and south- east of the catchment occupying a total combined area of approximately 460,000 km2. While the CLA consists of three equivalent hydrogeological units (Montejinni Limestone, Tindall Limestone and Gum Ridge Formation), only the Montejinni Limestone occurs beneath about 12,000 km2 in the east of the catchment (about 15% of the total catchment area) (Figure 2-10). As the CLA extends outside the catchment, the catchment boundary does not represent any groundwater flow boundary or divide. The CLA is an interconnected and complex regional-scale karstic aquifer exhibiting a high degree of variability in physical properties but is one of the largest and most productive aquifers in the catchment (Randal, 1973; Tickell and Rajaratnam, 1995b). Its complexity arises from the variability and interconnectivity between fractures, fissures and karsts, which influence groundwater flow processes across the aquifer. The CLA in the Victoria catchment is poorly characterised and to date has only been developed for stock and domestic use and community water supply at Top Springs (Department of Environment Parks and Water Security, 2022; Randal, 1973; Yin Foo and Matthews, 2001). Elsewhere outside of the catchment, groundwater resources have been developed for groundwater-based irrigation (see Section 1.5.2). Recharge to the CLA occurs either directly in the aquifer outcrop or where it is unconfined beneath overlying Cretaceous or Cenozoic strata. The CLA outcrops around and to the south of Top Springs and is elsewhere unconfined beneath a veneer of overlying Cretaceous and Cenozoic strata (see Figure 2-6). Recharge processes include a combination of: (i) localised preferential infiltration of rainfall and streamflow via sinkholes directly in the aquifer outcrop (Department of Environment Parks and Water Security, 2022; Fulton and Knapton, 2015; Knapton, 2009; 2020; Randal, 1973; Yin Foo and Matthews, 2001), and (ii) broad diffuse infiltration of rainfall through the overlying Cretaceous and Cenozoic strata, which vertically leaks to the underlying CLA in these areas (Department of Environment Parks and Water Security, 2022; Fulton and Knapton, 2015). The CLA discharges via a combination of: (i) intermittent lateral outflow to streams (Armstrong River and Bullock, Cattle and Montejinni creeks) where they are incised into the aquifer outcrop (Department of Environment Parks and Water Security, 2022; Randal, 1973), (ii) perennial localised spring discharge (Old Top, Lonely, Palm and Horse springs) (Department of Environment Parks and Water Security, 2019b; 2022); (iii) evapotranspiration via riparian and spring-fed vegetation (ELA, 2022); and (iv) groundwater extraction for stock and domestic use, including community water supply (Department of Housing and Community Development, 2017). Groundwater flow in the aquifer system is complex due to the variability in the frequency, distribution and connectivity of karstic features across the aquifer and the spatial variability in recharge and discharge across large areas. At a local scale, groundwater flow can occur via preferential flow in connected holes, karsts and caverns, but across the aquifer extent, regional flow occurs via interconnected karstic features acting as a porous medium (Knapton, 2020). Groundwater flow in the Victoria catchment occurs at an intermediate- to local-scale from the east just outside of the Victoria catchment to the west towards spring complexes around Top Springs (ELA, 2022). In the Wiso Basin, regional flow east of the catchment boundary is from south to north towards the Flora River (ELA, 2022; Knapton, 2020; Yin Foo and Matthews, 2001). Bore yields are variable due to the complex nature of the karstic aquifer. Few properly constructed production bores have been installed in the CLA in the catchment and only limited pumping test have been conducted (Short, 2021; Tickell and Rajaratnam, 1995a). However, bore yields from stock and domestic bores often range between 2 and 10 L/second, indicating that higher yields may be achievable from appropriately constructed production bores (Tickell and Rajaratnam, 1995a). Elsewhere, east of the catchment, it has been found that where production bores have been installed, bore yields can commonly be more than 10 L/second (NTLIS, 2020; Yin Foo and Matthews, 2001). Long-term pump tests (24 to 72 hours) have only been conducted outside of the catchment, and aquifer hydraulic properties have been estimated. Transmissivity for the Montejinni Limestone part of the CLA ranges from 0.82 to 7900 (m2/day), and hydraulic conductivity ranges from 0.02 to 121.5 (m/day) (ELA, 2022). Groundwater quality in terms of salinity ranges from fresh (<500 mg/L TDS) to slightly brackish (<2500 mg/L TDS) and the chemical composition of groundwater is mostly of a calcium (Ca), magnesium (Mg), bicarbonate (HCO3) water-type (ELA, 2022). Figure 2-10 Simplified regional hydrogeology of the Victoria catchment To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Geological faults data from Department of Industry, Tourism and Trade (2010) Spring data from Department of Environment Parks and Water Security (2019b) Sinkhole data from Department of Environment Parks and Water Security (2014) A map of a large area Description automatically generated Dolostone aquifers Dolostones aquifers are hosted in the Bullita and Limbunya groups of the Birrindudu Basin across the centre and south of the Victoria catchment (Figure 2-10). Proterozoic dolostones host productive karstic intermediate-scale to local-scale aquifers. However, information for them is sparse and they are therefore poorly characterised. Dolostone aquifers of the Bullita Group outcrop in the centre of the catchment around Timber Creek and Yarralin and to a lesser extent in the south around Kalkarindji. Dolostone aquifers of the Limbunya Group mostly outcrop in the south of the catchment, west of Daguragu and Kalkarindji, and near Limbunya, which sits just outside and to the south of the catchment boundary (Figure 2-10). Outcropping and subcropping areas of the aquifers occur across a combined total area of 7000 km2. The most significant dolostone aquifers are hosted in the Skull Creek and Timber Creek formations of the Bullita Group between Timber Creek and Yarralin. For the Limbunya Group, it is the Campbell Springs and Pear Tree dolostones around Daguragu and Limbunya (Figure 2-10). Like the CLA, dolostone aquifers are complex due to the variability and interconnectivity between fractures, fissures and karsts across their spatial extent. Groundwater resources in the aquifers have to date only been developed for stock and domestic water supplies and for the water supply at Timber Creek (see Section 1.5.2). The variability in the nature and interconnectivity between karstic features influences the physical properties of the aquifers and groundwater flow processes across their spatial extent. Where aquifers are unconfined in either outcropping or subcropping areas beneath overlying Cenozoic strata, recharge is spatially variable and is inferred to occur via a combination of: (i) localised preferential infiltration of rainfall or streamflow where streams traverse the outcrop via sinkholes, fractures and faults; and (ii) broad diffuse infiltration of rainfall through the overlying Cenozoic strata, which vertically leaks to the underlying aquifers. Elsewhere, dolostone aquifers are confined by overlying Proterozoic sandstones and shales of the Auvergne and Tijunna groups respectively, or the APV (Figure 2-10). These overlying units influence the spatial variability in recharge to, and discharge from the aquifers (Figure 2-10). Dolostone aquifers are inferred to discharge via a combination of: (i) intermittent lateral outflow to streams (East Baines River and Crawford, Giles and Middle creeks) where they are incised into the aquifer outcrop (Tickell and Rajaratnam, 1998); (ii) perennial localised spring discharge at Kidman, Crawford and Dead springs across the Bullita Group (Department of Environment Parks and Water Security, 2019b), and Depot, Farquharson and Wickham springs across the Limbunya Group (Jolly and Jackson, 2004; Tickell and Rajaratnam, 1996); (iii) evapotranspiration via riparian and spring-fed vegetation, and (iv) groundwater extraction for stock and domestic use, including community water supply at Timber Creek. Information on the directions and scale of groundwater flow in the aquifers is sparse, and groundwater flow is anticipated to be complex due to the variability in the amount and connectivity of karstic features across the aquifer and the spatial and temporal variability in annual recharge and discharge. Groundwater flow is inferred to generally occur from the elevated parts of the outcropping areas radially towards the outcrop margins where spring complexes occur. Bore yields are variable due to the complex nature of the karstic aquifer but yields often range from 5 to 15 L/second (Moser, 1993; Pearson, 1985). However, where appropriately constructed production bores have been installed and pumping tests carried out for community water supply, yields have been as high as about 40 L/second (Pearson, 1985). Groundwater quality expressed as salinity is generally fresh (<500 mg/L TDS) but can be subtly brackish in places (<2000 mg/L TDS) (Pearson, 1985; Tickell and Rajaratnam, 1996). Pumping tests were conducted in the dolostone aquifer for community water supply at Timber Creek and Yarralin. Pumping tests in localised parts of the unconfined dolostone aquifer near Yarralin yielded estimates of transmissivity ranging from 75 to 500 m2/day and a specific yield of approximately 0.09 (Britten, 1986). In a confined part of the aquifer near Timber Creek, transmissivity ranged between 280 and 430 m2/day and storativity was 1.3 x 10-5 to 1.3 x 10-4 (Pearson, 1985). Sandstone aquifers Sandstone aquifers are mostly hosted in Proterozoic sandstone of the Auvergne Group in the Victoria Basin, particularly the Jasper Gorge Sandstone, but also in the Proterozoic Seale Sandstone of the Wattie Group of the Birrindudu Basin. The Jasper Gorge Sandstone outcrops and subcrops beneath minor patches of Cenozoic sediments extensively across an area of about 16,000 km2 in the north and west of the catchment (Figure 2-10). The Seale Sandstone is more localised, outcropping across a much smaller area (<250 km2) around Daguragu. These sandstones are weathered and faulted in places, hosting fractured rock aquifers containing local-scale groundwater systems of variable productivity and water quality. These localised aquifers provide an important source of water for stock and domestic use, though information for them is sparse. The most productive parts of the aquifers occur where the sandstone is heavily fractured in and around fault zones (Moser, 1994; Sanders and Rajaratnam, 1995c). Groundwater storage and flow primarily occurs via the secondary porosity features such as fractures, faults and joints (Moser, 1994; Sanders and Rajaratnam, 1995c; 1995d). Recharge is inferred to occur as: (i) localised infiltration of rainfall and some streamflow (where streams traverse the sandstone) into vertical fractures and joints, or (ii) broad diffuse infiltration of rainfall through the overlying Cenozoic strata which then vertically leaks to the underlying sandstone aquifers, which are unconfined in these areas. The main known discharge mechanisms are bores extracting groundwater for stock and domestic use, evapotranspiration from shallow watertables, and discharge to streams. Bore yields are variable depending on the degree and interconnectivity of fractures and joints around the bore casing. Bore yields can often be low (<2 L/second) where secondary porosity features are infrequent. However, where fracturing and jointing are common, yields of between 10 and 30 L/second can be achieved (Moser, 1994; Sanders and Rajaratnam, 1995a; Sanders and Rajaratnam, 1995d). Despite some reasonable bore yields in places, aquifer storage is limited to secondary porosity features, rendering them mostly suitable for stock and domestic use. The transmissivity of the unconfined sandstone aquifer near Daguragu/Kalkarindji was estimated as 1000 m2/day (Jolly, 2002). Pumping tests published in bore reports estimate similar transmissivities in the western area of the aquifer, but around Bulla transmissivities are less than 100 m2/day. Water quality for these aquifers is also variable, ranging between fresh (~500 mg/L TDS) to brackish (~9000 mg/L TDS) (Moser, 1994; Sanders and Rajaratnam, 1995b; Sanders and Rajaratnam, 1995c). Basalt aquifers Basalt aquifers are hosted in the Cambrian basalt, particularly the APV of the Kalkarindji Igneous Province, and occur across large parts of the east, south and to a lesser extent the west of the Victoria catchment (Figure 2-10). These basalt rocks are highly heterogenous and occur in association with sandstone and chert interbeds or basal agglomerate. They host fractured rock aquifer systems that supply small quantities of groundwater mainly used for stock and domestic purposes. These aquifers are highly variable in composition and host local-scale flow systems. Most groundwater storage and flow results from the size and connectivity of secondary porosity features such as joints, fractures or faults, except where porous sandstone, chert or agglomerate occur. Recharge occurs as localised infiltration of rainfall and some streamflow (where streams traverse these geological units) through the weathered, fractured and jointed basalt. Recharge also occurs as broad diffuse infiltration of rainfall through the overlying Cenozoic strata in the south of the catchment, that then vertically leaks to the underlying basalt aquifers, which are unconfined in these areas. Where basalt underlies limestone in the east of the catchment, the basalt aquifers are recharged in places from vertical leakage from the overlying CLA. The main discharge mechanisms are: (i) bores extracting groundwater for stock and domestic use, (ii) evaporation from shallow watertables, (iii) lateral discharge to streams, and (iv) localised discharge at discrete springs. Individual bore yields are variable but often low (<2 L/second). Pumping tests published in bore reports estimate transmissivity values for the unconfined parts of the aquifers to range between 0.2 and 632 m2/day, but most are less than 10 m2/day (Department of Environment Parks and Water Security, 2021). Where transmissivity values exceed 100 m2/day it coincides with bores in the basalt aquifers underlying and hydraulically connected to the overlying CLA along the eastern margin of the Victoria catchment. Water quality is variable, ranging from fresh (~500 mg/L TDS) to brackish (~3000 mg/L TDS) (Tickell and Rajaratnam, 1998). These aquifers offer little potential for future groundwater resource development beyond stock and domestic purposes. The exception to this maybe where they occur in conjunction with, and are connected to, the CLA hosted in the overlying Montejinni Limestone. Siltstone and shale aquifers Siltstone and shale aquifers are hosted in Proterozoic rocks of the Auvergne Group of the Victoria Basin, and the Tijunna Group of the Birrindudu Basin. The most prominent siltstone and shale aquifers in the Victoria catchment include the Angalarri Siltstone and Saddle Creek Formation of the Auvergne Group, and the Stubb Formation of the Tijunna Group. They outcrop or subcrop beneath overlying Cenozoic strata across an area of about 16,500 km2 in the centre and north of the catchment (Figure 2-10). These fine textured units host only partial fractured rock aquifers that are highly localised and contain minor groundwater resources that are very low yielding. Very little information is available for these aquifers other than from sparse stock and domestic bores (Sanders and Rajaratnam, 1995c; 1995d). Pumping tests published in bore reports typically estimate transmissivities of less than 10 m2/day in the shale aquifer, however a couple of pump tests estimated transmissivities greater than 100 m2/day north of Amanbidji (Department of Environment Parks and Water Security, 2021). Recharge to these partial aquifers is inferred to occur via broad diffuse infiltration of rainfall and streamflow where streams traverse the outcropping areas of these units into the upper highly weathered parts of the siltstones and shales. Where these units subcrop beneath overlying Cenozoic strata, recharge is inferred to occur via diffuse vertical leakage from Cenozoic strata to the underlying aquifers, which are unconfined in these areas. The main known discharge mechanism for the aquifers is from bores extracting groundwater for stock and domestic use. Though it is inferred that discharge is also likely to occur via: (i) evapotranspiration from shallow watertables, and (ii) lateral seepage to streams. These aquifers are highly variable in composition and are very low yielding (often <2 L/second). They contain highly variable water quality, and salinity ranges from fresh (<500 mg/L TDS) to brackish (i.e. ~9000 mg/L TDS) (Sanders and Rajaratnam, 1995b; Sanders and Rajaratnam, 1995c). These partial aquifers offer little to no potential for future groundwater resource development beyond stock and domestic purposes. Even developing them for stock and domestic purposes can be challenging due to poor bore yields and highly variable water quality. Surficial aquifers Surficial unconfined aquifers are hosted in sediments and rocks such as Cretaceous sandstone, siltstone and claystone of the Carpentaria Basin and unconsolidated Cenozoic alluvial and colluvial deposits of clay, silt, sand and gravel. Cretaceous rocks and sediments host basal sandstone aquifers, and Cenozoic alluvium hosts surficial aquifers that occasionally occur in patches in association with minor parts of rivers, creeks and their floodplains and channels across parts of the catchment. However, these aquifers have a limited extent and there is little information about them, so they are poorly characterised. Aquifers hosted in the Cretaceous rocks are mostly comprised of sandstone but only a few bores exist in these aquifers. Around Yarralin, the Wickham River alluvium has been developed for water supplies to the local community (Britten, 1986). Recharge to these aquifers occurs via either diffuse rainfall infiltration through overlying regolith to Cretaceous aquifers or localised infiltration of rainfall or streamflow directly into alluvial aquifers. The main discharge mechanisms are: (i) bores extracting groundwater for stock and domestic use, (ii) evaporation from shallow watertables, and (iii) discharge as lateral outflow to rivers and creeks, or vertical leakage to underlying aquifers. Individual bore yields are highly variable, ranging from less than 1 L/second to approximately 10 L/second (Britten, 1986). Pump tests have been conducted on production bores for Yarralin’s water supply. Transmissivities for the alluvium range between 300 and 500 m2/day, and specific yield ranges between 0.03 L/second and 0.20 L/s (Britten, 1986). Water quality as salinity is also highly variable, ranging from fresh (~500 mg/L TDS) to brackish (~13,000 mg/L TDS) (Britten, 1986). These aquifers have limited spatial extent and storage, and mostly offer little potential for future groundwater resource development beyond stock and domestic purposes. 2.5 Surface water hydrology The Victoria River and its tributaries, the most substantial of which are the Baines, Wickham, Armstrong, Camfield and Angalarri rivers, define a catchment area of 82,400 km2 (Figure 2-11). Tidal variation at the mouth of the Victoria River is up to 8 m, and these tides propagate upstream to just downstream of Timber Creek (Power and Water Authority, 1987). The catchment has a north−south rainfall gradient that influences the local hydrological response. The Camfield River in the drier far south of the catchment has an estimated mean runoff coefficient of 5%, while the Angalarri River in the north-east of the catchment has an estimated mean runoff coefficient of 17%. Rainfall, runoff and streamflow in the Victoria catchment are variable between years and within years. Approximately 82% of all runoff in the Victoria catchment occurs in the 3 months from January to March. In some locations, such as gauges 8110007 (Coolibah Homestead) and 8110013 (Dashwood Crossing), there is some evidence of near perennial flow. Perennial flow is also likely at gauge 8110074 on Montejinni Creek (where monitoring has been discontinued). In most other cases, flow is ephemeral. Mean annual flow at the catchment outlet of the Victoria River is estimated at 6990 GL, while median annual flow is 5730 GL. Annual variation is high, and annual flow is estimated to range between 800 and 23,000 GL. Flow is highly seasonal, and 93% of all flow occurs in the months December to March, inclusive. Intense seasonal rains from monsoonal bursts and tropical cyclones in the December to March period create flooding in parts of the Victoria catchment and inundate large areas of floodplains on both sides of the Victoria River and its two major tributaries, the Baines and Angalarri rivers. This is an unregulated catchment, and its overbank flow is generally governed by the topography of the floodplain. Flooding is widespread at the junction of Victoria and Baines rivers, downstream of Timber Creek. Figure 2-11 Streamflow observation data availability in the Victoria catchment and median annual streamflow 2.6 Water dependent ecosystems As the Victoria catchment has a clear north-south climatic gradient extending from high rainfall coastal areas to arid inland plains, it’s contrasting water regimes from north to south support a wide range of ecosystems with varying water dependencies. The Victoria River and its tributaries typically flow for only 6 months of the year following the onset of the wet-season. When the monsoon rains cease, the waterways dry up leaving For more information on this figure please contact CSIRO on enquiries@csiro.au disconnected waterholes that persist for various lengths of time depending on their depths and connection to groundwater (Tickell and Rajaratnam, 1998). During the dry-season the remaining waterholes and riparian vegetation become refuges for fauna in the region. Prior to European settlement the Victoria catchment was dominated by thinly wooded grasslands with steep-sided rivers and creeks, that were densely covered with reeds and contained a rich variety of annual vines and shrubs (Lewis, 2002). The introduction of cattle to the Victoria catchment in the 1800s and the proliferation of feral animals (mainly donkeys and horses) transformed the waterways. Heavy grazing on floodplains reduced the distribution and diversity of native vegetation, damaged the soils and led to extensive erosion during flooding (Lewis, 2002). Mangroves that line the estuarine part of the Victoria River are fragmented and are no longer dense or provide habitat for Black flying foxes (Pteropus Alecto) which used to be abundant. Further inland, the riverbanks are no longer steep nor densely vegetated by reeds. Once reliable billabongs no longer hold much water after the rain (Lewis, 2002). Despite historic damage to the waterways, the wetlands in the northern catchment support waterbirds during the dry-season and many rare and endangered species (Kirby and Faulks, 2005a). Floodplain habitats and riparian vegetation across the catchment support much higher species diversity and abundance than surrounding grasslands (Kirby and Faulks, 2005a). Many of these ecosystems are likely to be supported by surface water inundation and/or connection to groundwater, particularly towards the end of the dry-season or during drought (Figure 2-12). Permanent surface water bodies are scarce across the Victoria catchment. Most rivers are ephemeral with only disconnected pools remaining in most river channels by the end of the dry- season. The Victoria catchment contains two Directory of Important Wetlands in Australia (DIWA) wetlands (Figure 2-12): (i) the Legune wetlands maintained by surface water inflows from local creeks and wet-season floods; and (ii) the Bradshaw Field Training Area inundated by wet-season floods and enhanced by high tides. Both wetlands occur in the northern part of the catchment and provide important dry-season habitat for waterbirds (Stratford et al., 2024). Groundwater- dependent vegetation fringes the pools that are supported by persistent discharge or shallow watertables. Surface water dependent vegetation occupies areas that are seasonally inundated and often retain water for prolonged periods (e.g. paperbark swamps). Little is known about subterranean groundwater-dependent ecosystems (GDEs) in the Victoria catchment, but stygofauna were identified in caves in one study near East Baines River (Moulds and Bannink, 2012), and shallow karstic and alluvial aquifers present within the catchment may provide suitable habitat. Figure 2-12 presents the current state of knowledge about the type and distribution of GDEs and surface water-dependent ecosystems in the Victoria catchment. Terrestrial GDEs were mapped based on occurrences of vegetation species that are known to only grow where they have access to groundwater (known GDE: Eucalyptus camaldulensis, Melaleuca argentea, Barringtonia acutangular) and mapped persistence of vegetation throughout seasons (Castellazzi et al., 2024) based on remote sensing analysis and ground-truthing. There are many other vegetation species present in the Victoria catchment that may also be dependent on access to groundwater however they are unconfirmed and therefore not included in Figure 2-12. Aquatic GDEs were mapped using existing datasets: (i) groundwater-fed springs (Department of Environment Parks and Water Security, 2019b); and (ii) persistent waterholes (Doody et al., 2017). Mapped subterranean GDEs are limited to the identification of fauna that exclusively live in underground aquifers in only one location in the Victoria catchment (Moulds and Bannink, 2012). Mapped ecosystems that require surface water inundation include rivers, wetlands, swamps, land subject to inundation and vegetation species that are known to require surface water inundation (Eucalyptus camaldulensis, paperbark swamp species, monsoon vine forest species that are diagnostic of flood inundation; (Stratford et al., 2024). Figure 2-12 Spatial distribution of known and moderate to high potential of ecological assets related to groundwater including Directory of Important Wetlands, groundwater-dependent ecosystems identified in the GDE atlas and known spring locations Data sources: Nationally important wetlands – Environment Australia (2001); known and potential GDEs – Doody et al. (2017) Part IIMethods 3 Regional desktop and modelling assessment of the Victoria catchment Section 3 summarises the regional desktop and modelling assessment of the Victoria catchment and the methods that were used including: (i) the mapping of aquifers, (ii) the collation of key groundwater datasets and their attribution to different aquifers, (iii) groundwater recharge estimation and, (iv) groundwater discharge mapping. Groundwater investigations in this study were conducted using several approaches at a variety of scales (local, intermediate and regional-scale). A regional assessment of the Victoria catchment was performed as desktop analyses or modelling using historical hydrogeological information related to all aquifers in the catchment. The regional assessment was conducted to compile spatial datasets of geology, aquifer properties, bore locations and construction, and key groundwater attributes (water levels, water quality and bore yield) for aquifers in all hydrogeological units across the catchment. This baseline information was used to provide a regional overview of the available hydrogeological data across the catchment, specifically to: • generate geology, hydrogeology and aquifer type maps at the catchment-scale (Section 2) • collate and summarise key groundwater attribute data (depth to groundwater, salinity, indicative bore yield) to identify spatial trends in groundwater levels and quality, as well as aquifer hydraulic properties • derive spatial estimates of groundwater recharge across the catchment • derive spatial maps of groundwater–surface water connectivity and groundwater discharge across the catchment • generate spatial datasets to underpin further targeted field (design of a groundwater sampling and monitoring program), desktop (piezometric cross-sections, potentiometric surfaces, spatial maps of depth to aquifers and groundwater) and numerical modelling investigations of the Cambrian Limestone Aquifer (CLA) and Proterozoic dolostone aquifers (PDAs) described in detail in Section 4. 3.1 Geology, hydrogeology and aquifer-types Analysis was undertaken to collate and review publicly available data for digital spatial datasets on surface and basement geology and point locations with lithological and/or stratigraphic logs to generate geology, hydrogeology and aquifer-type maps as well as attribute aquifers to individual groundwater bores. Multiple datasets were collated from various sources including: (i) the Geoscience Exploration and Mining Information Systems (GEMIS) online database (https://geoscience.nt.gov.au/gemis/ntgsjspui/community-list) for accessing geoscientific reports and data kept by the Northern Territory Geological Survey (NTGS); (ii) Natural Resources Maps (NR Maps) online (https://nrmaps.nt.gov.au/nrmaps.html) web mapping tool for accessing and mapping natural resources data; (iii) the Northern Territory Government Open Data Portal, which contains datasets made available from Northern Territory Government agencies; and geoscientific data available from Geoscience Australia (https://www.ga.gov.au/data-pubs). In addition, data and information from publicly available literature were also used where applicable. Spatial data were then used to generate surface geology, regional geology, regional hydrogeology and aquifer type map figures presented in Sections 2.3 and 2.4. Lithological and/or stratigraphic logs for all available groundwater bores, exploration and stratigraphic holes and petroleum wells drilled in the Victoria catchment were also sourced from the Northern Territory Government Open Data Portal (Department of Environment Parks and Water Security, 2019a; Department of Industry Tourism and Trade, 2000; 2010) and used to overlay these point locations on various digital geology datasets. Available lithological and/or stratigraphic logs were then used in conjunction with bore construction (i.e. screened interval or open hole section) to attribute an aquifer to each site that contained bore construction information. This required using: (i) existing interpretation of lithology recorded in drillers logs, (ii) existing lithology and/or stratigraphic logs generated by geologists or hydrogeologists during historical drilling investigations, or (iii) interpretation from experts in the project team to assign an aquifer where lithology logs had little existing interpretation (sand, broken rock etc.) or where complimentary data such as groundwater chemistry were available. Where no log was available from drilling records, the aquifer was assigned as unknown. The aquifer attribution spatial dataset was then used to: (i) symbolise and map key aquifer data (water level, water quality, and bore yield) into different categories that represent the major and minor aquifers across the catchment, (ii) attribute aquifers to time-series water level and aquifer hydraulic property datasets, (iii) generate a spatial dataset of chloride concentration in groundwater for use in deriving spatial estimates of groundwater recharge, and (iv) generate aquifer-specific spatial datasets for undertaking further targeted field, desktop and modelling investigations of the CLA and PDAs described in detail in Section 4. 3.2 Groundwater levels Depth to groundwater is an important attribute for assessing local and regional trends in water levels, identifying recharge and discharge areas and understanding the spatial changes in depths from which water would have to be pumped to the surface for use. The primary source of static groundwater level data for groundwater bores across the Victoria catchment was the Northern Territory Government Open Data Portal which provides a direct link to the digital spatial groundwater database for the entire Northern Territory (Department of Environment Parks and Water Security, 2019a). In addition to digital data from the groundwater database, drilling records (hand-written, typed or digital) were also accessed via NR Maps (Department of Environment Parks and Water Security, 2021) to undertake the aquifer attributions described in Section 3.1. These drilling records also include observations of groundwater level, groundwater salinity and bore yield data from testing undertaken when the bores were first constructed and installed. Depending on how old the drilling records are, some data have not always been digitised and added to the territory-wide groundwater database. Therefore, where groundwater level observations were absent in the territory-wide database, observations from the drilling records were obtained and digitised. Static groundwater level observations were then linked to the aquifer attribution dataset in Section3.1 and then symbolised and mapped in classes in ArcGIS by aquifer where aquifer information was available. Available temporal groundwater level data were also reviewed by accessing the Northern Territory Water Data Portal (Department of Environment Parks and Water Security, 2024). Where temporal groundwater level data were available, the groundwater bores were attributed an aquifer using the aquifer attributions described in Section 3.1. Where an appropriate temporal range (>5 years) of water level data was available, hydrographs were generated to review rainfall recharge relationships. 3.3 Groundwater salinity Groundwater salinity is an important attribute for assessing the suitability of groundwater for different uses (i.e. domestic use, stock water, irrigation water or industrial applications) as well as identifying local and regional trends in spatial changes in salinity that may be associated with groundwater flow processes. Salinity data in the form of total dissolved solids (TDS) was collated from the territory-wide groundwater database accessed via the Northern Territory Government Data Portal (Department of Environment Parks and Water Security, 2019a). Prior to mapping and evaluating the salinity data, a review of the validity and quality of the analytical data was undertaken by calculating the charge balance error (CBE). Only water samples returning a CBE ±10% were used. The salinity data was then linked to the aquifer attribution dataset in Section 3.1 and symbolised and mapped in ArcGIS by aquifer where aquifer information was available and mapped in classes suitable for different water uses including stock water and irrigation. 3.4 Bore yields Bore yield is an attribute that provides an indication of the influence the physical properties of the rocks and sediments of different aquifers have on controlling the storage and flow of groundwater at different locations. It should be noted that long term (12–48 hour) pump tests on production bores provide the best indication of bore yield at a certain location in an aquifer. However, the majority of bore yield data in most locations is obtained by short term (i.e. a few hours) discharge testing by air lifting or a submersible pump on small diameter (i.e. <150 mm) stock and investigation bores. In most cases, the bore construction and size of the air compressor or submersible pump limit the yield from these bores. Nevertheless, the limited testing does provide a useful indication of aquifer potential. Bore yield data were collated from the territory-wide groundwater database accessed via the Northern Territory Government Data Portal (Department of Environment Parks and Water Security, 2019a). In addition to digital data from the groundwater database, drilling records (hand-written, typed or digital) were also accessed via NR Maps (Department of Environment Parks and Water Security, 2021) to undertake the aquifer attributions described in Section 3.1. Drilling records were reviewed for bore yield data and where the territory-wide database did not have a bore yield observation for a specific bore site, data from the drilling records were used where available. Bore yield data were then linked to the aquifer attribution dataset in Section 3.1 and then symbolised and mapped in different classes in ArcGIS by aquifer where aquifer information was available. 3.5 Aquifer hydraulic properties Aquifer hydraulic property data provide an indication of an aquifer’s physical properties including both subsurface hydraulic conditions and their ability to store and transmit water. They are also important to reliably predict both current states (water balances) and future states (predicted changes in water balances due to drawdown or streamflow and spring flow depletion from groundwater extraction) of groundwater flow systems. Hydraulic conditions include aquifer confinement status (confined, semi-confined (leaky), unconfined), dimension of flow (ranging from linear to radial to spherical) and flow domain type (single such as a porous aquifer, or dual domain such as a fractured rock aquifer) depending on the aquifer’s physical properties. These properties include transmission and storage capabilities (described by transmissivity and storage coefficient parameters, respectively), primarily of aquifers and, less commonly, of aquitards. Long-term (24 to 78 hour) constant-rate discharge pumping tests provide the most direct and scale-appropriate means of estimating subsurface hydraulic conditions and properties. Though short-term (a few hours) step tests can also be useful in some applications. Hydraulic conditions can be identified from qualitative assessment of measured time–drawdown responses and their temporal derivatives, which are compared to established diagnostic reference plots (Renard et al., 2009). Hydraulic properties are estimated by matching modelled solutions to measured time– drawdown responses. Confounding processes, including borehole storage and casing effects, can also be identified from measured time–drawdown responses. In the NT, locations of historical pumping tests were first collated by Nguyen and Tickell (2014). This dataset has been periodically updated since first publication. Historically, the estimation of subsurface hydraulic properties from these tests was limited to aquifer transmissivity and, to a much lesser extent, storativity. This was largely due to use of the Cooper and Jacob (1946) approximation of the Theis (1935) solution. A limited number of publications have collated, digitised and reinterpreted selected tests identified by Nguyen and Tickell (2014) associated with the CLA and aquifers hosted in adjacent hydrogeological units. This includes the aquifer test data review recently for the Strategic Regional Environmental and Baseline Assessment (SREBA) project by Amery and Tickell (2022) and the hydrogeological assessment of the CLA by Taylor et al. (2023). In this study, all historical pumping tests undertaken in the Victoria catchment were collated, reviewed and summarised by aquifer and is presented in Section 2.4.2. 3.6 Recharge estimation The method used for this study is an evolution of that used in the groundwater components of the Northern Australia Water Resource Assessment (NAWRA) project (Taylor et al., 2018a; Taylor et al., 2018b; Turnadge et al., 2018), the Bioregional Assessments (Crosbie et al., 2018) and Geological and Bioregional Assessment projects (Crosbie and Rachakonda, 2021) and most recently in the Sydney region (Wilkins et al., 2022b) and Great Artesian Basin (Crosbie et al., 2022). It includes three steps (Figure 3-1): • estimating recharge using the chloride mass balance method (CMB) at a point scale. This includes estimating the chloride in rainfall, runoff and groundwater • regression kriging of the point estimates of recharge. This involves upscaling recharge using regression equations, kriging the residuals of the point scale recharge and aggregating the regression rasters and residual rasters to form the regression kriging upscaled recharge rasters • evaluating the uncertainty in the spatial recharge estimates using 1000 replicates of the two processes above. These three steps are described in detail in the following sub-sections. Results are found in Section 5.4. Figure 3-1. Schematic diagram of the methods used showing the calculation of the point recharge, the upscaling using regression kriging and the reporting of the results at the scale of the region and surface geology group The area under investigation for recharge using the CMB method is substantially larger than the Victoria catchment. It includes the extent of the Wiso, Birrindudu and Victoria basins as the aquifers under investigation extend outside the Victoria catchment. Also included is the remainder of the catchment of the Fitzmaurice River and the NT portions of the catchment of the Keep and Ord rivers for the same reason (Figure 3-2). For more information on this figure please contact CSIRO on enquiries@csiro.au Stochastically generate chloride deposition surface (single realisation for all points) Stochastically generate chloride in runoff(each point individually generated) Stochastically generate chloride in groundwater(each point individually generated) Calculate point rechargeDevelop regression equation using covariatesCalculate recharge residualsKrige residuals to regular gridUpscale recharge to regular grid using covariatesAdd recharge and residuals gridRaster Stack of 1000 replicates of upscaled recharge estimatesSummary of replicates as 5th, 50thand 95thpercentilesResults aggregated by aquifer and surface geologyRepeat 1000 times Figure 3-2 Region used for estimating recharge using the chloride mass balance (CMB) method For more information on this figure please contact CSIRO on enquiries@csiro.au 3.6.1 Point scale chloride mass balance The CMB method (Anderson, 1945) is the most widely used approach for estimating net recharge, both globally (Scanlon et al., 2006) and in Australia (Crosbie et al., 2010). It is popular because it is robust over many climate zones and is cost effective, requiring only analyses of chloride in groundwater and rainfall. It is relatively insensitive to the mechanism of recharge, whether that be diffuse recharge through the soil matrix, by-pass flow through macropores or even captured surface water in sinkholes (Alcalá et al., 2011; Bazuhair and Wood, 1996). At its simplest, the CMB method can be used to estimate recharge (R, measured in mm/y) knowing only the chloride deposition from rainfall (D, measured in kilograms per hectare per year) and the chloride concentration of the groundwater (Clgw, measured in mg/L ): 𝑅𝑅=100 𝐷𝐷 𝐶𝐶𝑙𝑙𝑔𝑔𝑔𝑔 (1) This works because the chloride deposited on the land surface from the atmosphere is excluded from evaporation and transpiration and therefore the chloride becomes concentrated as it travels through the soil profile to the groundwater. This evapoconcentration allows estimation of the recharge rate. Wood (1999) listed the important assumptions behind estimating recharge using the CMB method: 1. Chloride in groundwater originates from rainfall on the aquifer and not from flow from underlying or overlying aquifers. 2. Chloride is conservative in the system. 3. Steady-state conditions are assumed in that the fluxes of chloride and water have not changed over time. 4. There is no recycling of chloride within the aquifer. These assumptions are discussed in the following paragraphs. For much of the study area, the chloride in groundwater is due to rainfall. In some alluvial areas there is additional chloride added to the groundwater system through losing streams and recharge due to overbank flooding. Chloride in groundwater observations from these areas need to be identified and removed from the analysis. In confined aquifers there can be some mixing of groundwater from layers above and below, so to avoid these complications, only groundwater- chloride observations from outcropping areas are in the analysis. Chloride is conservative in the system when there are no sources or sinks in the system. There is chloride export from the system through runoff, and this is accounted for by incorporating surface runoff into Equation (1). The steady state assumption is difficult to meet in areas that have undergone recent land use change (Cartwright et al., 2007b) such as the clearing of native vegetation for agriculture. There has been limited land clearing in the areas under investigation and so the assumption is made that groundwater observations are from the current land use. To avoid ‘old’ water and only sample ‘young’ water, chloride in groundwater observations from outcrop areas and in bores that are less than 100 m deep are used. Recycling of chloride occurs when groundwater is evaporated or transpired and then returned to the groundwater system. Recycling can occur in groundwater discharge areas (Bazuhair and Wood, 1996), flow through lakes (Howcroft et al., 2017) and through irrigation with groundwater (Wood and Sanford, 1995). These areas can be easily identified as having actual evapotranspiration greater than rainfall and so any chloride in groundwater observations from these areas can be excluded. Chloride in rainfall The chloride deposition rate (kg per ha per yr) has been obtained from a national gridded product at a 5 km resolution (Wilkins et al., 2022a). Because of the sparsity of observational data concerning chloride in rainfall, the uncertainty in the result is high in certain regions. Figure 3-3 shows the chloride deposition rate in the study area. The mean (μ), standard deviation (σ) and skewness (g) from 1000 replicate models are used to provide a Pearson Type III distribution of chloride deposition at each point location in the study area (Figure 3-3). The deposition according to the Pearson Type III distribution is given as (Pilgrim, 1987): 𝐷𝐷=𝜇𝜇+𝐾𝐾𝑌𝑌.𝜎𝜎 (2) where KY is a frequency factor calculated from g and a standard normal deviate (z): 𝐾𝐾𝑌𝑌=2 𝑔𝑔􀵤􁉄􁉀𝑧𝑧−𝑔𝑔 6􁉁𝑔𝑔 6+1􁉅 3−1􀵨. (3) The standard normal deviate is generated stochastically for each replicate and used to generate the chloride deposition rate for input into the recharge estimation at each point location. The same standard normal deviate is used for each bore location in a replicate to maintain spatial consistency in the recharge estimates. The same process is repeated 1000 times to generate the inputs for the 1000 replicates. Figure 3-3 The (a) mean, (b) standard deviation, and (c) skewness of the chloride deposition across the study region The blue squares indicate points where chloride deposition has been measured. Data source: Wilkins et al., 2022a) Chloride in runoff The chloride exported in runoff can be accounted for by modifying Equation (1) by assuming that the chloride exported in surface runoff is proportional to the runoff coefficient. The runoff coefficient (RC), which is the proportion of rainfall that becomes stream flow, can be obtained from the output of the Australian Landscape Water Balance model (Vaze et al., 2013) on a gridded basis and therefore can then be used to estimate RC at each point within the study area (Figure 3-4). To account for runoff, Equation (1) can be modified to: 𝑅𝑅=100𝐷𝐷(1−𝛼𝛼.𝑅𝑅𝑅𝑅) 𝐶𝐶𝐶𝐶𝑔𝑔𝑔𝑔 . (4) Here α is a scale factor, which is included because large rainfall events that result in large runoff volumes are often below average in chloride concentration, and therefore do not reduce chloride deposition proportionally. For this study, α is sampled stochastically from a uniform distribution with a value between 0.33 and 0.66 (Crosbie et al., 2018) during the uncertainty estimation (the second step of Figure 3-1). The value of α is equal for each bore in a replicate to maintain spatial consistency but is sampled individually for each replicate. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 3-4 Runoff coefficient across the study area. Chloride in groundwater Measurements of chloride in groundwater were sourced from two datasets: • the Northern Territory groundwater database (Department of Environment Parks and Water Security, 2019a) • the Bureau of Meteorology’s (BoM) National Groundwater Information System (NGIS) There were too many individual observations of chloride in groundwater to enable quality assurance and quality control (QA/QC) to be undertaken on each observation, so an automated procedure was employed to select the observations used in estimating recharge. This procedure was based on previous work (Crosbie et al., 2018; Crosbie and Rachakonda, 2021) and involved the following steps: For more information on this figure please contact CSIRO on enquiries@csiro.au 1. Duplicate records were removed. 2. The mean value of chloride in groundwater was calculated for each location where multiple analyses had been undertaken. 3. Any location where the excess water (rainfall minus actual evapotranspiration) was less than −25 mm/year was excluded. Negative excess water means the actual evapotranspiration is greater than the rainfall, which indicates an additional source of water and chloride, for example groundwater discharge, irrigation or flooding etc. These violate the assumptions of the chloride mass balance method (see Section 3.6.1 above). 4. Any bore that was greater than 100 m deep was excluded. The ideal sampling point for estimating recharge would be immediately below the watertable where water has recharged the groundwater store recently. Monitoring bores are not constructed this way, so limiting the depth is a compromise to ensure that water is sampled from outcrop areas and is as recent as possible. 5. Chloride in groundwater concentrations of less than 2 mg/L were excluded, as these measurements are probably not representative of groundwater. These analyses are closer to the concentration of rainfall and probably indicate a mis-transcribed value entered in the database, or a sample contaminated with rainfall through short circuiting of the bore casing. 6. Chloride in groundwater concentrations of greater than 2000 mg/L were excluded as these data are probably not representative of groundwater in recharge areas. These are more likely to be discharge areas, or downstream of discharge areas. 7. Bores that were located on alluvium as mapped in Geoscience Australia’s surface geology maps (Raymond, 2018) were excluded, as the alluvium is likely to have additional chloride present that is not directly from rainfall. This includes areas subject to flooding, irrigation or groundwater discharge from deeper or adjoining layers. 3.6.2 Upscaling point estimates of recharge The upscaling of the point estimates of recharge using regression kriging (Hengl et al., 2004) includes three steps (detailed in subsequent sections): 1. developing a regression equation to predict recharge across the study area using covariates that can be mapped on a regular grid 2. kriging the residuals between the point estimates of recharge and the regression equation estimates of recharge to provide a surface of residuals 3. adding the residual surface to the regression surface, which provides a spatial estimate of recharge that is informed locally by point data and away from point data is dependent upon a global relationship between the covariates and recharge. This process is repeated for 1000 stochastically generated replicates to quantify the uncertainty in the recharge estimates, as outlined in Figure 3-1. 3.6.3 Regression equations Previous studies have shown that recharge is better approximated by a log-normal distribution rather than a normal distribution (Cook et al., 1989; Eriksson, 1985), and this extends to its relationship with rainfall (Petheram et al., 2002). Reviews of recharge in Australia and globally have shown that recharge estimates are dependent upon rainfall, soil type and vegetation (Crosbie et al., 2010; Kim and Jackson, 2012; Scanlon et al., 2006) and these have been used successfully as covariates to upscale modelled point estimates of recharge (Crosbie et al., 2013). Surface geology has also been used as a covariate in upscaling recharge estimates (Crosbie et al., 2015; Crosbie et al., 2018). Based on these previous studies, the covariates used are: • annual mean rainfall over the 30-year period 1991-2020 (Jones et al., 2009) (Figure 3-5a) • mean clay content of the top 2m of the soil profile (Grundy et al., 2015) (Figure 3-5b) • Normalised Difference Vegetation Index (NDVI) as a measure of vegetation density (BoM, 2019) (Figure 3-5c) • simplification of the surface geology (Raymond et al., 2012) (Figure 3-5d) Figure 3-5 Covariates used in upscaling (a) rainfall, (b) clay content of the soil, (c) Normalised Difference Vegetation Index (NDVI) and (d) simplified geology. The regression equation used in the upscaling uses the covariates of log transformed rainfall (log10𝑃𝑃), clay content (C) and NDVI as a measure of vegetation density (V), and is: log10𝑅𝑅=𝛽𝛽𝑔𝑔𝑔𝑔𝑔𝑔+𝛽𝛽1.log10𝑃𝑃+𝛽𝛽2.𝐶𝐶+𝛽𝛽3.𝑉𝑉. (5) where βgeo, β1, β2 and β3, are fitting parameters fitted through least squares regression (which will be different for each of the 1000 replicates). The βgeo parameter is a set of three values representing the intercept value for each of the three geology classes (high, medium and low). Now consider a single replicate (that is a single choice for the chloride deposition and runoff) and the most successful covariates. To form the regression equation for this replicate, bootstrapping is used to reduce the effect of outliers and estimate the uncertainty in the regression equation (Efron and Tibshirani, 1994). That is, the same number of observations are randomly selected from the population of all observations, with replacement. This ensures that the same number of For more information on this figure please contact CSIRO on enquiries@csiro.au observations is used in the regression equation for each replicate but not every point will be included in each replicate, and some points will be included more than once. This minimises the bias in the regression equation that can occur due to outliers in the dataset. 3.6.4 Extracting recharge values for zones of interest The 1000 m resolution rasters produced may be used to estimate recharge aggregated over any area, average recharge rates have been extracted for the major aquifers of interest (Figure 2-10) and also the simplified surface geology (Figure 3-5d). 3.7 Identifying potential groundwater discharge areas using remote sensing Identifying areas of groundwater discharge has conventionally involved extensive observations in the field, this includes identifying the presence of phreatophytes, springs and seeps on the ground or increases in baseflow along river reaches. These approaches are labour intensive and impractical at the regional scale. As an alternative, remote sensing products offer an opportunity to identify and delineate the areas of potential groundwater discharge for further investigation in the field. 3.7.1 Digital Earth Australia The Digital Earth Australia (DEA) suite of products is derived from the Landsat Datacube produced by Geoscience Australia (Lewis et al., 2017). This dataset includes all Landsat imagery at a 25 m resolution dating back to 1987. There is a waterbodies dataset (Krause et al., 2021) and a Water Observations from Space (WOfS) dataset (Mueller et al., 2016) that are both potentially useful. The WOfS dataset classifies each pixel in each Landsat image as being inundated with water or not. The dataset contains a summary layer that is the proportion of images from the Datacube that a pixel is inundated, in this way areas that are permanently inundated can be discriminated from areas that are infrequently inundated (and dryland areas). The DEA waterbodies dataset is a polygon representation of contiguous areas that are inundated in more than 10% of images with a minimum area of 5 pixels (~0.3 ha). By identifying permanent waterbodies there is the potential to narrow the search for groundwater discharge areas as they are likely to be supported by groundwater during the dry season. The maximum proportion of time inundated from each pixel within a waterbody has been assigned to each polygon. This method can establish the permanence of a waterbody, but it has an issue with scale: a large waterbody will be assigned a degree of permanence based on a single 25 x 25 m pixel. Any waterbodies that can hold water throughout the dry-season can be further investigated for the source of water. This could be surface water that replenishes a deep waterhole during the wet-season and the depth of water is greater than the evaporation through the dry-season leading to the permanent presence of water. Alternatively, it could be a continuous discharge of groundwater throughout the year providing a permanent presence of water. This process would also identify anthropogenic water storages such as instream or farm dams. 3.7.2 Excess water The CMRSET v2.2 dataset (Guerschman et al., 2022; McVicar, 2022) uses high-resolution / low- frequency satellite data (e.g., Landsat and Sentinel-2) blended with low-resolution / high- frequency satellite data (e.g., MODIS and VIIRS) to produce a monthly actual evapotranspiration product at a resolution of 30 m from 2002 onwards. By subtracting the annual mean actual evapotranspiration (AET) from the annual mean rainfall (P), excess water is calculated. Areas with positive values of excess water (i.e. P>AET) are exporting water elsewhere, either through runoff or through groundwater recharge. Areas with negative values of excess water (i.e. P 11). The SrCO3 precipitate was then acidified and purified cryogenically into aliquots of carbon dioxide (CO2) for measurement by AMS (Fallon et al., 2010). Values were reported with a measurement precision of ±0.2% pmC. Samples for 13C were analysed using precipitated SrCO3 and freeze-drying samples under vacuum and immediately weighing approximately 1.6 mg into a 5 mL exetainer vial. The vial was flushed with helium and acidified with 0.2 mL of greater than 101% phosphoric acid (H3PO4). The CO2 was measured on a Sercon 20- 20 IRMS in continuous flow mode. The precision was 0.1‰ and was based on duplicate/triplicate measurements of unknown groundwater samples. Samples for dissolved noble gases were analysed at CSIRO’s Noble Gas Facility, Waite Campus, Adelaide. Dissolved gases were first separated from water using an offline extraction system. Gas subsamples were then analysed on a fully automated facility that conducts: (i) drying of all gases, (ii) raw gas analysis to determine the dinitrogen gas (N₂) to Ar ratio on a quadrupole mass spectrometer, (iii) separation of noble gases from reactive gases using a variety of reactive getter systems, (iv) separation of the noble gases by cryogenic techniques, and (v) measurement of gas volumes and their isotopic composition using a spinning rotor gauge, quadrupole mass spectrometers, and a high-resolution Helix MC noble gas mass spectrometer (Suckow et al., 2019). All noble gas concentrations are given in cubic centimetre of gas per gram (ccSTP/g where STP stands for standard temperature and pressure and the gram is related to the weight of the sample). Post processing of all noble gas measurements was done using the LabData laboratory information management and database system (Suckow and Dumke, 2001). 4.2.9 Interpretation of general chemistry Multiple approaches can be used for interpretating the ionic composition of a water sample, from simple ion plots and piper diagrams (Peeters, 2014; Piper, 1944) to more complex geochemical speciation modelling (Parkhurst and Appelo, 2013). In this study, the charge balance error (CBE) was initially calculated for each water sample to evaluate the viability of each sample with respect to analytical errors or preservation of the sample from field collection. Where the CBE was acceptable (±10%), ion plots were derived, and the ionic composition of groundwater and surface water compared to the seawater dilution line to evaluate if the sources of ions in water were from rainfall of marine origin or from other sources (e.g. water–rock interactions). Piper diagrams were used to plot groundwater and surface water samples to evaluate the hydrochemical evolution of groundwater to characterise the nature (i.e. inter-aquifer, aquifer–aquitard and groundwater– surface water connectivity) and scale of groundwater systems (i.e. local, intermediate or regional). Spatial maps of water quality were also derived to identify any spatial evolution of groundwater in the CLA, DCA and adjacent aquifers. RockWare Inc’s AqQA program was used to generate piper diagrams and to assess the salinity hazard for different water samples. PHREEQC (Parkhurst and Appelo, 2013) was used to: (i) determine the saturation indices (SI) for groundwater with respect to calcite, gypsum, and dolomite and (ii) ascertain the chemical speciation of aqueous CO2 and HCO3 in relation to the total dissolved inorganic carbon (TDIC) in groundwater. The latter was necessary for interpreting 14C values in groundwater for the potential addition of dead carbon from either mixing with atmospheric CO2 during recharge or isotopic exchange during flow through the carbonate aquifers, and to derive an MRT. 4.2.10 Interpretation of environmental tracers A range of methods exist to interpret different environmental tracers observed in groundwater including decaying tracers (radioactive isotopes) such as 3H and 14C, accumulating tracers such as 4He, or those with known temporal concentrations in the atmosphere such as CFCs. This includes the use of multiple one- and two-dimensional analytical solutions or lumped parameter models (LPMs) that can be parametrised using aquifer thickness and geometry (Appelo and Postma, 1996; Małoszewski and Zuber, 1982; Vogel, 1967; Zuber, 1986). In some cases, more complex numerical groundwater flow and solute transport models have been used where detailed information on aquifers is known (Reilly et al., 1994; Salamon et al., 2006; Sanford, 2011; Voss and Wood, 1994). In this study, existing hydrogeological conceptual models for the CLA (Bruwer and Tickell, 2015; ELA, 2022; Jolly et al., 2004; Karp, 2008; Knapton, 2004; 2009; 2020; Randal, 1973; Taylor et al., 2023; Tickell and Bruwer, 2018) and PDAs (Pearson, 1985; Tickell, 1998; Tickell and Rajaratnam, 1998) were validated or refined by interpreting measured tracer concentrations in groundwater from shallow unconfined parts of both aquifers (along topographic gradients and previously reported hydraulic gradients) and their adjacent aquifers, and comparing them to concentrations in mapped discharge areas (springs and seepage zones along streamlines). In addition, measured tracer concentrations in groundwater were used for estimating MRTs and areal or spatially averaged recharge rates for a range of porosities and depths within each aquifer. Measured tracer concentrations also collected in springs/surface water were used to characterise the spatial locations and sources of discharge/groundwater–surface water connectivity from the CLA and PDAs (see Section 4.2.12). 4.2.11 Groundwater recharge rates and residence times Groundwater residence times Residence times for groundwater flow and mixing were estimated by interpreting measured tracer concentrations with LPMs. The LPMs tested and evaluated in this study were: • the piston flow model, which assumes flow from the recharge to the discharge area with no mixing along the flow path (see Figure 4-10a, which is adapted from Jurgens et al. (2012)) • the exponential model, in which residence times for groundwater are stratified and increase logarithmically from the watertable to the base of the aquifer (Appelo and Postma, 1996; Vogel, 1967) (see Figure 4-10b, which is adapted from Jurgens et al. (2012)) The LPMs were used to predict the MRT for a given tracer at a specified depth in the aquifer. They used a few key assumptions and parameters: • Aquifer geometry was simplified based on the saturated thickness of aquifers estimated from measurements of groundwater levels at the time of field sampling and the base of the aquifer determined from lithological logs that were incorporated on to hydrogeological cross-sections (see Section 6.1). • Elevations of groundwater bores were taken from either surveyed elevation data or use of the 1- second SRTM-derived hydrologically enforced digital elevation model. • Mean annual recharge temperature was based on observations from the Bureau of Meteorology weather stations at Top Springs for the CLA and Auvergne for the PDAs (see Section 2.2). • Groundwater salinity was based on the mean total dissolved solids (TDS) estimated from the analyses of major and minor ions in groundwater (see Section 6.2.5). • Transient measurements of 3H in precipitation and atmospheric concentrations of CFCs were obtained from existing published databases (Bullister, 2015; Tadros et al., 2014). In this study, the predicated residence time distribution outputs from LPMs served as a basis for determining residence times from the tracers. Where multiple tracers are sampled at a given site, the most reliable tracer was identified and a simple model with only one free parameter (such as the piston flow model or the exponential model) was used to derive a MRT from that single tracer. MRTs were predicted assuming a steady-state groundwater system and advective groundwater flow and the transient input datasets described above based on the use of the convolution of a time-varying input signal with a response function (Małoszewski and Zuber, 1982). That is: 𝐶𝐶𝑜𝑜𝑜𝑜𝑜𝑜(𝑡𝑡)=􀶱𝐶𝐶𝑖𝑖𝑖𝑖(𝑡𝑡ʹ)⋅𝑔𝑔(𝑡𝑡−𝑡𝑡ʹ)⋅𝑒𝑒−𝜆𝜆(𝑡𝑡 – 𝑡𝑡ʹ)𝑑𝑑𝑑𝑑ʹ 𝑡𝑡 −∞ (7) where Cout(t) is the time-dependent 3H concentration in this example at a given sampling site, Cin(t’) is the 3H time series in precipitation in this example, t – t’ is the time difference between the concentration in precipitation and groundwater sampling at the bore, and λ is the radioactive decay constant of 3H. Figure 4-10 Schematic cross-section representations of advective flow in the (a) piston flow model and (b) exponential model in idealised unconfined aquifers Grey dots represent a sandstone aquifer. Nabla (inverted delta) represents the depth of the watertable below the land surface. Grey lines represent the evolution of groundwater flow lines from their origin at the watertable to the base of the aquifer and side of the cross-section. Figure source: Figure (a) is adapted from Figure 1 in Jurgens (2012) and Figure (b) is adapted from Figure 2 in Jurgens (2012) Groundwater recharge Recharge rates were derived using multiple approaches: (i) the CMB method was employed where appropriate (i.e. where certain assumptions can be met) as described in Section 3.6, and (ii) environmental tracers were applied where appropriate (i.e. adequate bore construction, valid analytical results, available groundwater-level and aquifer property information). Advective flow in unconfined aquifers All groundwater samples in this study were collected from shallow unconfined portions of the aquifers. Mean annual recharge rates were therefore estimated using the MRTs derived for the most reliable tracers from either the PFM or EMM LPM described above. A key assumption here was that the depth of the sample is small relative to the total aquifer thickness. Accordingly, the recharge rate R (mm/year) can be given by the following equation from Cook and Böhlke (2000): 𝑅𝑅= 𝑧𝑧𝑧𝑧 𝑡𝑡 (8) where z is the depth of the sample below the watertable, taken as the mid-point of the screen (m), ε is aquifer porosity and t is the MRT (years) based on the concentration of 3H or CFC-12, for For more information on this figure please contact CSIRO on enquiries@csiro.au example, at a specified depth collected within an aquifer. MRTs derived from tracers were used as input into equation (7). 4.2.12 Groundwater–surface water interactions Water sampling at key spring complexes from either individual spring vents or lateral seepage zones was conducted in conjunction with the groundwater sampling described in Section 4.2.3, to better characterise the sources of discharge from the CLA and PDAs. Across the spatial extent of the CLA and PDAs, spring complexes and lateral seepage zones often occur at aquifer margins or margins of the outcropping and subcropping areas of the aquifers where contact with adjacent hydrogeological units occurs. As these GDEs are ecologically and culturally important, its crucial to characterise their water sources. In addition, it provides the basis for better constraining the water balance for the aquifers and understanding if the springs have potential to be impacted by reduced water availability from future groundwater resource development. The NT springs dataset (Department of Environment Parks and Water Security, 2019b) was overlayed on the simplified regional hydrogeology presented in Figure 2-10 in Section 2.4 and the geology and existing water source data for several key springs were reviewed in relation to the CLA and PDA to identify candidate sites for water sampling of chemistry and environmental tracers. In addition, remotely sensed data identifying potential discharge areas described in Section 3.7.3 was also overlayed on the hydrogeology and used in conjunction with the NT springs dataset to design the spring sampling program. Cambrian Limestone Aquifer Very few investigations have characterised the source of spring discharge at key spring complexes and lateral outflow or seepage to lower reaches of streams on the western margin of the CLA in the far east of the Victoria catchment (Figure 4-11). This is mostly because there has been little development of groundwater from the aquifer other than for stock and domestic use and water supply to the community at Top Springs. Reconnaissance investigations by Tickell and Rajaratnam (1998), including isotope sampling by Tickell (1998) provided an initial characterisation that has been incorporated into the NT springs database (Department of Environment Parks and Water Security, 2019b). However, historical investigations by Randal (1973) and more recent investigations by Eco Logical Australia (ELA, (2022) and Amery and Tickell (2022) highlight that localised discharge from the CLA may be an important component of the water balance for the aquifer. In addition, historical investigations by Randal (1973) and more recent investigations by Taylor et al. (2023) have indicated that the CLA can be vertically connected in places to the underlying APV highlighting the potential for some springs near the margin of the CLA and APV potentially sourcing their water from the CLA. Figure 4-11 Target area for spring and surface water sampling of potential groundwater discharge locations for the Cambrian Limestone Aquifer in the Victoria catchment To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) To better characterise the spatial locations of localised spring discharge the NT springs dataset (Department of Environment Parks and Water Security, 2019b) and remotely sensed estimates of perennial discharge described in Section 3.7.3 were reviewed in relation to the hydrogeology along the western margin of the CLA in the east of the catchment (Figure 4-11). Candidate sites for a water sampling campaign were identified with a focus on spring complexes occurring along the western margin of the CLA and parts of the APV near the western margin of the CLA associated with the Armstrong and Camfield rivers and their tributaries that traverse both aquifers (Figure 4-11). Water samples were collected from springs and lateral outflow or seepage zones using a 12V submersible pump placed directly into the spring or reach of the stream below the water surface. Using the same approach as for the groundwater sampling, springs and/or streams were monitored for general chemistry (pH, temperature, EC and dissolved oxygen (DO)) using a YSI multi-parameter probe and the alkalinity of water was also measured in the field (see sections 4.2.5 and 4.2.7). Water samples were collected at spring and stream sites across the CLA and APV where access was possible. Water samples were collected during a spring sampling trip conducted between 19th and 21st of October 2023. Samples were collected for major and minor ions and environmental tracers including: 2H,18O, 87Sr/86Sr, 3H and noble gases (He, Ne, Ar, Kr, Xe). Proterozoic dolostone aquifers Similar to the CLA, there have been very few investigations that have characterised the source of spring discharge at key spring complexes and lateral outflow or seepage to reaches of streams in the centre of the Victoria catchment where the PDAs outcrop and subcrop between Timber Creek and Yarralin (Figure 4-12). This is mostly because there has been little development of groundwater from the aquifer other than for stock and domestic use and water supply to the communities at Timber Creek and Yarralin. Reconnaissance investigations by Tickell and Rajaratnam (1998), including isotope sampling by Tickell (1998) provided an initial characterisation that has been incorporated into the NT springs database (Department of Environment Parks and Water Security, 2019b). To better characterise the spatial locations of localised spring discharge the same approach was used as described above for the CLA using the NT springs dataset (Department of Environment Parks and Water Security, 2019b) and remotely sensed estimates of perennial discharge described as being reviewed in relation to the hydrogeology in the centre of the catchment (Figure 4-12). Candidate sites for a water sampling campaign were identified with a focus on spring complexes occurring on the margins of the outcropping and subcropping areas of the PDAs and where tributaries of the Victoria River traverse these areas in the centre of the catchment between Timber Creek and Yarralin (Figure 4-12). Water samples were collected from springs and lateral outflow or seepage zones using a 12V submersible pump placed directly into the spring or reach of the stream below the water surface. Springs and/or streams were also monitored for general chemistry (pH, temperature, EC and DO) using a YSI multi-parameter probe and the alkalinity of water was also measured in the field (see sections 4.2.5 and 4.2.7). Water samples were collected at spring and stream sites across the PDAs where access was possible. Water samples were collected during the spring sampling trip described above conducted between 19th and 21st of October 2023. Samples were collected for major and minor ions and environmental tracers including: 2H,18O, 87Sr/86Sr, 3H and noble gases (He, Ne, Ar, Kr, Xe). Figure 4-12 Target area for spring and surface water sampling of potential groundwater discharge locations for the Proterozoic dolostone aquifers in the Victoria catchment To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) A map of the earth Description automatically generated 4.3 Numerical flow modelling Unless stated otherwise the following contextual and methods summary comes from a companion technical report on groundwater flow modelling by Knapton et al. (2024). The CLA and PDAs are the two largest, most productive and potentially most promising aquifers within and beneath the Victoria catchment for future groundwater-based development (Pearson, 1985; Tickell and Rajaratnam, 1998). Parts of these aquifers coincide with land recently identified as potentially suitable for agricultural intensification (Thomas et al., 2024). To better understand the potential opportunities and risks associated with future groundwater resource development, water resource modelling was undertaken to provide new information that could potentially help inform future water resource planning, investment, and management across parts of the CLA in the eastern Victoria catchment considered potentially suitable for groundwater-based irrigation. CSIRO engaged CloudGMS Pty Ltd to run, process and evaluate the results of multiple future hypothetical groundwater development and climate scenarios using the existing finite element groundwater model (DR2 model) of the CLA (Knapton, 2020). As the PDAs are yet to be developed, are data sparse, and require further investigation, there is no existing groundwater flow model for the aquifers, so groundwater flow modelling was only undertaken for the CLA. The specific objectives of this modelling investigation were to provide indicative modelling to assess the possible effects of changes in rainfall and potential evaporation and future hypothetical development of groundwater resources on the availability of groundwater for environmental requirements of surface water resources (e.g. environmental receptors such as several spring complexes in the Top Springs region) and existing water users along the eastern margin of the Victoria catchment. To do this, four scenarios were simulated using the DR2 groundwater model of the CLA. The climate data used as input to the groundwater model was sourced from climate analyses undertaken in a companion technical report on the climate of the Victoria catchment by McJannet et al. (2023). The locations for future hypothetical groundwater development are outlined in Section 4.3.4. This section provides a succinct summary of the methods used for the numerical modelling, and Section 6.3 provides a summary of key results. Further details on model construction, calibration, sensitivity analysis and predictions are provided in Knapton et al. (2024). 4.3.1 Modelling time period The CLA is mostly a regional-scale groundwater flow system, so across most parts of the aquifer it may take a few hundred to several hundreds of years, before the system re-establishes a quasi- equilibrium state, where the groundwater flow patterns stabilise following a change in state, such as climate or development. In the Victoria catchment, only a small thin portion of the western part of the CLA exists along the eastern edge of the Victoria catchment (12,000 km2) where flow paths are much shorter (<30 km). Consequently, the impacts of hypothetical future groundwater development or future climate are likely to occur over shorter timescales (a few years to tens of years). Modelling scenarios were conducted to examine how different levels of groundwater development and/or a change in projected climate might impact water resources in the CLA over a specific period in the future (2055-2065), using historical and future climate data. Prior to running any hypothetical groundwater development and climate scenarios, a warmup period of 109-years of historical climate data was used to prime the CLA groundwater model to 2019. For the projected 2060 model state, the models were run for 159 years comprising a warm- up period of 109-years using the historical climate data (1910 to 2019) and thereafter using the 50-year historical and future climate and groundwater development inputs to evaluate the 2059 to 2069 representative conditions (nominally representing 2060 conditions). The 50-year historical and future climate and groundwater development inputs are based on the time series of rainfall and potential evapotranspiration (PET) from the period 1910 to 1960. Thereafter, using the historical and future climatic sequences and hypothetical groundwater development inputs, changes in: (i) the annual water balance across different parts of the CLA, (ii) groundwater levels and groundwater drawdown propagation across different parts of the CLA, and (iii) changes in spring flow and evapotranspiration (ET) were evaluated for the period 2055 to 2065, representative of 2060 conditions. The approximately 40-year time period (2060) was considered a pragmatic time period over which to consider the impacts of changes in climate and groundwater extraction because: (i) it is equivalent to more than twice the length the investment period of a typical agricultural enterprise, (ii) it is about the service life of an appropriately constructed groundwater production borefield, (iii) it is about five times the length of the current period over which NT water licences are assigned, and (iv) it is consistent with the time period over which future climate projections have been evaluated. 4.3.2 Historical and future climate datasets Establishing future climate scenarios The historical and future climate sequences were used to assess four different scenarios: • Scenario A – historical climate and current groundwater development • Scenario B – historical climate plus current and future hypothetical groundwater development • Scenario C – future climate (dry, mid and wet) and current groundwater development • Scenario D – future climate (dry, mid and wet) plus current and future hypothetical groundwater development. Future climate sequences were generated using methods outlined in the companion technical report by McJannet et al. (2023). Briefly, the potential impacts of future climate change were evaluated within a sensitivity analysis framework. Future climate sequences were evaluated using seasonal scaling factors from selected global climate models to scale the historical climate data. The entire climate sequence was then re-scaled using an annual scaling factor representing a percentage change from the long-term annual rainfall and PET. This provided a method to generate future climate datasets based on the sensitivity to climate change (±10% rainfall and PET). Percentage change in long-term annual rainfall and PET were based on the 10, 50 and 90% exceedance values shown in McJannet et al. (2023). These datasets were applied to the groundwater models, resulting in changes to mean annual recharge. The following variations are used in the future climate scenarios C and D: • Scenario Cdry represents a reduction in the long-term mean annual rainfall by 10% and an increase in the long-term mean annual PET by 10%. • Scenario Cmid represents a reduction in the long-term mean annual rainfall by 2% and an increase in the long-term mean annual PET by 7.5%. • Scenario Cwet represents an increase in the long-term mean annual rainfall by 10% and an increase in the long-term mean annual PET by 5%. • Scenario D includes all of the variants of Scenario C of the climate change scenarios (i.e. Cdry, Cmid, Cwet) with the addition of proposed variations in future hypothetical groundwater development, nominally called scenarios Ddry, Dmid, Dwet. In considering changes to the hydrological regime (groundwater-level drawdown and groundwater flux) under different scenarios, all results were assessed relative to current conditions (i.e. Scenario A, which is based on historical climate and current groundwater development). 4.3.3 Adopted model scenarios and naming conventions The first scenario (Scenario A) represents ‘recent climate’ scenarios and is based on the 50-year historical climate sequence without any future hypothetical groundwater development, only current levels of groundwater development. The A scenario was used as the baseline against which scenarios B9, B12, and B15 described below and shown in Table 4-2 assessments of relative change were made. The second scenario (Scenario B) is also a ‘recent climate’ scenario, using the same 50-year climate sequences as Scenario A. It assumes growth in groundwater development using the levels of hypothetical future development shown in Table 4-2. Scenario B assesses water availability under hypothetical future groundwater development. The third scenario (Scenario C) represents a ‘future climate’ with current development. It is based on the 50-year future climate sequences derived from scaling rainfall and PET described in Section 4.3.2. Scenario C uses the current level of groundwater development. The fourth scenario (Scenario D) considers ‘future climate’ and future development. It uses the same future climate sequences as Scenario C but assumes growth in groundwater use according to the hypothetical future development shown in Table 4-2. Table 4-2 Summary of modelling scenarios A, B, C and D for the Cambrian Limestone Aquifer using 109 years of historical climate and combinations of current and hypothetical future groundwater development CLA A Historical climate and current development B9 Historical climate and current development + 3 x 3 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to an additional 9 GL/y) B12 Historical climate and current development + 3 x 4 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to an additional 12 GL/y) B15 Historical climate and current development + 3 x 5 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to an additional 15 GL/y) Cdry Cdry corresponding to a 10% reduction in mean annual rainfall and 10% increase in potential evaporation relative to the historical climate (1910–2019) and current development CLA Cmid Cmid corresponding to a 2% reduction in mean annual rainfall and 7.5% increase in potential evaporation relative to the historical climate (1910–2019) and current development Cwet Cwet corresponding to a 10% increase in mean annual rainfall and 5% increase in potential evaporation relative to the historical climate (1910–2019) and current development Ddry9 Cdry climate and current development + 3 × 3 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 9 GL/y) Ddry12 Cdry climate and current development + 3 × 4 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 12 GL/y) Ddry15 Cdry climate and current development + 3 × 5 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 15 GL/y) Dmid9 Cmid climate and current development + 3 × 3 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 9 GL/y) Dmid12 Cmid climate and current development + 3 × 4 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 12 GL/y) Dmid15 Cmid climate and current development + 3 × 5 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 15 GL/y) Dwet9 Cwet climate and current development + 3 × 3 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 9 GL/y) Dwet12 Cwet climate and current development + 3 × 4 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 12 GL/y) Dwet15 Cwet climate and current development + 3 × 5 GL/y hypothetical future enterprises (i.e. total hypothetical future extraction sums to additional 15 GL/y) 4.3.4 Locations for hypothetical future groundwater development Locations for hypothetical future groundwater development across the CLA (Figure 4-13) were selected based on spatial analyses of land suitability grids derived by Thomas et al. (2024) overlain on: (i) the spatial hydrogeology data for the extent of the CLA (Department of Environment Parks and Water Security, 2008; Department of Industry Tourism and Trade, 2024) (see Figure 4-13), and (ii) the spatial water plan and management zone data (Department of Environment Parks and Water Security, 2019d) (see Figure 4-13). In addition, the DR2 (CLA) Leapfrog geological model was used to: (i) extract gridded depth to the top of the CLA, (ii) extract gridded depth to groundwater, and (iii) extract multiple hydrogeological cross-sections to evaluate the saturated thickness. From this, based on potential water availability in the current Georgina Wiso Water Allocation Plan (Northern Territory Government, 2023), three hypothetical locations were chosen where there is: (i) potentially suitable land for agricultural intensification; (ii) a suitable aquifer exhibiting a saturated thickness greater than 20 m; (iii) suitable groundwater quality for irrigation (≤1000 mg/L TDS; (iv) a threshold drilling depth to intersect the aquifer of less than 200 mBGL; and (vi) a depth to groundwater of less than 100 mBGL. Each location was placed at greater than 10 km from existing licensed water users and key GDEs. This area is between Top Springs, south-east towards Cattle Creek, a tributary of the Camfield River. Figure 4-13 Spatial locations for hypothetical groundwater extraction sites across the Cambrian Limestone Aquifer (CLA) relative to land that is potentially suitable for agricultural intensification, current and proposed water management zones and groundwater discharge features (springs) for the aquifer Also shown as the blue circles are sites for reporting modelled groundwater levels Data sources: Water allocation plan areas (Department of Environment Parks and Water Security, 2019c); water management zone areas (Department of Environment Parks and Water Security, 2019d) 4.3.5 Mean annual water balances Mean annual water balances for the different modelling scenarios described in Section 4.3.5 are reported for: (i) for the entire of the DR2 model domain within the Victoria catchment, and (ii) two water management zones, the Wiso Water Management Zone related to the current Georgina Wiso Water Allocation Plan, and the proposed Flora Tindall Water Management Zone related to the proposed Flora Tindall Water Allocation Plan. Mean annual water balances are reported for the ten-year period 2055-2065 and water balance components are reported in gigalitres per year. 4.3.6 Groundwater levels Groundwater levels are documented for six sites across the CLA. The locations of the reporting sites are presented in Figure 4-13. The mean modelled groundwater level for each of the sites was calculated to provide a simple measure of the effects of each scenario on the groundwater systems of the CLA. The mean groundwater level for each of the scenarios was calculated from the ten-year period 2055 to 2065 (representing 2060) and are reported in mAHD. 4.3.7 Groundwater drawdown To demonstrate the spatial extent of the effects associated with each scenario, the change in groundwater elevation or groundwater drawdown has been calculated for the ten-year period 2055 to 2065 (representing 2060) of each scenario and presented as contours. Drawdowns calculated for Scenarios B, C and D use Scenario A as the reference elevation. 4.3.8 Groundwater discharge Groundwater discharge to springs and ET are presented as hydrographs in m3/day for the areas in the eastern Victoria catchment within the DR2 model. The discharge at these features was calculated by summing fluxes at the Dirichlet BC nodes used to represent the springs and diffuse ET. Mean groundwater discharge has also been calculated to provide simple measures that reflect changes to the discharge regime under each scenario. The mean groundwater discharge for the projected 2060 conditions was calculated from the ten-year period 2055-2065 (representing 2060) and are reported as gigalitres per year. Part III Results 5 Regional assessment of the Victoria catchment Section 5 summarises the results of the regional assessment of the catchment 5.1 Groundwater levels 5.1.1 Static groundwater levels Using the aquifer attribution dataset described in Section 3.1, Table 5-1 summarises the available groundwater level data for bores in aquifers hosted in different hydrogeological units across the Victoria catchment including a 50 km ‘buffer zone’ to the east of the catchment, incorporating some of the Wiso Basin (Department of Environment Parks and Water Security, 2019a). This buffer was included because the Cambrian limestone extends outside and to the east of the catchment boundary which does not represent the groundwater flow boundary. Table 5-1 Summary of groundwater level data for bores in aquifers hosted in different hydrogeological units of the Victoria catchment HYDROGEOLOGICAL UNIT QUATERNARY ALLUVIUM CAMBRIAN LIMESTONE CAMBRIAN BASALT PROTEROZOIC SANDSTONE PROTEROZOIC SHALE PROTEROZOIC DOLOSTONE Range min (mBGL) 2.8 5.0 -0.7 -1.8 -0.4 0.6 Range max (mBGL) 85 136 131 107 21 63 Mean (mBGL) 15 73 19 16 10 19 Median (mBGL) 16 76 11 12 10 13 Count 60 85 330 125 20 72 There is approximately 1470 bores across the Victoria catchment, and approximately 1630 bores with the inclusion of the 50 km buffer zone east of the catchment boundary. Of these bores, there was static standing water level (SWL) data available for just over 1000 bores across the catchment and 50 km buffer zone to the east, approximately 349 of those bores did not have a lithological log available in drilling records and could not therefore be attributed to an aquifer. Most bores with SWL data come from local-scale aquifers hosted within the Cambrian basalt (n=330), most of which are installed in the Antrim Plateau Volcanics (APV) (Figure 5-1). The second largest group of bores with SWL data come from local-scale aquifers hosted in the Proterozoic sandstone (n=125), where most bores are installed in the Jasper Gorge and Seale sandstones (Figure 5-2). The Cambrian limestone had SWL data available from 85 (n=85) bores, almost all of which are exclusively installed in the Montejinni Limestone which hosts the regional-scale Cambrian Limestone Aquifer (CLA) (Figure 5-1). There were also a large number of bores with SWL data installed in intermediate-scale aquifers hosted in the Proterozoic dolostone (n=72), where most bores are installed in the Skull Creek and Timber Creek formations in the centre of the catchment and the Campbell Springs and Pear Tree dolostones in the south of the catchment (Figure 5-1). Figure 5-1 Static groundwater levels for the major aquifers of the Victoria catchment Symbols represent different aquifers that bores are installed in, the colour indicates the groundwater level classes. Groundwater level data source: Department of Environment Parks and Water Security (2019a) Bores installed in aquifers hosted in both the Quaternary alluvium (n=60) and Proterozoic shale (n=20), had the fewest SWL data as these hydrogeological units host highly variable local-scale aquifers that are very data sparse (Figure 5-2). Despite there being no available lithological logs for the 330 ‘unknown’ sites, their spatial distribution suggests the majority are likely to be installed in the outcropping and subcropping areas of the Cambrian basalt (Figure 5-2). For more information on this figure please contact CSIRO on enquiries@csiro.au There are very few clear spatial trends in SLW across the Victoria catchment which is not surprising given it is dominated by highly variable local-scale aquifers hosted in Cambrian basalt, and Proterozoic sandstone and shale (see Section 2.4). The only clear spatial trend in SWL data is for bores installed in the regional-scale CLA hosted in the Cambrian limestone along the eastern margin of the catchment. Groundwater levels in the CLA in areas around Top Springs and as well as further south near the Camfield River appear to be shallowest (between zero and 50 mBGL) and increase in depth towards the eastern catchment boundary to depths greater than 50 mBGL (Figure 5-1). Figure 5-2 Static groundwater levels for the minor aquifers of the Victoria catchment Symbols represent different aquifers that bores are installed in, the colour indicates the groundwater level classes. Groundwater level data source: Department of Environment Parks and Water Security (2019a) For more information on this figure please contact CSIRO on enquiries@csiro.au 5.1.2 Temporal groundwater levels Groundwater hydrographs were generated for 8 bores with an appropriate time series (i.e. >5 years of data) available from the Northern Territory Water Data Portal (Department of Environment Parks and Water Security, 2024) and interpreted in terms of wet-season rainfall responses during observation periods. Observations from bores were historically taken as part of regular monitoring of groundwater levels in aquifers hosted in the Proterozoic dolostones, specifically the Skull Creek and Timber Creek formations near Timber Creek (Figure 5-4). Observations were also available from several bores installed in alluvial aquifers hosted in the Quaternary alluvium of the Wickham River near Yarralin (Figure 5-4). At both locations, historical observations were part of regular monitoring of groundwater extraction for community water supplies at both locations. Figure 5-3 Locations of bores with sufficient time series water level data for generating hydrographs To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Quaternary rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Proterozoic dolostone aquifers Hydrographs for four groundwater bores installed in the Proterozoic dolostone aquifers (PDAs), specifically two bores installed in the Timber Creek Formation (RN023446 and RN023848), and two bores installed in the Skull Creek Formation (RN008645 and RN023445) near Timber Creek, are shown in Figure 5-4 (see Figure 5-3 for their locations). Observations of groundwater levels shown in the hydrographs during the period 1985 to 1999 featured wet-season responses that ranged in magnitude from 0.3 to 4.3 m (Figure 5-4). Relatively small wet-season responses were observed at RN008645 the most northern bore installed in the Skull Creek Formation (0.3 to 2.5 m) (Figure 5-4a). Moderate magnitude responses were observed at RN023446 (0.9 to 3.7 m), the closest bore to Timber Creek and installed in the Timber Creek Formation (Figure 5-4b). Similar to For more information on this figure please contact CSIRO on enquiries@csiro.au RN023446, moderate magnitude responses were observed at RN023848 (0.7 to 4.3), the most southern bore installed in the Timber Creek Formation (Figure 5-4c). Moderate magnitude responses (1.1 to 3.9 m) were also observed at RN023445, which is not far from RN023848 but is installed in the Skull Creek Formation (Figure 5-4d). Overall, there was a slight net increase in groundwater levels exhibited in all four hydrographs during the observation period due to larger and more frequent wet-season rainfall during the 1990s, compared to the 1980s (Figure 5-4). Figure 5-4 Groundwater hydrographs for temporal static standing water level observations from the Proterozoic dolostone aquifers near Timber Creek between 1984 and 1999 Data source: Northern Territory (NT) water data portal (Department of Environment Parks and Water Security, 2024) For more information on this figure please contact CSIRO on enquiries@csiro.au 050100150200048121620Daily rainfall (mm) Hydraulic head/Ground suface (mAHD) (a) RN008645050100150200048121620Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (b) RN023446050100150200141822263034Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (c) RN0238480501001502001416182022241984198519861987198819891990199119921993199419951996199719981999Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (d) RN023445Groundwater hydraulic headGroundlevel sufaceDaily rainfall Alluvial aquifers Hydrographs for four groundwater bores installed in the Quaternary alluvium of the Wickham River are shown in Figure 5-5 (see Figure 5-3 for their locations). Observations of groundwater levels shown in the hydrographs during the period 1985 to 1999 featured wet-season responses that ranged in magnitude. Yarralin township during the period 1985 to 1999 featured wet-season responses that ranged in magnitude from 0.3 to 1.5 m (Figure 5-5). All four bores (RN022823, RN022826, RN023844 and RN022824) had relatively small wet-season responses were observed (Figure 5-5a to d). Overall, there was minimal net change in groundwater levels during the observation period, though groundwater levels have a very subtle decline in the last 5 years of the 1990s when the magnitude and frequency of wet-season rainfall was lower around Yarralin than for Timber Creek during the same period. Figure 5-5 Static groundwater level hydrographs for the Yarralin area 1994 to 1999 Data source: Northern Territory (NT) water data portal (Department of Environment Parks and Water Security, 2024) For more information on this figure please contact CSIRO on enquiries@csiro.au 0501001502002508286909498102Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (b) RN0228260501001502002508286909498102Daily rainfall (mm) Hydraulic head (mAHD) (c) RN0238440501001502002508286909498102Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (a) RN02282305010015020025082869094981021984198519861987198819891990199119921993199419951996199719981999Daily rainfall (mm) Hydraulic head (mAHD) (d) RN022824Groundwater hydraulic headGround surfaceDaily rainfall 5.2 Groundwater salinity Using the aquifer attribution dataset described in Section 3.3, Table 5-2 summarises the available groundwater salinity data for bores in aquifers hosted in different hydrogeological units across the Victoria catchment. There was available salinity data of an acceptable quality (charge balance error (CBE) within ±10%) for 628 bores across the catchment and 50 km buffer zone to the east, but 247 of those bores could not be attributed to an aquifer. Most bores with available salinity information comes from local-scale aquifers hosted in the Cambrian basalt (n=182), where most bores are installed in the APV (Figure 5-6). The next largest group of bores with available salinity information is from local-scale aquifers in the Proterozoic sandstone (n=62), where most bores are installed in the Jasper Gorge and Seale sandstones (Figure 5-7). There was also a reasonable amount of salinity information for the intermediate-scale aquifers hosted in the Proterozoic dolostone (n=52), where most bores of are installed in the Skull Creek and Timber Creek formations. There is sparse salinity information for the regional-scale CLA hosted in the Cambrian limestone (n=28), where most bores are installed in the Montejinni Limestone (Figure 5-6). There is only sparse salinity information for aquifers hosted in the Quaternary alluvium (n=47), and Proterozoic shale (n=17), as there has been little drilling of these highly heterogenous local-scale aquifers (Figure 5-7). Table 5-2 Summary of groundwater salinity data for bores in aquifers hosted in different hydrogeological units of the Victoria catchment HYDROGEOLOGICAL UNIT QUATERNARY ALLUVIUM CAMBRIAN LIMESTONE CAMBRIAN BASALT PROTEROZOIC SANDSTONE PROTEROZOIC SHALE PROTEROZOIC DOLOSTONE Range min (mg/L) 275 290 165 85 90 280 Range max (mg/L) 12830 4310 6130 9130 940 1960 Mean (mg/L) 1173 899 634 986 500 577 Median (mg/L) 440 655 469 495 470 452 Count 47 28 182 62 17 52 Major aquifers in the Victoria catchment contain mostly fresh groundwater (mostly <1000 mg/L TDS) (Figure 5-6). However, minor parts of the major aquifers contain some brackish to moderately saline groundwater (>3000 mg/L TDS). Salinity in the regional-scale CLA hosted in the Cambrian limestone varies between fresh in the northern part of the aquifer around Top Springs to slightly brackish in the southern part of the aquifer, with mean and median groundwater salinities were 899 and 655 mg/L, respectively (Figure 5-6 and Table 5-2). Of all of the major aquifers, the intermediate-scale aquifers hosted in the Proterozoic dolostone contain the most consistently fresh groundwater with mean and median salinities of 577 and 452 mg/L, respectively (Figure 5-6 and Table 5-2). Local-scale aquifers hosted in the Proterozoic sandstone have some fresh groundwater across the northern higher rainfall parts of the catchment compared to southern parts of the catchment in lower rainfall areas near Amanbidji where the aquifers are saline (Figure 5-7). Groundwater salinity for local-scale aquifers hosted in the Cambrian basalt is typically fresh (mean 634 and median 469 mg/L), with some isolated brackish patches (Figure 5-6). Groundwater salinity for local-scale aquifers hosted in the Quaternary alluvium and Proterozoic shale seem to contain mostly fresh groundwater, though isolated parts are brackish to saline (Figure 5-7). However, these aquifers are data sparse. Figure 5-6 Groundwater salinity for the major aquifers of the Victoria catchment Symbol shape indicates different aquifers that bores are installed in, the colour indicates the groundwater salinity classes as total dissolved solids (TDS). Salinity data source: Department of Environment, Parks and Water Security (2014) A map of a large area with rivers and rivers Description automatically generated Figure 5-7 Groundwater salinity for the minor aquifers of the Victoria catchment Symbol shape indicates different aquifers that bores are installed in, the colour indicates the groundwater salinity classes as total dissolved solids (TDS). Salinity data source: Department of Environment, Parks and Water Security (2014) A map of a river Description automatically generated 5.3 Bore yields Using the aquifer attribution dataset described in Section 3.1, Table 5-3 summarises the available groundwater bore yield data for bores in aquifers hosted in different hydrogeological units across the Victoria catchment. There was available bore yield data for 1162 bores across the catchment and 50 km buffer zone to the east, 409 of those bores could not be attributed to an aquifer. The local-scale aquifers hosted in the Cambrian basalt have the most bore yield data (n=366), most bores of which are installed in the APV (Figure 5-8). The next largest group is for the local-scale aquifers hosted in the Proterozoic sandstone (n=135), most bores of which are installed in the Jasper Gorge and Seale sandstones (Figure 5-9). The regional-scale CLA hosted in the Cambrian limestone has a modest amount of bore yield data (n=82), most of which is for bores exclusively installed in the Montejinni Limestone (Figure 5-8). The intermediate-scale aquifers hosted in the in Proterozoic dolostone have a similar amount of information for bore yields as the CLA (n=81), most of which is for bores installed in the Skull Creek and Timber Creek formations and Campbell Springs and Pear Tree dolostones (Figure 5-8). The local-scale aquifers hosted in the Quaternary alluvium and Proterozoic shales have very sparse bore yield information, 68 and 21 bores respectively (Figure 5-9). Table 5-3 Summary of groundwater bore yield data for bores in aquifers hosted in different hydrogeological units of the Victoria catchment HYDROGEOLOGICAL UNIT QUATERNARY ALUVIUM CAMBRIAN LIMESTONE CAMBRIAN BASALT PROTEROZOIC SANDSTONE PROTEROZOIC SHALE PROTEROZOIC DOLOSTONE Range min (L/s) 0.1 0.1 0.0 0.1 0.1 0.1 Range max (L/s) 11 8.0 40 38 5.0 38 Mean (L/s) 2.4 2.4 2.8 4.2 2.3 4.0 Median (L/s) 2.0 2.0 2.0 2.0 2.0 1.8 Count 68 82 366 135 21 81 Overall, most aquifers in the Victoria catchment exhibit low bore yields (median and mean yields of <2.5 L/second), except for aquifers hosted in the Cambrian limestone, and Proterozoic dolostones and sandstones (Figure 5-8). However, most aquifers remain sparsely tested using appropriately constructed production bores (casing diameter >150 mm, screened intervals >10 m) and long-term pumping tests (24-to-72-hour constant-rate discharge tests) due to the lack of groundwater development across the catchment. As discussed in Section 3.4, the bore yield data collated in this study is only indicative as most bore testing is by short term (i.e. a few hours) discharge testing by air lifting or a submersible pump on small diameter (i.e. <150 mm) stock and investigation bores (not long-term (24 to 72 hour) pumping tests). However, these values provide a good indication of the potential for the aquifer to yield water at a sufficient rate for different purposes. Figure 5-8 Groundwater bore yields for the major aquifers of the Victoria catchment Symbol shape indicates different aquifers that bores are installed in, the colour indicates bore yield classes. Bore yield data source: Department of Environment, Parks and Water Security (2014) A map of a large area with red and black dots Description automatically generated Figure 5-9 Groundwater bore yields for the minor aquifers of the Victoria catchment Symbol shape indicates different aquifers that bores are installed in, the colour indicates bore yield classes. Bore yield data source: Department of Environment, Parks and Water Security (2014) 5.4Recharge estimation This section containsthe results of estimating rechargeusingthe chloridemass method. It followsthe sameformat as themethods (Section 3.6). The results arepresented at the point scale,then upscaled to theentire study area, andfinally recharge is extracted for various areas of interest. 5.4.1Point scale chloride massbalance Thedatabase of chloride in groundwater measurements contains 1322 locationswith at least onechloride in groundwater observation (Figure5-10). The chloride in groundwater ranged from 2 to24,000 mg/L with a median of 48 mg/L. At thesepoints the mean chloridedeposition in rainfallranged from 21 kgperhaper year alongthe coast, to 1.4 kilogramsper hectareper year further inland. Of these1322 points, 1035were retained after being assessed againstthe criteria in Section3.6.1. The median of the 1000replicates of point recharge at these1035 points is shown in Figure5-10. It ranges between 0.1 to472 mm/year, witha mean of 20 mm/year and a median of 7 mm/year, demonstratingthe skewed distribution of recharge. Figure5-10(a)The chloride in groundwater observationswithin the study region and(b)the median of the pointscale estimates of recharge derived from them Data source: chloride in groundwater data was sourced from the NorthernTerritory groundwater database (Department of Environment Parks andWater Security, 2019a) Chapter5 Regional assessment of the Victoriacatchment|101 5.4.2 Upscaling point estimates of recharge Regression equations The median of the 1000 replicates of point recharge is used in an exploration of the covariates chosen for upscaling (Figure 5-11 and Figure 5-12). There is a positive correlation between rainfall and recharge (Figure 5-11a), as rainfall increases so does recharge. Clay content of the top 2 m of the soil profile also shows a positive correlation (Figure 5-11b). This is not the expected result. Recharge is expected to be higher with a lower clay content, a negative correlation, but this relationship cannot be seen here without accounting for the correlation with rainfall first. The relationship with the Normalised Difference Vegetation Index (NDVI) is also not as expected as a higher vegetation density should result in lower recharge. The positive correlation seen (Figure 5-11c) is due to the relationship between rainfall and NDVI. As higher vegetation density is expected with higher rainfall, the relationship with rainfall needs to be accounted for before the negative correlation with recharge can be seen. The three geology classes show the expected result, the regression line for the high recharge class is above the regression line for all points combined, the low recharge class has a regression line lower than all points combined and the medium class is close to the combined line (Figure 5-12). Figure 5-11 Point-scale relationships between log recharge and (a) rainfall, (b) clay content of the soil, and (c) Normalised Difference Vegetation Index (NDVI) For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-12 Point scale relationships between log rainfall and log recharge by geology class Black line is the regression through the points for that class and the red line is the regression through all points for the three geology classes combined The combined results of the covariates in predicting recharge were analysed through multiple linear regressions. The parameter results are shown as boxplots in Figure 5-13. The value of βgeo for the high recharge class is greater (less negative) than the values for the medium and low recharge geology classes indicating that the high recharge class does indeed have higher recharge than the other classes all other variables being equal. The coefficients for the clay content and NDVI are negative as they should be, as increases in clay content or NDVI should lead to decreased recharge. The results of the regression equation can be measured by the R2 as a measure of the goodness-of-fit of the models. The R2 had a mean of 0.49 and a range of 0.36 to 0.61 for the 1000 replicates. This means that the regression equation is only able to explain (on average) 49% of the variance in the recharge estimates. This is probably a reflection of a noisy dataset rather than a poor model and is similar to other predictions of recharge using regression analysis (Crosbie et al., 2018; Crosbie and Rachakonda, 2021). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-13 Coefficients used in the regression equations for upscaling the 1000 replicates (a-f), (g) the R2 for each of the 1000 replicates The line in the centre of the box is the median, the box represents the interquartile range (the 25th to 75th percentiles), the whiskers represent the 10th and 90th percentiles and the dots are any data points outside of the 10th and 90th percentiles. NDVI = Normalised Vegetation Index Kriging residuals The point estimates of recharge can be upscaled using the fitted regression equations and the spatially mapped covariates. The mean of the 1000 replicates is shown in Figure 5-14a. The spatial trends in the recharge follow the rainfall trends with the highest recharge in the areas with the highest rainfall and vice versa. The residuals of the fitted recharge from the regression equation were kriged to create a surface, the mean of the 1000 replicates is shown in Figure 5-14b. This surface has a mean of zero and has a mean absolute residual of the upscaled surface of 0.10. In areas without data points, the residual surface tends to zero. This is by design, as in these areas the recharge estimates will be reliant upon the regression equations. In areas with data points the regression estimates will be ‘corrected’ toward the point values of recharge. The variogram used has a nugget of ~0.1 which prevented over-fitting: this can be seen in the lack of ‘bullseyes’ around each data point. The range of the variograms is ~40 km, this is the distance that each data point can influence. If some structure can be found in the patterns of the residual surface, then this could be brought into the regression equations to reduce the residuals and increase the fit of the regression equations, but the cause of the patterns could not be identified in this study. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-14 (a) The median value of the 1000 replicates of upscaled recharge using the regression equation and (b) the median of the residuals kriged to a regular grid also showing the points used Aggregating recharge rasters When the residuals are added back to the regression estimates of recharge, the final set of regression-kriging upscaled recharge estimates is created. These are displayed as the 5th, 50th and 95th percentiles of the 1000 replicates in Figure 5-15. Over the entire study area, the 50th percentile surface of recharge has a mean of 11 mm/year, the 5th percentile has a mean of 6 mm/year and the 95th percentile has a mean of 21 mm/year (Figure 5-16). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-15 The 5th, 50th and 95th percentiles of upscaled recharge from the 1000 replicates using regression kriging For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-16 The 5th, 50th and 95th percentiles of constrained recharge for the Victoria catchment The 50th percentile upscaled recharge estimates on a 0.05° (~1000m) grid are compared to the median point estimates of recharge in Figure 5-17. This shows that the mean R2 for the 1000 replicates has increased from 0.49 for the regression equation to 0.73 for the regression kriging demonstrating a much better fit to the point estimates of recharge from the CMB method. Figure 5-17 (a) A scatterplot of the 50th percentile of the recharge calculated at a point scale versus the 50th percentile of the regression kriging upscaled recharge estimate (red line is 1:1 for reference), and (b) the R2 of all 1000 replicates for the point estimates of recharge versus the upscaled estimates of recharge as a boxplot For more information on this figure please contact CSIRO on enquiries@csiro.au For more information on this figure please contact CSIRO on enquiries@csiro.au 5.4.3 Extracting recharge values for zones of interest Recharge rates were extracted for the outcropping areas of aquifers hosted within the regional hydrogeological units of the Victoria catchment, as shown in Figure 2-10 (Table 5-4). The recharge over the Proterozoic dolostone (27 mm/year) is much greater than the recharge over the Cambrian Limestone (7 mm/year) as it is in a higher rainfall zone. Table 5-4 Average recharge rates over each of the major aquifers. The 50th percentile is outside the brackets and the 5th and 95th percentiles give a range for the uncertainty within the brackets MAJOR AQUIFERS RECHARGE (MM/Y) Quaternary alluvium 15 (8 – 28) Cambrian limestone 7 (3 – 13) Cambrian basalt 12 (6 – 24) Devonian-Carboniferous sandstone 20 (8 – 63) Proterozoic sandstone 20 (10 – 38) Proterozoic shale 27 (14 – 48) Proterozoic dolostone 27 (15 – 49) The mean recharge rates aggregated to the simplified surface geology classes (Figure 3-5d) is shown in Table 5-5. The highest recharge was seen in the Palaeozoic or older (partial) aquifer (including the Proterozoic dolostone) as this is in the highest rainfall part of the catchment (around Timber Creek). Table 5-5 Mean recharge rates over the simplified surface geology classes (Figure 3-5d). The 50th percentile is outside the brackets and the 5th and 95th percentiles give a range for the uncertainty within the brackets SIMPLIFIED SURFACE GEOLOGY CLASS RECHARGE (MM/Y) Palaeozoic or older partial aquifer 39 (21 - 70) Volcanics 15 (7.5 - 29) Alluvium colluvium 14 (7.5 - 28) Cenozoic low permeability sediments 9 (4.7 - 17) Aquitard 19 (8.7 - 37) Sediments undifferentiated 5.4 (2.4 - 11) Palaeozoic or older aquifer 37 (21 - 66) Cenozoic aquifer 2.8 (0.9 - 7.6) Quaternary sediments undifferentiated 4.7 (1.4 - 11) 5.4.4 Uncertainty Estimates of groundwater recharge using the CMB method in this study represent the spatial variability in recharge across the land surface and are a good starting point for estimating a water balance arithmetically or using a groundwater model. However, none of the methods accounts for aquifer storage (available space in the aquifer), so it is unclear whether the aquifers can accept these rates of recharge on an annual basis. The methods also do not account for potential preferential recharge from streamflow or overbank flooding, or through karst features such as dolines and sinkholes that occur across parts of the Victoria catchment. Therefore, the key features of an aquifer must be carefully conceptualised before simply deriving a recharge flux based on the surface area of an aquifer outcrop and an estimated recharge rate. 5.5 Identifying potential groundwater discharge areas using remote sensing 5.5.1 Digital earth Australia The Digital Earth Australia water bodies dataset (Krause et al., 2021) identifies 2015 waterbodies that are greater than 0.3 ha in size and contain water more than 10% of the time (Figure 5-18). Only 126 of these water bodies contain water more than 75% of the time and can be considered (semi-) permanent. The majority of these occur along the Victoria River and its tributaries and are associated with the narrow alluvium along the riparian zone. This dataset has not identified the springs that are of interest associated with the regional scale (CLA), specifically the Montejinni Limestone and the PDAs, specifically the Skull Creek and Timber Creek formations. This may be a scale issue as the resolution of the data is 25 x 25 m with a minimum of 5 pixels and the springs are generally smaller than this. Figure 5-18 Water bodies in the Victoria catchment identified from Digital Earth Australia and the proportion of time that water bodies are inundated from Water Observations from Space The size of the water bodies has been exaggerated so that they can be seen at the catchment scale. Data source: Krause et al. (2021) 5.5.2 Excess water The map of excess water across the Victoria catchment (Figure 5-19) shows which areas are exporting water (green) and which are importing water (purple). Areas that are importing water are potentially groundwater discharge areas (or floodplains, irrigation areas etc). There is a large area following the highway from Top Springs to Kalkarindji that appears to have much higher actual evapotranspiration (AET) than rainfall. These areas are on the APV downgradient from the Montejinni Limestone. They are not in topographic lows and so appear to be overestimated AET rather than potential groundwater discharge areas. The other large area that can be seen with a high negative excess water is on the coast in the north-west of the catchment, this area is potentially periodically inundated by sea water. Figure 5-19 Excess water across the Victoria catchment 5.5.3 Potential discharge areas over whole catchment There were 30,025 polygons identified in the Victoria catchment as potential groundwater discharge features. Of these, 4,254 were larger than 0.5 ha and were considered further. It is these larger potential discharge areas that are important at this regional scale when considering a water resource assessment. Smaller discharge features can potentially be identified down to 900 m2 pixel size, but these would be very localised in scale and difficult to consider at the regional scale. Not all of these 4254 polygons are groundwater discharge features. Any permanent water bodies that are above the watertable and act as recharge features will also have been identified. Without an adequate depth to watertable map of the catchment these features cannot be algorithmically excluded and need to be manually excluded. Each polygon was inspected individually and assigned to one of five categories (Figure 5-20, Table 5-6). The ‘perennial groundwater discharge’ category included 1096 polygons for a total area of 1537 ha. These features are predominantly springs and soaks in the mid to upper reaches of catchments and are related to geological contacts. The ‘seasonally varying’ category included 1054 polygons for a total area of 3946 ha. These features are mostly associated with the alluvium in the Victoria River and its major tributaries. They are conceptualised as being recharged by surface water during the wet-season and then discharging this water during the dry-season through evapotranspiration, there may also be a component of this groundwater discharge from local or regional flow from other geological units. There were no polygons identified in the ‘recharge’ category. The ‘coastal’ category included 954 polygons for a total area of 13,014 ha. These are in the estuarine part of the Victoria River and along the coast of Joseph Bonaparte Gulf. These areas may have a component of groundwater discharge along with the evapotranspiration of surface water and sea water. The ‘mis-identified?’ category included 1150 polygons for a total of 2350 ha. These features seem to have anomalously high October evapotranspiration without a geological explanation. Table 5-6 Summary of areas identified as potential groundwater discharge areas CLASS COUNT AREA (HA) Perennial groundwater discharge 1096 1537 Seasonally varying 1054 3946 Recharge 0 0 Coastal 954 13,014 Mis-identified? 1150 2350 Total 4254 20,849 Figure 5-20 Areas of potential groundwater discharge across the Victoria catchment. The size of the polygons has been greatly exaggerated to allow them to be seen at this scale Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) The Springs database of Northern Territory Government (Department of Environment Parks and Water Security, 2019b) is the best comparison dataset to assess the performance of this method. This database has been compiled over decades of field work where springs have been identified in the field, it does not purport to be complete or to capture all the major springs. Of the 72 springs identified, only one is classified as ‘major’: Wickham Spring. Wickham spring was correctly identified here as a potential groundwater discharge area. If the 0.5 ha criterion is ignored then the median distance between the springs in the Northern Territory Government database and those identified here is 660 m. Considering the accuracy of the mapping in the database (some springs locations are noted as approximate, others as being 100s of metres upstream) it can probably be concluded that half of the springs were correctly identified. Some non-identification of springs could be scale dependent, a pixel in the remote sensing data is 900 m2, small seeps and soaks will not be identified. A better test for springs identified here would be field verification of the more than 1000 springs identified that are not in the Northern Territory Government database. 5.5.4 Summary of potential discharge areas from remote sensing The mapping of water bodies from DEA and assessing their permanence using Water Observations from Space (WOfS) is useful for identifying water bodies that are larger than 0.3 ha. The pixel size is 25 m from Landsat and therefore minor pools and water holes within streams are often too small to be identified using this platform (along much of its length the Victoria River is not identified as a water body). Excess water calculated using CMRSET data identified many areas that had evapotranspiration that were much greater than rainfall but some of these were not groundwater discharge areas (mostly streamlines and wetlands receiving run-on). The use of October evapotranspiration from CMRSET provided a much more useful guide to the location of groundwater discharge areas. Areas that are still using a lot of water at the end of the dry-season have another source other than rainfall, in a lot of cases this will be groundwater. The method developed here identified 1096 potential springs across the Victoria catchment compared to the 72 springs that have been documented from field studies. This method successfully identified about half of the springs in the database, there are expected to be differences due to the scale of investigation. The next step is field verification of the springs that are not in the Northern Territory Government database. 6 Targeted field, desktop and modelling investigations Section 6 presents the results of targeted field, desktop, and modelling investigations of: (i) the Cambrian Limestone Aquifer (CLA) hosted in the Montejinni Limestone, and its potential connectivity with the underlying Antrim Plateau Volcanics (APV) and several springs along the western margin of the CLA; and (ii) the Proterozoic dolostone aquifers (PDAs), particularly the Skull Creek Formation, being one of the most prominent dolostone-rich hydrogeological units of the PDAs and its potential connectivity with the Battle, Bynoe and Timber Creek formations and several springs along the outcropping/subcropping margin of the PDAs in the centre of the catchment of the Victoria River. 6.1 Hydrogeological framework 6.1.1 Airborne electromagnetics Regional elevation surfaces Two elevation surfaces (100 mAHD and zero mAHD) of the inverted bulk conductivity across all three airborne electromagnetic (AEM) surveys are presented in Figure 6-1 and Figure 6-2. Figure 6-1 Bulk conductivity of the subsurface for an elevation slice at 100 mAHD for all three airborne electromagnetic survey areas Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) Changes in bulk conductivity are broadly associated with changes in the simplified outcropping and subcropping geology shown in both figures. Broadly: (i) less conductive areas (<50 mS/m) are associated with the Cambrian limestone along the east of the Victoria catchment and the Proterozoic dolostone in the centre and south of the catchment, and (ii) more conductive areas (50 to 200 mS/m) are associated with some parts of the Cambrian basalt and most of the Proterozoic shales. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-2 Bulk conductivity of the subsurface for an elevation slice at zero mAHD for all three airborne electromagnetic survey areas Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) Conductivity-depth sections Four conductivity-depth sections are presented in Figure 6-3 to Figure 6-6. These lines have been selected as they intersect parts of the outcropping and subcropping areas of both the CLA and PDAs and where lithology and standing water level (SWL) data for groundwater bores are either along the flight lines or close by. Note that a 5 km search radius has been used for the borehole intersections. For more information on this figure please contact CSIRO on enquiries@csiro.au Cambrian Limestone Aquifer Figure 6-3 presents a conductivity-depth section for a south to north flight line intersecting mostly the Cambrian limestone along the eastern margin of the Victoria catchment, with some Cambrian basalt at the northern end. The modelled bulk conductivity shows a broad correlation between shallow resistive areas coinciding with a veneer of unsaturated surficial cover. Across most of the line, a three-layer representation can be extracted, broadly a near surface resistor (unsaturated sand and sandstone associated with surficial quaternary to Cretaceous cover), followed by a conductor (unsaturated clay and mudstone), followed by a resistor (saturated basement rocks). The deeper resistive unit can be either limestone and dolostone, or basalt, and indicates areas of low salinity groundwater hosted in both the CLA and APV, as can be seen from the groundwater bore data. The southernmost half of this section exhibits more variability with potentially a five- layer structure in parts, with a resistive layer (unsaturated cover) between two conductive layers. The presence of a number of boreholes in this area indicates a possible link between this resistive unit and groundwater hosted in a variety of hydrogeological units (limestone, dolostone, chert and basalt) comprising mostly of the CLA. Figure 6-3 Conductivity-depth section from survey AusAEM1, Line 1010004 Groundwater bores and their attributed data (lithology logs and standing water level (SWL)) where available are presented along the section. The location of streams and outcropping and subcropping geology are also presented and annotated accordingly. Figure 6-4 presents a conductivity-depth section for a west to east flight line in the southern part of the catchment traversing parts of the Proterozoic dolostone and sandstone along the western end of the section, and parts of the Cambrian basalt and limestone on the eastern end of the section. The modelled bulk-conductivity shows the complexity of topography and geological structure in this part of the catchment. Little groundwater bore data coincides with this section. There are two bores at the eastern end of the section intersecting groundwater in the CLA and APV (areas of <50 mS/m). There are no bores along the western part of the section where topography and subsurface structure are dynamic. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-4 Conductivity section from survey AusAEM2, Line 5004004 Groundwater bores and their attributed data (lithology logs and standing water level (SWL) where available are presented along the section. The location of streams and outcropping and subcropping geology are also presented and annotated accordingly. Proterozoic dolostone aquifers Figure 6-5 presents a conductivity-depth section for a west to east flight line traversing the outcropping and subcropping Proterozoic dolostone in the centre of the catchment. The modelled bulk-conductivity shows a strong correlation between the shallow subsurface geology. Areas of Proterozoic shale on the extreme western end of the section and mid to latter half of the eastern end of the section exhibit areas of higher bulk conductivity (>100 mS/m). Proterozoic dolostones associated with topographic rises in a small part of the western end of the section exhibit areas of near surface resistivity (<10 mS/m) overlying areas of low bulk-conductivity (<50 mS/m) where groundwater is hosted in the PDAs (Skull Creek Formation). Figure 6-5 Conductivity section from survey CR19980231, Line 1251 Groundwater bores and their attributed data (lithology logs and standing water level (SWL)) where available are presented along the section. The location of streams and outcropping and subcropping geology are also presented and annotated accordingly. For more information on this figure please contact CSIRO on enquiries@csiro.au For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-6 presents a conductivity-depth section for a west to east flight line that also traverses the outcropping and subcropping Proterozoic dolostone in the centre of the catchment. The modelled bulk conductivity shows a good correlation between near surface conductivity and surface geology. Outcropping and subcropping areas of Proterozoic shale at the extreme ends of the section correlate with areas of higher (>100 mS/m) near-surface bulk conductivity. Outcropping and subcropping areas of Proterozoic dolostone exhibit near-surface areas of low bulk conductivity (<20 mS/m) overlying areas of slightly higher subsurface bulk conductivity (20 to 50 mS/m) associated with low salinity groundwater hosted in the PDAs (sandstone and siltstone of the Skull Creek Formation). Figure 6-6 Conductivity section from survey CR19980231, Line 1601. Groundwater bores and their attributed data (lithology logs and standing water level (SWL)) where available are presented along the section. The location of streams and outcropping and subcropping geology are also presented and annotated accordingly. 6.1.2 Hydrogeological cross-sections The locations of three regional-scale hydrogeological cross-sections are shown in Figure 6-7. Cross- section A–A’ traverses outcropping and subcropping parts of the CLA in the far east of the Victoria catchment traversing areas of the aquifer from south-east to the north-west both within and outside of the catchment boundary (adjacent the eastern boundary) (Figure 6-7). Cross-sections B–B’ and C–C’ traverse outcropping and subcropping parts of the PDAs from north-west to south- east adjoining the CLA on the eastern end (Figure 6-7). The three cross-sections are presented in Figure 6-8, Figure 6-9 and Figure 6-10. These cross-sections summarise the two-dimensional spatial context of several key hydrogeological units at a regional scale. They provide the vertical and horizontal framework that underpins part of the hydrogeological conceptual model for groundwater flow in the unconfined parts of the CLA and PDAs. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-7 Locations of three regional-scale hydrogeological cross-sections traversing the Cambrian Limestone Aquifer (CLA: A–A’) and the Proterozoic dolostone aquifers (PDAs: B–B’ and C–C’) To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Geological faults data from Department of Industry, Tourism and Trade (2010) Spring data from Department of Environment Parks and Water Security (2019b) Sinkhole data from Department of Environment Parks and Water Security (2014)Cambrian Limestone Aquifer A south-west to north-east cross-section (A–A’) traversing outcropping and subcropping parts of the Montejinni Limestone (Cmm) which hosts the CLA in the far east of the Victoria catchment is presented in Figure 6-8. Key features of the cross-section include: (i) spatial changes in the occurrence and thickness of the surficial Cretaceous strata (KI), (ii) spatial changes in the thickness of the CLA (Cmm), (iii) spatial changes in the topography of the underlying APV (Cla), and (iv) no difference or vertical gradient between groundwater levels in the CLA and APV. In the south- western parts of the CLA the aquifer outcrops and has a thin saturated thickness (<20 m) at bore RN042689 due to a structural high in the underlying APV. Further along the cross-section heading north-east at bores RN026478 and RN025855 just to the east of Top Springs, the saturated thickness in the CLA is greater (>50 m) where the underlying APV dips and the overlying Cretaceous strata is thin (<25 m). At the north-eastern end of the cross-section, the saturated thickness of the CLA thins out again (<20 m) as the underlying APV is at its highest point along the cross-section, and the overlying Cretaceous strata is thickest (ranging between 50 and 75 m). The spatial changes in all three hydrogeological units have an influence on both vertical and horizontal flow within the CLA whereby: (i) increases in the thickness and lithology (particularly claystone and siltstone) of the overlying surficial Cretaceous strata reduces vertical recharge to the underlying CLA, (ii) the fractured and karstic nature of the Montejinni Limestone enhances vertical leakage between the CLA and underlying APV, and (iii) topographic high points in the low permeability APV underlying the CLA influence horizontal flow within the CLA across the aquifer. Figure 6-8 Hydrogeological cross-section A–A’ traversing from south-west to north-east through the Cambrian Limestone Aquifer around the eastern margin of the Victoria catchment To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cenozoic rocks and sediments has been removed. Cross-section location is shown in Figure 6-7, traversing areas inside and outside of the Victoria catchment. For more information on this figure please contact CSIRO on enquiries@csiro.au Proterozoic dolostone aquifers The first of two north-west to south-east cross-sections, cross-section B–B’ traversing the outcropping to subcropping parts of the Proterozoic dolostone that hosts the PDAs from north- west around Timber Creek to south-east around Top Springs is presented in Figure 6-9. Key features of this cross-section include: (i) spatial changes in the occurrence in the outcropping and subcropping areas and gently to steeply dipping nature of the PDAs of the Bullita Group (Pba, Pby, Pbs and Pbt) between Timber Creek on the north-western edge of the cross-section and Wallaby Spring mid-section near RN026125, (ii) the occurrence of localised discharge areas for the PDAs (Victoria River, Skull Creek and Kidman Springs), and (iii) spatial changes in topography along the entire length of the cross-section and its influence on the depth of the watertable across aquifers hosted in different hydrogeological units. Across the extent of the PDAs from the north-west near Timber Creek to mid cross-section near Wallaby Spring, geological structure and topography are dynamic. The cross-section traverses’ anticlines of Mount Dempsey between RN034279 (near the Victoria River) and RN027348 (near Skull Creek), Sundown and Lounger Hill between Skull Creek and Kidman Springs and the Fitzgerald Range between Kidman Springs and Wallaby Spring. Along this north-western part of the cross-section, the PDAs dip gently steeply in the subsurface and change from unconfined to confined where the overlying shale (Pc) and sandstone (Paj) of the Tijunna Group of the Birrindudu Basin and Auvergne Group of the Victoria Basin respectively overlie the PDAs. In addition, the structure and topography influence both depth to the watertable across the PDAs and the occurrence of localised discharge areas at topographic lows points where the watertable is shallow (<10 mBGL). Spatial changes in the occurrence of anticline features and presence of the overlying shale (Pc) and sandstone (Paj) influence changes in the confinement status of the PDAs and vertical and horizontal flow within them. Figure 6-9 Hydrogeological cross-section B–B’ traversing from north-west around Timber Creek through parts of the Proterozoic dolostone aquifers (PDAs) to the south-east around Top Springs To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cenozoic rocks and sediments has been removed. Cross-section location is shown in Figure 6-7, traversing areas inside and outside of the Victoria catchment. The second north-west to south-east cross-section C–C’, traverses the outcropping to subcropping parts of the Proterozoic dolostone that hosts the PDAs from the north-west near Whitewater Falls to the south-east traversing the APV and western edge of the CLA, and is presented in Figure 6-10. Similar to cross-section B–B’, key features of cross-section C–C’ include: (i) spatial changes in the occurrence in the outcropping and subcropping areas and gently to steeply dipping nature of PDAs either side of the anticline of Stokes Range (between RN030196 and RN008988 near the Wickham River), (ii) the eroded anticline east of the Wickham River, and (ii) the presence of the Proterozoic shale (Pc) of the Tijunna Group mid cross-section. As for cross-section B–B’, spatial changes in the occurrence of anticline features and presence of the overlying shale (Pc) and sandstone (Paj) throughout cross-section C–C’ influence changes in the confinement status of the PDAs and vertical and horizontal flow within them. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-10 Hydrogeological cross-section C–C’ traversing from north-west around Whitewater Falls through parts of the Proterozoic dolostone aquifers (PDAs) to the south-east near the western margin of the Cambrian Limestone Aquifer (CLA) To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cenozoic rocks and sediments has been removed. Cross-section location is shown in Figure 6-7, traversing areas inside and outside of the Victoria catchment. 6.1.3 Depth to top of key hydrogeological units Cambrian Limestone Aquifer Figure 6-11 shows the spatial interpolated depth to the top of Cambrian limestone below the natural surface. Over 2600 groundwater bores, mineral exploration holes or stratigraphic wells were used in the Leapfrog hydrostratigraphic model that forms the geometry of the layers in the DR2 FEFLOW groundwater model by Knapton (Knapton, 2020). Only a partial extent of the interpolated surface for the limestone has been shown beyond the Victoria catchment as well as parts of the surface to the east and north of the catchment into the Wiso and Daly basins respectively. This output represents the general depth required to drill and intersect the CLA hosted in the Cambrian limestone (almost exclusively the Montejinni Limestone), noting that at a local scale the aquifer can be interconnected with the overlying surficial veneer of Cretaceous rocks and sediments but can equally be unsaturated too. For more information on this figure please contact CSIRO on enquiries@csiro.au The surface of the CLA beneath the Victoria catchment is generally flat and only exhibits minor dips in places. Depth to the top of the CLA in the subsurface along the eastern margin of the Victoria catchment is generally shallow (<50 mBGL) (Figure 6-11). To the north-east of Top Springs, depth to the top of CLA increases to about 120 mBGL where overlying Cretaceous rocks are more extensive (Figure 6-11) (see Figure 2-6 in Section 2.3.2 for information on surface geology). Depth to the top of the CLA generally increases (>150 mBGL) east of the catchment boundary out into the central Wiso Basin where the overlying Cretaceous rocks are thicker. Depth to the CLA also increases (>150 mBGL) where the aquifer dips below mean sea level in the Daly Basin in the far north (Figure 6-11). Figure 6-11 Interpolated surface of the depth to the top of the Cambrian limestone Only a partial spatial extent of the limestone is shown beyond the Victoria catchment boundary. Depths are in metres below the ground surface. Stratigraphic data represents the location of a bore with stratigraphic data to obtain information about changes in geology with depth. Data source: Gridded depth to top of the Cambrian Limestone Aquifer is sourced from Knapton (2020). A map of a river Description automatically generated 6.2 Groundwater recharge and flow 6.2.1 Regional groundwater levels Cambrian Limestone Aquifer There are no nested observation bores specifically constructed to investigate vertical gradients between the CLA, specifically the Montejinni Limestone and aquifers hosted in the underlying APV within the Victoria catchment, nor further east into the Wiso Basin. An attempt has been made to investigate the possibility of vertical gradients by creating a reduced water level surface (RSWL) for the CLA in the catchment and extending east into the Wiso Basin. There were 204 bores identified with a standing water level observation within the Wiso Basin, these span recent decades and so do not represent a point in time and therefore contain considerable uncertainty in the interpreted flow direction. However, most observations are taken in the dry-season when bores were first drilled. These bores are not evenly distributed across the basin, they are concentrated in the north with sparse observations in the eastern and western margins and no bores in the centre of the basin (Tanami Desert). Generally, the flow direction in the Wiso Basin is from the south to the north (Figure 6-12a), with outflow to the Daly Basin in the north and the Georgina Basin in the east. The uncertainty in the interpolation, as measured by the standard deviation of the prediction, is greatest in areas without observations and least close to the bores (Figure 6-12b). Within the Victoria catchment in the northern part of the Wiso Basin there is radial flow away from the area around Top Springs (Figure 6-12a), this is consistent with the recent potentiometric surface created by Amery and Tickell (2022). There are no bores screened close the basin margin and so the interpolation via kriging has extrapolated the RSWL surface in this area based on the trends observed from bores down gradient. There are some bores in this region that have been drilled through the Montejinni Limestone without striking water and have been screened in the APV. This would suggest that the interpolated RSWL surface in this area is too high and should sit within the APV and not the CLA. Further south within the Victoria catchment there appears to be a low point in the RSWL surface in the vicinity of Cattle Creek and a large clay pan (Figure 6-12a). While the water level contours would suggest that this is a regional discharge area, there is no extensive development to suggest that it is caused by groundwater extraction and the watertable is too deep for evapotranspiration. There is also the possibility that this is an artefact of the data. If the bores in this region were measured during a dry period with a low watertable and the surrounding bores were measured during a wet period with a high watertable, it would appear as though there is a potentiometric low point where there isn’t one. Figure 6-12 Interpolated reduced standing water level surface for the Cambrian Limestone Aquifer in the Victoria catchment and Wiso Basin showing the (a) prediction (mAHD), and (b) standard deviation around that prediction in (metres) GWL = depth to groundwater. mAHD = metres Australian Height Datum. Cambrian basalt aquifers The extent of the APV is much greater than the area of interest here so the scale was limited to the combined extent of the Victoria catchment and Wiso Basin. There were 330 bores identified with a standing water level observation within the APV, these span recent decades and so do not represent a point in time and therefore contain considerable uncertainty in the interpreted flow direction. The APV bores within the footprint of the Wiso Basin are clustered along the northern and eastern boundaries (Figure 6-13a). Within the footprint of the Wiso basin the flow direction is generally from the south-west to the north-east. Within the Victoria catchment the flow direction is generally following topography. The uncertainty in the interpolation, as measured by the standard deviation of the prediction, is greatest in areas without observations and least close to the bores (Figure 6-13b). The shape of the RSWL surface in the area around Top Springs within both the Wiso Basin and the Victoria catchment is quite different in the APV compared to that in the CLA (Figure 6-13a). In the APV there is a groundwater flow divide with some flow toward the Victoria catchment and some flow toward the Daly Basin, this is consistent with the watertable map drawn by Randal (1973). This flow divide better conceptualises the springs that occur on the western edge of the Montejinni Limestone within the Victoria catchment. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-13 Interpolated reduced standing water level surface for the Antrim Plateau Volcanics using only bores screened in the APV showing the (a) prediction, and (b) standard deviation around that prediction RSWL = reduced standing water level. mAHD = metres Australian Height Datum. In isolation, the interpreted flow direction of the APV and CLA are different in the area around Top Springs but largely the same elsewhere (Figure 6-12a, and Figure 6-13a). To investigate whether a vertical gradient exists between the two layers the predicted piezometric surfaces were compared (Figure 6-14). Areas largely devoid of observations in one layer or the other will not have a reliable RSWL surface and so will not be discussed, the areas where water levels are available in both layers is largely confined to the northern half of the intersection between the Wiso Basin and the Victoria catchment. The western margin of the Wiso Basin appears to have a greater water level within the Montejinni Limestone than the APV, this would be indicative of downward flow. However, the bores closest to the margin are screened in the APV suggesting a greater reliability of the interpolated surface in the APV. To incorporate the uncertainty in the interpolation, 1000 replicates of the difference in water level were created using a normal distribution from the prediction and standard deviation. These were then evaluated as to whether the water level was greater in the Wiso Basin compared to the APV (Figure 6-14). There are two small areas in the north on the western margin of the Montejinni Limestone where the interpolated water level is greater in the CLA in more than 90% of the replicates. This would appear to show an area where a downward gradient can be observed but as discussed above is probably an interpolation artefact. There are two areas where there is less than 10% of replicates with the CLA water level above the APV water level. The area in the south-west can be dismissed as there are no bores screened in the Wiso Basin within 50 km. The other is the area around Cattle Creek that appeared to be discharge zone. It would be expected that a regional discharge area would have an upward gradient but there is no evidence that this is actually a regional discharge area so is possibly an artefact of having groundwater level measurements spanning decades rather than all from a discrete point in time. For more information on this figure please contact CSIRO on enquiries@csiro.au The groundwater level data that we have available is not sufficient to confidently say that there is a vertical hydraulic gradient between the CLA and APV. Without a vertical gradient we can assume that the two layers are connected and acting as a single water resource. There is some corroborating evidence for this is in the bore records where there are many bores that have been drilled through the CLA to the APV and screened in both aquifers to increase the yield. In addition, the chemistry of groundwater in both aquifers is similar where they coincide which often suggests that the water extracted from the APV has been in contact with the carbonates of the CLA (see Section 6.2.5). Figure 6-14 Difference in groundwater water levels between the Cambrian Limestone Aquifer and Antrim Plateau Volcanics expressed in (a) metres, and (b) proportion of replicates showing CLA greater than APV SWL = standing water level The assumption has been made that the APV and CLA are connected as one water source, therefore the standing water level data from both layers can be combined to create one RSWL surface representative of the connected aquifers. This allows more data points in the Top Springs area to characterise the scale and directions of groundwater flow. All available bores installed in the CLA and adjacent APV, including within a 50 km buffer zone east of the catchment boundary were used in conjunctions with remotely sensed discharge locations from analyses in Section 5.5.4 to interpolate a potentiometric surface for the CLA. In the area around Top Springs there is an inferred groundwater flow divide within the Victoria catchment (Figure 6-15), this means that some recharge from within the catchment is flowing out of the catchment toward the north and some of the flow is south-west back toward the Victoria River. This inferred flow divide is similar to that of Randal (1973) to the north-east of Top Springs but different to the south. The springs near the margin of the Montejinni Limestone (e.g. Old Top Spring, Lonely Spring, Palm Spring, Horse Spring) are between 10 and 25 km from the groundwater inferred flow divide indicating that they are probably fed from local to intermediate-scale groundwater flow rather than a longer regional flow path (>50 km). This is consistent with the current conceptualisation of these springs (see section 6.2.4 to 6.2.9). For more information on this figure please contact CSIRO on enquiries@csiro.au The area around Cattle Creek reflects the water levels trends evident in the separate RSWL surfaces for the CLA and APV in Figure 6-12 and Figure 6-13 where there appears to be a regional discharge area. The contours suggest the possibility of flow from the Wiso Basin into the Victoria catchment. The lack of identified springs or perennial flow in the creek does not support this interpretation. The hydraulic gradients in this area are very flat and an error of only a few metres would change the direction of flow. Resolving the flow direction in this area would require measuring the water level at all available bores on the same day, which was not possible within this study. Figure 6-15 Interpolated reduced standing water level for the Cambrian Limestone Aquifer Only a partial spatial extent of the CLA is shown beyond the Victoria catchment boundary. RSWL = reduced standing water level. SWL = standing water level. mAHD = metres Australian Height Datum. Aquifer extent data source: Department of Environment, Parks and Water Security (2008) For more information on this figure please contact CSIRO on enquiries@csiro.au 6.2.2 Depth to groundwater Cambrian Limestone Aquifer Changes in the depth to groundwater across the CLA, also referred to as depth to standing water level (SWL), exhibit similar spatial patterns to the depth to the top of the aquifer. For example, groundwater is shallow (>10 mBGL) along the western margin of the aquifer around and to the south of Top Springs (Figure 6-16) where groundwater discharges via: (i) intermittent lateral outflow to streams (Bullock, Cattle and Montejinni creeks) that are tributaries of the Camfield and Armstrong rivers where they are incised into the aquifer outcrop and (ii) via perennial localised spring discharge at spring complexes (Old Top, Lonely, Palm and Horse springs); and (iii) via evapotranspiration from groundwater-dependent vegetation (GDV) nearby streams and springs. However, groundwater level data is very sparse south of Cattle Creek representing an area of uncertainty in the interpolated surface. Depths to groundwater then increases subtly in a somewhat radial pattern north-east, east, and south-east from the western aquifer boundary toward the eastern margin of the Victoria catchment. Groundwater depth is approximately 50 mBGL along the eastern margin of the catchment and increases to greater than 100 mBGL out into parts of the Wiso Basin (Figure 6-16). For this reason, groundwater-dependent ecosystems (GDEs) associated with the CLA in the Victoria catchment are largely limited to the western margin of the aquifer around Top Springs. Figure 6-16 Interpolated depth to standing water level (SWL) surface for the Cambrian Limestone Aquifer Only a partial spatial extent of the CLA is shown beyond the Victoria catchment boundary. Depths are in metres below the land surface. SWL = standing water level Aquifer extent data source: Department of Environment, Parks and Water Security (2008) Proterozoic dolostone aquifers Despite the PDAs having insufficient information (aquifer interconnectivity, spatial and temporal groundwater level data for individual aquifers) to currently interpolate a depth to groundwater surface, a classified depth to standing water level map for the PDAs is shown in Figure 6-17. Across the most northern outcropping and subcropping area for the PDAs near Timber Creek, depth to groundwater in this northern higher rainfall parts of the PDAs is shallow (<10 mBGL) in areas where the elevation is ~100 mAHD (see Section 6.1.2). Further to the south-east around Yarralin, depth to groundwater is more variable that in the north near Timber Creek, though the depth to groundwater is still relatively shallow (<50 mBGL). Much further south in the catchment, where outcropping and subcropping areas occur to the west of Kalkarindji, depth to groundwater varies from shallow (<10 mBGL) to depths exceeding 75 mBGL (Figure 6-17). Figure 6-17 Classified depth the standing water level map for the Proterozoic dolostone aquifers in the centre and south of the Victoria catchment To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) A map of a large area Description automatically generated 6.2.3 Hydrological monitoring Field inspection of groundwater bores that the study had permission to access yielded few sites appropriate for installing digital temperature and pressure data loggers. This was limited to three bores in the east of the catchment in and around the CLA (Figure 6-18). Datasets from the data loggers were used to generate groundwater hydrographs shown in Figure 6-19. Unfortunately, the bores the study had access to installed in the PDAs in the centre of the catchment were not suitable for installing data loggers as they were all equipped with diesel-powered Mono pumps or solar-powered bores preventing both access to the top of the borehole but equally the bore casing for installation. Figure 6-18 Locations where temperature and pressure data loggers were installed in bores in aquifers hosted in the Cambrian limestone and Cambrian basalt To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) For more information on this figure please contact CSIRO on enquiries@csiro.au Cambrian basalt aquifers Two groundwater bores installed in aquifers hosted in the Cambrian basalt, specifically the Antrim Plateau Volcanics (APV), had digital temperature and pressure data loggers installed. These were, RN026127 to the north-west of Top Springs, and RN021978 just south-east of Top Springs (Figure 6-18). The data logger installed in RN026127 was installed during the first reconnaissance field trip in August 2022. A barometric pressure data logger was also installed in RN026127 as the pressure data loggers measure water and atmospheric pressure, and the water level data needs to be barometrically compensated. The data logger installed in RN021978 was installed during the groundwater sampling field trip in August 2023. Hydrographs for RN026127 and RN021978 for the monitoring period August 2022 to June 2024 are shown in Figure 6-19a, and Figure 6-19b, respectively (noting RN021978 had a shorter monitoring period). Observations of groundwater levels in both bores in relation to wet-season responses were greatly varied between sites, reflecting the heterogeneity in the physical properties of the local-scale aquifers hosted in the APV. Observations of groundwater levels in RN026127 exhibited virtually no wet-season response, more a delayed increase in groundwater levels of less than 0.5 m towards the end of the monitoring period (Figure 6-19a). A moderate magnitude (~3m) wet-season response was observed at RN021978, noting the bore casing appears to have been temporarily flooded causing about a 10 m increase in groundwater levels, though about 7 m of this is an artefact of preferential infiltration of water directly in the casing (Figure 6-19b). The range in wet-season responses ranged from (3.0 to 7.1 m) before groundwater levels slowly and subtly declined toward the end of the monitoring period (Figure 6-19b). The wet-season responses observed at RN021978 reflect the higher degree of fracturing and weathering in the basalt at this location compared to the basalt at RN026127. Overall, there was a net increase in groundwater levels at both bore sites, though more pronounced in RN021978 compared to RN026127. Cambrian Limestone Aquifer One groundwater bore installed in the CLA, specifically the Montejinni Limestone, had a digital temperature and pressure data logger installed. This was at RN020019 to the south-east of Top Springs and south-east of RN021978 (Figure 6-18). The data logger installed in RN020019 was installed during the groundwater sampling field trip in August 2023. The barometric pressure data logger installed in RN026127 was used to be barometrically compensated the water level data recorded at RN020019 (Figure 6-18). A hydrograph for RN020019 for the monitoring period August 2023 to June 2024 is shown Figure 6-19c. Observations of groundwater levels for RN020019 exhibited a range in magnitude of wet- season response between 1.4 and 6.5 m spanning the period January to April 2024 (Figure 6-19c). Overall, there was a net increase in groundwater levels for RN020019, with groundwater levels now subtly declining toward the end of the monitoring period (Figure 6-19). The wet-season responses exhibited by groundwater levels in RN020019 reflect the fractured and karstic nature of the Montejinni Limestone that outcrops at this location. Figure 6-19 Groundwater hydrographs for temporal groundwater level observations from the Cambrian basalt and limestone aquifers near Top Springs between August 2022 and June 2024 050100150200250170175180185190195Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (a) RN026127050100150200250160165170175180185Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (b) RN021978050100150200250185190195200205210Jul 2023Sep 2023Nov 2023Jan 2024Mar 2024May 2024Jul 2024Daily rainfall (mm) Hydraulic head/Ground surface (mAHD) (c) RN020019Groundwater hydraulic headGround surfaceDaily rainfall 6.2.4 Field water sampling campaigns Groundwater samples were collected at nine groundwater bores during a field trip in August 2023, with their locations shown in Figure 6-20. These bores were constructed and installed in aquifers hosted in three different hydrogeological units: (i) Cambrian Limestone, (ii) Cambrian basalt, and (iii) Proterozoic dolostone. Cambrian Limestone Aquifer Field sampling associated with the CLA involved locating and inspecting bores constructed in both the Cambrian limestone, specifically the Montejinni Limestone and the Cambrian basalt, specifically the Antrim Plateau Volcanics (APV). During a fieldtrip in August 2023 seven groundwater bores were visited and inspected for suitability for sampling, and five bores were sampled (Figure 6-20). Three of the bores sampled were constructed in the Montejinni Limestone: (i) RN041173 to the north-east of Top Springs, (ii) RN020019 just south-east of Top Springs, and (iii) RN026444 south of Top Springs (this bore is also partly constructed in the APV) (Figure 6-20). A data logger was installed in RN020019 as discussed in Section 6.2.3. RN037939 was also visited but was not suitable to sample due to a non-functioning solar-powered pump. Two of the bores sampled were constructed in the APV: (i) RN021978 just near Top Springs and the margin of the CLA, and (ii) RN026127 to the north-west of Top Springs (Figure 6-20). A data logger was installed in RN026127 as discussed in Section 6.2.3. RN005462 was also visited but was not suitable for sampling as the bore had been abandoned and was not suitable for sampling. Please see Appendix A for further details on bore location and construction details and a summary of the measured field parameters, and results of general chemistry and environmental tracers including noble gases in Apx Table A-1 to Apx Table A-5. Five spring complexes (Companion, Lonely, Palm, Old Top and Winari springs) were also sampled associated with the western margin of the CLA in the east of the Victoria catchment (Figure 6-20). The results of spring sampling are discussed in Section 6.2.14 on groundwater–surface water interactions. Figure 6-20 Groundwater and spring sampling locations associated with the Cambrian Limestone Aquifer To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous and Cenozoic rocks and sediments have been stippled. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) Proterozoic dolostone aquifers Field sampling associated with PDAs involved locating and inspecting bores constructed in the Proterozoic Dolostone, specifically the Skull Creek, Bynoe and Battle Creek formations. During a fieldtrip in August 2023 four groundwater bores were visited and inspected for suitability for sampling, and three bores were sampled (Figure 6-22). All three of the bores sampled were constructed in the Bullita group in three different formations: (i) RN007403 to the north-west of Kidman Springs, (ii) RN036195 near Kidman Springs, and (iii) RN033280 south-east of Yarralin (Figure 6-22). No data loggers were installed as all the bores had head works. See Appendix A for further details on bore location and construction details and a summary of the measured field parameters, and results of general chemistry and environmental tracers including noble gases in For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Table A-1 to Apx Table A-5. Five spring complexes (Kidman, Bulls Head, Crawford, Waterbag and Dead springs) were also sampled associated with the margins of the PDA in the centre of the Victoria catchment (Figure 6-21). The results of spring sampling are discussed in Section 6.2.14 on groundwater–surface water interactions. Figure 6-21 Groundwater and spring sampling locations in the Proterozoic dolostone aquifers To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data from Department of Environment Parks and Water Security (2019b) For more information on this figure please contact CSIRO on enquiries@csiro.au 6.2.5 Chemistry The spatial distribution of the major water-types for groundwater and spring samples collected across the CLA (specifically the Montejinni Limestone) and PDAs (specifically the Battle Creek, Bynoe and Skull Creek formations) and aquifers hosted in adjacent hydrogeological units is shown in Figure 6-22. In addition to groundwater samples collected in the field, the major water-types for select historical groundwater samples available via the NT groundwater database (Department of Environment Parks and Water Security, 2019a) have also been mapped in Figure 6-22 and used in subsequent data analyses to further characterise the major ion composition of groundwater in different aquifers and springs. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-22 Spatial distribution of water quality types for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and Proterozoic dolostone as well as aquifers hosted in adjacent hydrogeological units To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous and Cenozoic rocks and sediments have been stippled. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Geology data sources adapted from: Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data source: Department of Environment Parks and Water Security (2019b) Cambrian Limestone Aquifer The major ion composition for groundwater samples collected from the CLA (Montejinni Limestone) and aquifers hosted in adjacent hydrogeological units, specifically the APV are shown in Figure 6-22 and Figure 6-23. The ionic composition of groundwater in the CLA is reflective of the composition of the main rock types of the Montejinni Limestone (limestone and dolostone). Groundwater samples were either of a Ca–HCO3 or Mg–HCO3 composition (see Figure 6-22, Figure 6-23 and Table 6-1). The major ion composition for groundwater samples collected from aquifers hosted in the adjacent Cambrian basalt, specifically the APV had a similar composition to that of the CLA (either Ca–HCO3 or Mg–HCO3) in places where the samples were collected adjacent the western margin of the CLA reflecting some interconnectivity between the aquifers. The major ion composition of historical groundwater samples also shown in Figure 6-22, and Figure 6-23, Table 6-1 exhibit similar compositions. When combining the field data and historical data, some spatial hydrogeochemical trends emerge. Samples collected where the CLA underlies Cretaceous and Cenozoic strata tended to have a Mg–HCO3 and one sample a Na–HCO3 composition, possible due to lower recharge in these areas (Figure 6-22). Samples collected from areas where the CLA outcrop, tended to have a Ca–HCO3 composition, possible reflecting higher recharge in these areas (Figure 6-22). In addition, samples collected from the APV adjacent the western margin of the CLA tended to have a similar composition to that of groundwater in the CLA, further reflecting potential horizontal interconnectivity between the aquifers. Some of these trends are further reflected in samples collected from springs along the western margin of the CLA, all of which, except Companion Spring, exhibit a similar ionic composition to that of groundwater from the CLA. Where the major ion-composition of groundwater was of a Na–HCO3 composition, groundwater samples were either collected from bores screened only in the APV west of the margin of the CLA, or from bores screened in both aquifers but the overlying CLA was unsaturated at the time of sampling. Only Companion Spring had a Na–HCO3 composition similar to that of groundwater in parts of the APV. These sources of discharge to springs are consistent with the groundwater sources stated in the Springs of the NT dataset (Department of Environment Parks and Water Security, 2019b). Winari Spring was a spring complex/groundwater-fed reach identified along Townsend Creek from the results of the remote sensing analyses identifying potential groundwater discharge areas in Section 5.5.4. The ionic composition of Winari Spring was Mg– HCO3 similar to that of both the CLA and APV. Both hydrogeological units were observed outcropping and subcropping in the bed of Townsend Creek when water sampling was undertaken. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-23 Piper diagram showing major ion composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent hydrogeological units (Cambrian basalt) Samples are symbolised by aquifers or springs sampled. Filled shapes are samples collected in this study. Hollow shapes are historical data available from Department of Environment Parks and Water Security (2019a). ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. The salinity of groundwater samples collected in the field was mostly fresh, ranging from 109 to 683 mg/L total dissolved solids (TDS). The salinity of historical samples spanned a larger range, ranging between 289 and 1138 mg/L TDS (Table 6-1). The pH of groundwater samples indicated most samples were close to neutral or slightly alkaline to alkaline, ranging between 6.8 and 9.1 for the field samples, and between 6.8 and 8.0 for historical samples (Table 6-1). This further reflects the dominance of HCO3 in groundwater hosted in the carbonate rocks (Figure 6-24). Table 6-1 Summary of pH, salinity (total dissolved solids (TDS)), water-type and carbonate speciation and mineral saturation indices for groundwater samples collected from aquifers and springs hosted in the Cambrian limestone and adjacent hydrogeological units BORE NUMBER HYDROGEOLOGICAL UNIT FIELD pH SALINITY – TDS (mg/L) WATER TYPE AQUEOUS CO2 (mM) AQUEOUS HCO3 (mM) SI CALCITE SI DOLOMITE SI GYPSUM RN026127 Antrim Plateau Volcanics 9.1 109 Na–HCO3 0.00 0.37 –0.55 –1.88 –4.34 RN026444 ML/APV 6.8 671 Ca–HCO3 2.34 7.84 0.18 0.10 –3.27 RN021978 Antrim Plateau Volcanics 7.0 683 Mg–HCO3 1.33 7.81 0.19 0.53 –2.70 RN020019 Montejinni Limestone 6.8 652 Ca–HCO3 2.19 7.92 0.09 0.17 –3.39 RN041173 Montejinni Limestone 7.3 597 Ca–HCO3 0.58 6.12 0.32 0.65 –2.43 *RN020020 Montejinni Limestone 7.4 449 Ca–HCO3 0.97 8.17 0.51 1.00 –2.34 *RN024781 Montejinni Limestone 7.6 379 Na–HCO3 0.18 3.69 0.18 0.28 –1.93 *RN026206 Montejinni Limestone 7.6 494 Ca–HCO3 0.15 3.23 0.26 0.45 –1.67 *RN026445 Montejinni Limestone 7.5 380 Mg–HCO3 0.63 6.72 0.31 0.87 –2.26 *RN026551 Montejinni Limestone 8.0 289 Mg–HCO3 0.08 4.00 0.61 1.39 –2.66 *RN030877 Montejinni Limestone 7.4 709 Mg–HCO3 0.37 5.05 0.34 0.87 –1.25 *RN005423 Antrim Plateau Volcanics 7.5 483 Mg–HCO3 0.19 3.95 –0.11 0.66 –3.74 *RN005462 Antrim Plateau Volcanics 7.8 569 Mg–HCO3 0.17 7.25 0.88 2.11 –2.32 *RN020994 Antrim Plateau Volcanics 7.5 354 Mg–HCO3 0.39 6.46 0.5 1.17 –2.51 *RN029238 Antrim Plateau Volcanics 7.8 411 Ca–HCO3 0.21 7.11 0.94 1.85 –2.16 *RN029261 Antrim Plateau Volcanics 8.0 511 Ca–HCO3 0.14 7.58 1.08 2.33 –2.43 *RN030302 Antrim Plateau Volcanics 6.8 1138 Na–HCO3 0.30 1.04 –1.48 –4.14 –1.37 Companion Spring Antrim Plateau Volcanics 8.4 211 Na–HCO3 0.07 8.64 1.25 2.90 –3.37 Lonely Spring Montejinni Limestone 7.2 159 Ca–HCO3 0.93 7.33 0.45 0.53 –3.73 Old Top Spring Montejinni Limestone 7.5 169 Ca–HCO3 0.56 8.80 0.65 1.43 –3.60 Palm Spring Montejinni Limestone 7.6 197 Ca–HCO3 0.40 8.39 0.97 1.78 –3.21 Winari Spring Antrim Plateau Volcanics 8.4 84 Mg–HCO3 0.03 3.49 0.90 1.95 –3.79 Bore numbers with a (*) are historical data available from Department of Environment Parks and Water Security (2019a). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-24 Outcropping and subcropping Montejinni Limestone and Antrim Plateau Volcanics along a reach of Townsend Creek Photo source: CSIRO Plots of the major cation and anion concentrations relative to chloride concentration have been used to further investigate hydrogeochemical trends in groundwater hosted in the CLA and APV (Figure 6-25). All groundwater samples from the CLA are enriched in Ca, Mg and HCO3 relative to rainfall of marine origin, and most samples are also slightly enriched in Na (Figure 6-25). This trend reflects the carbonate-rich minerals of the limestones and dolostones that the Montejinni Limestone are comprised of. Small hydrogeochemical trends from recharge areas to discharge areas at the springs are evident from the somewhat exponential shift in Ca, Mg and HCO3 concentrations relative to chloride. These observations can be made because Cl/Br ratios provide no evidence of halite dissolution (chloride is only sourced from rainfall) (Figure 6-25). Na/HCO3 ratios for all groundwater samples are significantly lower than expected for rainfall of marine origin (Figure 6-25). Therefore, even though Na has been added to groundwater through processes such as cation exchange, the addition of Ca, Mg and HCO3 is far more pronounced. These trends point towards water–rock interactions and the primary source of these ions being from carbonate dissolution. PHREEQC modelling indicates that all groundwater samples from both the PDAs and APV have a significant portion of their dissolved inorganic carbon in the form of aqueous HCO3 compared to aqueous CO2 (Table 6-2). Nearly all groundwater samples from the CLA and APV are over saturated with respect to calcite and dolomite, except for three samples from the APV (Table 6-1). Gypsum dissolution is unlikely to be a source of Ca, as most groundwater samples exhibit Ca/SO4 ratios much higher than unity, and all samples are below saturation for gypsum (Figure 6-25 and Table 6-1). 0.02.04.06.08.00.00.51.01.52.0Na/Cl (meq/L) Cl (meq/L) (a) 0204060801000.00.51.01.52.0Ca/Cl (meq/L) Cl (meq/L) (b) 0204060801001200.00.51.01.52.0HCO3/Cl (meq/L) Cl (meq/L) (c) 01503004506007500.00.51.01.52.0Cl/Br (meq/L) Cl (meq/L) (e) 0501001502002500.01.02.03.04.0Na/HCO3 (meq/L) Cl (meq/L) (f) 0.010.020.030.040.050.060.00.00.51.01.52.0(Ca+Mg)/HCO3(meq/L) Cl (meq/L) Montejinni LimestoneAntrim Plateau VolcanicsCLA/APVSpringsSeawater dilution(g) 01002003004005006000.00.51.01.52.0Ca/SO4 (meq/L) Cl (meq/L) Montejinni LimestoneAntrim Plateau VolcanicsCLA/APVSpringsSeawater dilution(h) 01020300.00.51.01.52.0Mg/Cl (meq/L) Cl (meq/L) (d) Companion SpringLonely SpringPalm SpringLonely SpringOldTopSpringLonely SpringOldTopSpring Figure 6-25 Major ion ratio plots for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units (Cambrian basalt) Samples are symbolised by aquifers or springs sampled. Filled shapes are samples collected in this study. Hollow shapes are historical data available from Department of Environment Parks and Water Security (2019a). ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Proterozoic dolostone aquifers The major ion composition for groundwater samples collected from the PDAs (Battle Creek, Bynoe and Skull Creek formations) and aquifers hosted in adjacent hydrogeological units, specifically the APV are shown in Figure 6-26. The ionic composition of groundwater in the PDAs is reflective of the composition of the main rock types (dolostone, sandstone and siltstone). Groundwater samples were either of Ca–HCO3 for bores generally screened in dolostone (Bynoe and Skull Creek formations, or Mg–HCO3 composition for bores generally screened in siltstone (Battle Creek Formation) (Figure 6-26 and Table 6-2). This major ion composition was equally reflected in all but one historical sample (RN005867) which has a 50 m screen across both limestone and siltstone. Unlike the CLA, there are few clear spatial trends in the hydrogeochemical evolution of groundwater given the paucity of data. However, the data tends to indicate mixing resulting from interconnectivity between aquifers, particularly the Bynoe and Skull Creek formations. The hydrogeological cross-section shown in Section 6.1.2 for the PDAs, provides an indication of topographically driven flow where groundwater flow paths in the aquifer outcrops are short (<20 km). The Battle Creek Formation appears more like a partial aquifer that may confine parts of the underlying Bynoe and Skull Creek formations given its different ionic composition (Figure 6-26). The ionic composition of spring samples from Crawford, Bulls Head and Kidman springs correlates with groundwater from the Bynoe and Skull Creek formations. The ionic composition of spring samples collected from Dead and Waterbag springs correlates with groundwater from the Battle Creek Formation (Figure 6-26). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-26 Piper diagram showing major ion composition for groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent hydrogeological units Samples are symbolised by aquifers or springs sampled. Filled shapes are samples collected in this study. Hollow shapes are historical data available from Department of Environment Parks and Water Security (2019a). Groundwater that was collected had a range in salinity that is fresh (<800 mg/L TDS) regardless of aquifer (Table 6-2). The salinity of historical samples reviewed for the same aquifers was also fresh regardless of the aquifer sampled. The pH of field samples ranged from close to neutral in the Bynoe and Skull Creek formations, to alkaline in the samples collected from aquifers hosted in the Battle Creek Formation and APV (Table 6-2). Similar to the CLA, the pH of groundwater samples reflects that of the carbonate rocks that hosts the aquifers in this part of the catchment. Table 6-2 Summary of pH, salinity (total dissolved solids (TDS)), water-type and carbonate speciation and mineral saturation indices for groundwater samples collected from aquifers hosted in the Cambrian limestone and adjacent hydrogeological units BORE NUMBER HYDROGEOLOGICAL UNIT pH SALINITY – TDS (mg/L) WATER TYPE AQUEOUS CO2 (mM) AQUEOUS HCO3 (mM) SI CALCITE SI DOLOMITE SI GYPSUM RN033280 Battle Creek Formation 7.5 590 Mg–HCO3 0.39 7.03 0.39 1.11 –2.65 RN036194 Bynoe Formation 6.9 460 Ca–HCO3 1.18 5.61 0.00 –0.07 –2.39 RN007403 Skull Creek Formation 7.1 693 Ca–HCO3 1.38 8.34 0.36 0.74 –2.35 RN031740 Antrim Plateau Volcanics 8.2 764 Na–HCO3 0.29 7.50 0.71 1.56 –2.52 *RN005867 Skull Creek Formation 7.5 529 Mg–HCO3 0.58 9.97 0.84 1.90 –2.64 *RN006494 Battle Creek Formation 8.0 279 Mg–HCO3 0.09 5.06 0.74 1.64 –2.63 *RN006677 Skull Creek Formation 7.6 300 Ca–HCO3 0.26 5.54 0.54 1.17 –2.70 *RN008030 Battle Creek Formation 8.5 459 Mg–HCO3 0.03 5.73 0.90 2.67 –2.86 *RN026814 Bynoe Formation 7.4 424 Ca–HCO3 0.60 8.17 0.68 1.44 –2.48 *RN032170 Bynoe Formation 7.2 287 Ca–HCO3 0.65 5.47 0.16 0.43 –2.61 Bulls Head Spring Skull Creek Formation 7.2 130 Ca–HCO3 0.73 5.98 0.33 0.44 –3.13 Crawford Spring Skull Creek Formation 7.3 175 Ca–HCO3 0.91 8.80 0.55 1.00 –3.71 Dead Spring Antrim Plateau Volcanics 7.9 226 Mg–HCO3 0.25 10.65 1.01 2.44 –3.69 Kidman Springs Skull Creek Formation 7.8 69 Ca–HCO3 0.10 3.09 0.34 0.66 –4.04 Waterbag Spring Battle Creek Formation 8.3 284 Mg–HCO3 0.11 10.91 1.09 2.94 –2.60 Bore numbers with a (*) are historical data available from Department of Environment Parks and Water Security (2019a). Plots of the major cation and anion concentrations relative to chloride concentration for groundwater hosted in the PDAs and adjacent hydrogeological units are shown in Figure 6-27. Groundwater samples from the PDAs are enriched in Ca, Mg and HCO3 relative to rainfall of marine origin, and most samples are also slightly enriched in Na (Figure 6-27). However, samples from the Battle Creek Formation which is rich in siltstone are less enriched in Ca compared to the dolostone aquifers (Bynoe and Skull Creek formations) which contain dolomite. These compositions reflect the carbonate-rich minerals of the limestones and dolostones that comprise the transmissive parts of the PDAs (mostly Skull Creek Formation and to a lesser extent the Bynoe Formation). Small hydrochemical trends from recharge areas to discharge areas at the springs is evident from the somewhat exponential shift in Ca, Mg and HCO3 concentrations relative to chloride (Cl/Br ratios provide no evidence of halite dissolution) (Figure 6-27). Na/HCO3 ratios for all groundwater samples are significantly lower than expected for rainfall of marine origin and the like the CLA, the addition of Ca, Mg and HCO3 in the PDAs is far more pronounced as a results of carbonate dissolution. PHREEQC modelling indicates that all groundwater samples from both the CLA and APV have a significant portion of their dissolved inorganic carbon in the form of aqueous HCO3 compared to aqueous CO2 (Table 6-2). All groundwater samples from the PDAs and APV are over saturated with respect to calcite and dolomite, except for one sample from the PDAs (Bynoe Formation) (Table 6-2). Gypsum dissolution is unlikely to be a source of Ca, as most groundwater samples exhibit Ca/SO4 ratios much higher than unity, and all samples are below saturation for gypsum (Figure 6-27 and Table 6-2). For more information on this figure please contact CSIRO on enquiries@csiro.au -1.02.05.08.011.014.00.00.51.01.52.0Na/Cl (meq/L) Cl (meq/L) (a) 0102030400.00.51.01.52.0Ca/Cl (meq/L) Cl (meq/L) (b) 01020304050600.00.51.01.52.0HCO3/Cl (meq/L) Cl (meq/L) (c) 01503004506007500.00.51.01.52.0Cl/Br (meq/L) Cl (meq/L) (e) 0.010.020.030.040.050.060.00.00.51.01.52.0(ca+Mg)/HCO3(meq/L) Cl (meq/L) Battle CreekBynoeSkull CreekSpringsAntrim Plateau VolcanicsSeawater dilution(g) 01002003004005000.00.51.01.52.0Ca/SO4 (meq/L) Cl (meq/L) Battle CreekBynoeSkull CreekSpringsAntrim Plateau VolcanicsSeawater dilution(h) 051015200.00.51.01.52.0Mg/Cl (meq/L) Cl (meq/L) (d) 0501001502002500.00.51.01.52.0Na/HCO3 (meq/L) Cl (meq/L) (f) Dead SpringWaterbag SpringBulls Head SpringCrawford SpringKidman SpringsBulls Head SpringKidman Springs Figure 6-27 Major ion ratio plots for groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone aquifers and aquifers hosted in adjacent hydrogeological units Samples are symbolised by aquifers or springs sampled. Filled shapes are samples collected in this study. Hollow shapes are historical data available from Department of Environment Parks and Water Security (2019a). 6.2.6 Stable hydrogen and oxygen isotopes Cambrian Limestone Aquifer The stable hydrogen and isotopic compositions of groundwater samples from the CLA (Montejinni Limestone) and aquifers hosted in adjacent hydrogeological units (APV) are shown in Figure 6-28 and Figure 6-29. For comparison, the isotopic composition of rainfall near Darwin (the closest rainfall isotopic data available) spanning 1963 to 2010 (IAEA/WMO, 2023) and the local meteoric water line (LMWL) for Darwin is also shown in Figure 6-28. The range in isotopic composition for groundwater samples collected in this study is from −8.4 to −6.1‰ for δ18O, and from −59 to −44‰ for δ2H. For the samples specifically collected from the Montejinni Limestone, δ18O ranges from −8.2 to −7.2‰ and δ2H ranges from −55 to −49‰. For samples collected in this study from the APV, δ18O ranges from −8.4 to −6.1‰ and δ2H ranges from −58‰ to −44‰. The similarity of these ranges for most samples reflects: (i) their correlation with the most depleted wet-season rainfall being the source of recharge to the aquifer; and (ii) their interconnectivity and mixing between aquifers in both hydrogeological units. The exception to this is for two samples: (i) one sample from the CLA (RN041173) which plots further to the right of the LMWL; and (ii) two samples from the APV (RN026127) which are both more enriched. Evaporative enrichment has occurred during recharge to sample RN041173 from the CLA as it was collected in an area where the CLA does not outcrop, and recharge occurred diffusely through overlying Cretaceous cover. The same has occurred for the sample from the APV (RN026127) which is from a localised low- yielding aquifer in the APV well west of the margin of the CLA. These results are consistent with historical samples collected by Tickell (1998) shown in Figure 6-28 and Figure 6-29. Generally, the most depleted samples are associated with localised recharge to the CLA in the east of the catchment and samples west of the margin of the CLA collected from the APV in the Cambrian basalt are more enriched from these highly heterogenous localised aquifers. For more information on this figure please contact CSIRO on enquiries@csiro.au -70-60-50-40-30-20-10.0-8.0-6.0-4.0-2.00.0d2H (‰ VSMOW) d18O (‰ VSMOW) Montejinni LimestoneML/APVAntrim plateau volcanicsSpring or RiverDarwin rainfallLMWL - DarwinRN041173RN026127PalmSpringLonelySpringCompanion SpringWinari Spring Figure 6-28 Stable hydrogen and oxygen isotope composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent hydrogeological units (Cambrian basalt) compared to rainfall Symbols indicate the aquifer or spring sampled. Filled shapes are samples collected in this study. Hollow shapes are historical data available from Tickell (1998). Inferred evaporation line in red. LMWL = local meteoric water line; VSMOW = Vienna Standard Mean Ocean Water; ML = Montejinni Limestone; APV = Antrim Plateau Volcanics. Rainfall data source: IAEA/WMO (2023) The isotopic composition of samples collected from springs associated with the CLA and APV varied across a large range, spanning from –7.4‰ to –1.5‰ for δ18O and from –54‰ to –25‰ for δ2H. The springs with the most depleted isotopic composition are from Lonely, Palm and Old Top springs which receive groundwater from the CLA. The springs with the most enriched isotopic composition are from Companion Spring which receives groundwater from the APV and Winari Spring which receives groundwater from both the CLA and APV. The water in both Companion and Winari springs has undergone significant evaporative enrichment (Figure 6-28). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-29 Spatial distribution of stable hydrogen isotope composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed, except for the Quaternary alluvium. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. ML = Montejinni Limestone; APV = Antrim Plateau Volcanics. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008); spring data from Department of Environment, Parks and Water Security (2019b) Proterozoic dolostone aquifers The stable hydrogen and isotopic compositions of groundwater samples collected from the PDAs (Battle Creek, Bynoe and Skull Creek formations) and aquifers hosted in adjacent hydrogeological units (APV) are shown in Figure 6-29 and Figure 6-30. The groundwater isotopic composition is also shown in relation to Darwin rainfall and the LMWL for Darwin (Figure 6-30). The isotopic composition for all groundwater samples collected in this study ranges from −8.3‰ to −6.0‰ for δ18O and from −55‰ to −42‰ for δ2H. The sample collected from the Skull Creek Formation has the most depleted isotopic composition (−8.3‰ for δ18O and –55‰ for δ2H), followed by the sample collected from the Bynoe Formation (–7.9‰ for δ18O and –54‰ for δ2H). The sample collected from the Battle Creek Formation has a more enriched composition (–6.0 for δ18O and −42‰ for δ2H) (Figure 6-30). All samples plot close to the LMWL indicating that recharge to groundwater is localised with very little evaporation prior to entering the saturated zone. Only one historical sample was available for the outcropping subcropping area of the PDAs from Tickell (1998). Despite this sample from the Skull Creek Formation being more enriched (–4.8‰ for δ18O and –31‰ for δ2H) than samples collected in this study, it also plots close to the LMWL suggesting localised recharge occurred at this location. This is not surprising given the PDAs mostly occur in a higher rainfall zone in the catchment compared to that of the CLA. These samples exhibit little enrichment from evaporation, suggesting groundwater recharge is rapid and highly localised in the aquifer outcrop. For more information on this figure please contact CSIRO on enquiries@csiro.au -70-60-50-40-30-20-100-10.0-8.0-6.0-4.0-2.00.02.04.0δ2H (‰ VSMOW) δ18O (‰ VSMOW) Battle Creek FormationBynoe FormationSkull Creek FormationSpring or RiverDarwin rainfallLMWL - DarwinKidman SpringDead SpringWaterbag SpringCrawfordSpring Figure 6-30 Stable hydrogen and oxygen isotope composition for groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent hydrogeological units compared to rainfall Symbols indicate the aquifers or springs sampled. Filled shapes are samples collected in this study. Hollow shapes are historical data available from Tickell (1998). Inferred evaporation line in red. LMWL = local meteoric water line; VSMOW = Vienna Standard Mean Ocean Water. Rainfall data source: IAEA/WMO (2023) The isotopic composition of samples collected from springs associated with the PDAs varied across a large range, spanning from –7.6‰ to –3.0‰ for δ18O and from –51‰ to –6‰ for δ2H (Figure 6-30). The springs with the most depleted isotopic composition are from Bulls Head and Crawford springs which receive groundwater from the dolostone aquifers of the PDAs (Byone and Skull Creek formations). The springs with the most enriched isotopic composition and exhibiting clear evaporative enrichment are from Dead, Kidman and Waterbag springs (Figure 6-30). 6.2.7 Strontium isotopes Cambrian Limestone Aquifer The strontium (87Sr/86Sr) isotopic compositions of groundwater samples collected from the CLA (Montejinni Limestone) and aquifers hosted in adjacent hydrogeological units (APV) are presented in Figure 6-31 and Figure 6-32. The range in 87Sr/86Sr for groundwater samples in the Montejinni Limestone ranges from 0.7107 to 0.7158 (Figure 6-31). Whilst they are higher than 87Sr/86Sr isotopic composition of groundwater in carbonate aquifers elsewhere in Australia (0.7084 Dogramaci and Herczeg, 2002), this range is similar to the 87Sr/86Sr composition of groundwater found in other recharge areas for the CLA (0.7120 to 0.7190; northern aquifer margin in the Roper catchment overlying the Daly Basin; (Taylor et al., 2023), and may be representative of the mineralogy of the CLA in this region. Two samples collected from RN020019 and RN02644 have a similar composition, whereas the sample from RN041173 is much lower (0.7107) at RN041173. For the two former samples, this range reflects groundwater that has acquired strontium predominantly from inland rainfall (0.7150,) and silicate weathering (Bogard et al., 1967; Dogramaci and Herczeg, 2002; Hamilton, 1966; Seimbille et al., 1988). The composition of the sample at RN041173 may be more reflective of carbonate dissolution. For more information on this figure please contact CSIRO on enquiries@csiro.au 0.7100.7130.7150.71811010087Sr/86Sr1/Sr (mg/L) Montejini LimestoneML/APVAntrim Plateau VolcanicsSpringsRN026127RN041173RN021978RN020019 Figure 6-31 Strontium isotope composition relative to strontium concentration for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units Samples are symbolised by aquifers or springs sampled. Strontium concentration plotted on a logarithmic scale. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. The range in 87Sr/86Sr for groundwater samples in the APV is narrow ranging from 0.7112 to 0.7127 (Figure 6-31). The two samples from the APV (RN026127 and RN021978) have a similar strontium isotopic composition to samples collected in the APV underlying the CLA recently by Taylor et al. (2023) in the Roper catchment and Daly Basin. Generally, the strontium isotopic composition of groundwater collected from both the Montejinni Limestone are and APV are similar. This further reflects the hydrological interconnectivity of the aquifers hosted in these hydrogeological units in the Victoria catchment. This is consistent with findings from groundwater level investigations in Section 6.2.1 and hydrochemical trends in groundwater found in Section 6.2.5. The strontium isotopic composition of samples collected in springs ranges between 0.7139 to 0.7156 (Figure 6-31). This composition correlates with groundwater collected from the bores installed in the Montejinni Limestone and bores installed in the APV where it underlies the limestone. This provides another line of evidence that all springs sampled in this study in the eastern part of the Victoria catchment source their water from the CLA, except perhaps Companion Spring (see Section 6.2.5). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-32 Spatial distribution of strontium isotopic composition for groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and aquifers hosted in adjacent hydrogeological units To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous and Cenozoic rocks and sediments have been stippled. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Geology data sources adapted from: Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data source: Department of Environment Parks and Water Security (2019b) Proterozoic dolostone aquifers The strontium (87Sr/86Sr) isotopic compositions of all groundwater samples collected from the PDAs (Battle Creek, Bynoe and Skull Creek formations) and aquifers hosted in adjacent hydrogeological units (APV) are shown in Figure 6-32 and Figure 6-33. The 87Sr/86Sr in groundwater samples ranged from 0.7154 to 0.7365 with the highest values exhibited in bores installed in the northern part of the PDAs (Figure 6-32 and Figure 6-33). The higher 87Sr/86Sr values (>0.7300) in groundwater are associated with the Skull Creek and Bynoe formations which are partly comprised of sandstone and is typical of weathering of old silicate minerals (Harrington and Herczeg, 2003). The lowest value (0.7154) collected across the PDAs is from a bore installed in an aquifer hosted in the Battle Creek Formation. This value correlates with the expected range for inland rainfall and also those of silicate aquifers. While dolostone composition typically only contains minor amounts of silicate, the Proterozoic dolostones in the Victoria catchment are all partly comprised of sandstones and siltstones. Similarly high strontium isotope compositions were also found in groundwater recently collected from the dolostone Dook Creek Aquifer by Taylor et al. (2023). The other sample collected from the APV (RN031740) has a similar strontium isotopic composition more reflective of silicate aquifers (0.7107-0.7191) in other parts of Australia (Cartwright et al., 2007a). Whilst the APV are predominantly basaltic, RN031740 is screened into a sandstone part of the APV, hence its different composition. All springs sampled in and around the PDAs exhibited characteristically high 87Sr/86Sr values (0.7318 to 0.7351) aligning most closely with groundwater collected from both the Bynoe and Skull Creek formations (Figure 6-33). Springs occurring in the eastern parts of the PDAs (Dead and Waterbag springs; see Figure 6-32), at the interface with the Cambrian basalt correlate with the strontium isotopic composition that sits amongst the strontium isotopic composition for groundwater collected from both the Battle Creek Formation and APV (Figure 6-33). For more information on this figure please contact CSIRO on enquiries@csiro.au 0.7000.7100.7200.7300.74011010087Sr/86Sr1/Sr (mg/L) Battle Creek FormationBynoe FormationSkull Creek FormationSpringsAntrim Plateau VolcanicsDeadSpringWaterbag SpringKidman SpringsCrawford Spring Figure 6-33 Strontium isotope composition relative to strontium concentration for groundwater and spring samples collected from aquifers hosted in Proterozoic dolostones Samples are symbolised by aquifers or springs sampled. Strontium concentration plotted on a logarithmic scale. 6.2.8 Tritium Tritium (3H) sources in aquifers can cover the full spectrum of tritium present in precipitation ranging from either current cosmogenic background or from the time of the ‘bomb’ pulse from above-ground thermonuclear testing in the 1950s (Payne, 1972). In the Southern hemisphere, most of the 3H related to the bomb pulse has now dissipated or decayed. Modern Darwin precipitation is also now close to the pre-atomic testing cosmogenic background (~1.2 to 1.6 TU) (Tadros et al., 2014). Since the half-life of 3H is 12.4 years, groundwater systems containing measurable 3H can be used to provide semi-quantitative estimates of contemporary (spanning recent decades) mean annual recharge. Cambrian Limestone Aquifer Tritium (3H) concentrations in groundwater and springs collected from the CLA (Montejinni Limestone) and aquifers hosted in the adjacent hydrogeological units (APV) are presented in Figure 6-34 and Figure 6-35. Tritium in groundwater from all aquifers ranged from 0.059 to 1.3 TU, with one sample below the detection limit (0.025 TU). In samples collected from the Montejinni Limestone, the 3H concentration in groundwater varied from 0.067 to 0.63 TU and the concentration decreased with increasing depth below the watertable. In the sample collected from the APV, the 3H concentration in groundwater was 0.059 at RN021978 to non-detectable at RN026127 in the APV (Figure 6-34). The highest concentrations of 3H in groundwater were collected from RN0200149 and RN026444 installed in an outcropping area of the Montejinni Limestone where localised recharge has occurred (Figure 6-34). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-34 Spatial distribution of the tritium concentration for groundwater and spring samples from aquifers hosted in the Cambrian limestone and Proterozoic dolostone, as well as aquifers hosted in the adjacent Cambrian basalt To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous and Cenozoic rocks and sediments have been stippled. The lower left inset indicates the geographical extent of the map figure within the Victoria catchment. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Geology data sources adapted from: Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Spring data source: Department of Environment Parks and Water Security (2019b) One groundwater sample collected from the Montejinni Limestone at RN041173 had a low concentration of 3H (0.067). This could be attributed to the fact that 3H is mostly decayed at this depth near the base of the aquifer which may be comprised of mudstone. It also could be attributed to the fact the bore is screened immediately above the interface with the APV according to the lithology and construction logs. However, the bore could be partly screened in the transition from the CLA to the APV. In addition, the bore could be drawing water from mudstone parts of the Montejinni Limestone which may have lower concentrations of 3H. Tritium concentrations in springs samples collected along the western margin of the CLA were high compared to groundwater ranging from 0.60 to 1.1 TU (Figure 6-35). Though 3H concentrations in springs were mostly lower than that expected for contemporary rainfall (1.2 to 1.6 TU), concentrations in some springs were closer (Companion, Lonely and Winari springs). This suggests, based on the radioactive decay rate for 3H, that the sources of spring discharge occur from localised groundwater flow paths in the aquifer outcrop for the APV around Companion Spring, and the aquifer outcrop of the CLA (Montejinni Limestone) around, Lonely and Winari springs. These localised flow paths appear to have very short residence times of about a few years. In addition, slightly longer intermediate scale groundwater flow paths in the aquifer outcrop of the CLA with mean residence times (MRTs) in the order of about a few decades appears to be a source of discharge for Old Top and Palm springs. For more information on this figure please contact CSIRO on enquiries@csiro.au 01020304050600.00.30.50.81.01.3Depth below the watertable (m) 3H (TU) Montejinni LimestoneML/APVAntrim Plateau VolcanicsSpringWinari and Lonely springsPalm SpringRN041173RN026444RN021978RN020019Old Top Spring Figure 6-35 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Cambrian limestone and Cambrian basalt Samples are symbolised by aquifers and springs sampled. Springs are shown for comparison with groundwater. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Proterozoic dolostone aquifers Tritium (3H) concentrations in groundwater and springs collected from the PDAs (Battle, Bynoe and Skull Creek formations) and aquifers hosted in adjacent hydrogeological units (APV) are presented in Figure 6-34 and Figure 6-36. Tritium in groundwater from all aquifers ranged from 0.067 to 1.3 TU. In samples collected from the PDAs, the 3H concentration in groundwater varied from 0.067 to 0.50 TU, with the sample collected from the Bynoe Formation having the highest concentration (Figure 6-34). The range in 3H concentrations found in the PDAs is very similar to that found in the CLA, as is the decrease in concentration with depth below the watertable (Figure 6-36). Only one sample from the APV (RN031740) was collected adjacent the PDAs, and the groundwater at this location had a much higher 3H concentration (1.3 TU). However, the lithology of the APV at this location is a quartzose sandstone unlike the majority of the basaltic APV across the catchment. This indicates that localised recharge is frequently occurring at this location at a rate higher than for the PDAs as the concentration in groundwater is equivalent to that of contemporary rainfall (Tadros et al., 2014). For more information on this figure please contact CSIRO on enquiries@csiro.au 0102030405060700.00.30.50.81.01.31.5Depth below the watertable (m) 3H (TU) Battle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau VolcanicsSpringKidman SpringsBulls Head SpringCrawford Spring Figure 6-36 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Proterozoic dolostone and Cambrian basalt Samples are symbolised by aquifers and springs sampled. Springs are shown for comparison with groundwater. Similar to springs associated with the CLA, tritium concentrations in springs occurring around the subcropping and outcropping margins of the PDAs were mostly high compared to groundwater. Tritium concentrations mostly ranged from 1.0 to 1.4 TU, except for Bulls Head Spring having a concentration of 0.32 TU (Figure 6-36). Samples collected from Crawford, Dead, Kidman and Waterbag springs all had concentrations similar to that of contemporary rainfall near Darwin (1.2 to 1.6 TU). This suggests, based on the radioactive decay rate for 3H, that the sources of spring discharge occur from localised groundwater flow paths in the aquifer outcrop for the Battle Creek Formation and APV around Crawford and Dead springs, and the Bynoe and Skull Creek formations for Kidman and Waterbag springs (see Section 6.2.7). These localised flow paths appear to have very short residence times of about a few years. Bulls Head Spring has a much lower concentration of 3H suggesting the source of discharge comes from longer intermediate scale groundwater flow paths in the aquifer outcrop of the Bynoe and Skull Creek formations with residence times in the order of about a few decades. 6.2.9 Noble gases Cambrian Limestone Aquifer Concentrations of stable noble gases (Ne, Kr and Xe) from groundwater samples and spring samples collected from the CLA (Montejinni Limestone) and aquifers hosted in the adjacent hydrogeological units (APV) are summarised in Figure 6-37. Noble gases are plotted as one element against another, with the black line representing the solubility equilibrium at different temperatures and green lines starting at different solubility equilibrium temperatures demonstrate the addition of excess air. Noble gas concentrations indicate infiltration temperatures between about 25 and 34 °C across all aquifers sampled (Figure 6-37). Though samples collected from the Montejinni Limestone or where the APV underlies it indicate infiltration temperatures closer to the range of 30 to 34 °C. Only one sample, RN026127 collected from the aquifer hosted in the APV, exhibits a noble gas composition indicating a lower infiltration temperature of about 25 °C. These results are consistent with recharge associated with mean wet-season temperatures across the CLA from the semi-arid to arid zone (McJannet et al., 2023). All groundwater samples, regardless of aquifer, exhibit higher excess air than the springs sampled. This is not surprising given the fractured and karstic nature of the Montejinni Limestone within the CLA and the fractured nature of the basalt rocks of the APV. Air entrapment can be caused by watertable fluctuations, either from natural recharge and discharge processes or by groundwater extraction inducing drawdown and recovery at a given bore location. Excess air may be less likely to be evident in springs samples if samples have undergone some de-gassing prior to sampling, particularly where discharge processes can be highly variable ranging from diffuse seepage to localised preferential outflow at the same location. For more information on this figure please contact CSIRO on enquiries@csiro.au 1920212223242526272829303132333435363738396.0E-096.5E-097.0E-097.5E-098.0E-098.5E-099.0E-099.5E-091.0E-081.5E-071.7E-071.9E-072.1E-072.3E-072.5E-072.7E-072.9E-073.1E-073.3E-073.5E-07Xe (ccSTP/g) Ne (ccSTP/g) APVML/APVMLSpringsSolEqEx. Air 34CEx. Air 25C1920212223242526272829303132333435363738396.0E-096.5E-097.0E-097.5E-098.0E-098.5E-099.0E-099.5E-091.0E-084.0E-084.5E-085.0E-085.5E-086.0E-086.5E-087.0E-087.5E-088.0E-08Xe (ccSTP/g) Kr (ccSTP/g) (a) (b) RN026127RN026127 Figure 6-37 Measured concentrations of (a) xenon versus krypton and (b) xenon versus neon in groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt Samples are symbolised by aquifers or springs sampled. Sol. Eq. = solubility equilibrium; Ex. Air = excess air; ML = Montejinni Limestone; APV = Antrim Plateau Volcanics. Proterozoic dolostone aquifers Concentrations of stable noble gases from groundwater samples and spring samples collected from the PDAs (Battle, Bynoe and Skull Creek formations) and aquifers hosted in the adjacent hydrogeological units (APV) are summarised in Figure 6-38. Noble gas concentrations indicate infiltration temperatures between about 25 and 34 °C across all aquifers sampled (Figure 6-37). Though samples collected from the PDAs indicate infiltration temperatures closer to the 30 °C for the aquifer hosted in the Skull Creek Formation and about 34 °C for the aquifers hosted in the Battle Creek and Bynoe formations (Figure 6-38). Only one sample, RN031740 collected from the aquifer hosted in the APV, exhibits a noble gas composition indicating a lower infiltration temperature of about 25 °C (Figure 6-38). These results are consistent with recharge associated with mean wet-season temperatures for the central and northern parts of the Victoria catchment (McJannet et al., 2023). Similar to the springs associated with the CLA, the springs associated with the PDAs exhibit a composition reflective of less excess air. The dolostone-rich PDAs are also karstic and fractured and natural, or anthropogenic groundwater processes (groundwater extraction) will entrap excess air in groundwater. In addition, samples collected from springs may have undergone some de-gassing prior to sampling. For more information on this figure please contact CSIRO on enquiries@csiro.au 1920212223242526272829303132333435363738396.0E-096.5E-097.0E-097.5E-098.0E-098.5E-099.0E-099.5E-091.0E-084.0E-084.5E-085.0E-085.5E-086.0E-086.5E-087.0E-087.5E-088.0E-08Xe (ccSTP/g) Kr (ccSTP/g) (a) 1920212223242526272829303132333435363738396.0E-096.5E-097.0E-097.5E-098.0E-098.5E-099.0E-099.5E-091.0E-081.5E-071.7E-071.9E-072.1E-072.3E-072.5E-072.7E-072.9E-073.1E-073.3E-073.5E-07Xe (ccSTP/g) Ne (ccSTP/g) Battle Creek FormationBynoe FormationSkull Creek FormationSpringsSolEqEx. Air 34CEx. Air 25CAntrim Plateau Volcanics(b) Dead SpringDead SpringRN031740RN031740 Figure 6-38 Measured concentrations of (a) xenon versus krypton and (b) xenon versus neon in groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt Samples are symbolised by aquifers or springs sampled. Sol. Eq. = solubility equilibrium; Ex. Air = excess air. 6.2.10 Anthropogenic gases – chlorofluorocarbons, sulfur hexafluoride, bromotrifluoromethane Sulfur hexafluoride (SF6) and bromotrifluoromethane (H1301) Cambrian Limestone Aquifer Figure 6-39 summarises concentrations of SF6 and H1301 in groundwater collected from across the CLA (Montejinni Limestone) and aquifers hosted in adjacent hydrogeological units (APV). The presence of measurable SF6 and H1301 in groundwater indicates recent recharge is taking place directly in the outcrop but also where Cretaceous and Cenozoic cover occur over the CLA. This is consistent with 3H concentrations in groundwater spatially across the CLA, as well as with depth in the aquifer as described in Section 6.2.8. Concentrations of SF6 in groundwater samples range from 0.04 to 101 fMol/kg and similar to 3H in groundwater, decrease in concentration with increasing depth below the watertable (Figure 6-39a). The expected measurable SF6 concentration for groundwater in equilibrium with atmospheric concentrations (i.e. no excess air) is 0.15 to 1.7 fMol/kg. SF6 concentrations in samples collected from the Montejinni Limestone range from 0.13 to 101 fMol/kg (Figure 6-39). One sample from the Montejinni Limestone at RN020019 installed in the aquifer outcrop is affected by excess air, a second sample from the limestone at RN026444 is contaminated due to its excessive concentration (101 fMol/kg) (Figure 6-39). SF6 concentrations in samples collected from the APV range from 0.04 to 0.32 fMol/kg. The groundwater collected at RN021978 with a concentration of 0.32 fMol/kg also had measurable 3H. This bore is located along the margin of the Montejinni Limestone where groundwater discharges from the CLA to several spring complexes (see Figure 6-20). Concentrations of H1301 in groundwater collected from the CLA and APV range from 0 (below detection limit) to 0.76 fMol/kg (Figure 6-39b). Similar to 3H and SF6 compositions in groundwater, H1301 exhibits a decreasing trend in concentration with increasing depth below the watertable (Figure 6-39). The highest concentrations of H1301 found in samples collected from the CLA, and the lowest concentrations of H1301 were found in samples collected from the APV (Figure 6-39). For more information on this figure please contact CSIRO on enquiries@csiro.au 0204060800.00.11.010.0100.01000.0Depth below the water table (m) SF6 (fMol/kg) 0204060800.00.10.20.30.40.50.60.70.8Depth below the water table (m) H1301 (fMol/kg) Montejinni LimestoneML/APVAntrim Plateau Volcanics(a) (b) RN020019RN026444RN026127RN026444RN026127RN041173 Figure 6-39 Measured concentrations of (a) sulfur hexafluoride (SF6) (b) bromotrifluoromethane (H1301) in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt Samples are symbolised by aquifers sampled. Green shading on (a) represents expected measurable range in SF6 concentrations in groundwater assuming samples are in equilibrium with atmospheric concentrations and are unaffected by excess air. ML = Montejinni Limestone; APV = Antrim Plateau Volcanics. Proterozoic dolostone aquifers Figure 6-40 summarises concentrations of SF6 and H1301, in groundwater collected from aquifers hosted in the PDAs (Battle Creek, Bynoe and Skull Creek formations) and aquifers hosted in adjacent hydrogeological units (APV). Similar to samples collected from the CLA, the presence of measurable SF6 and H1301 in groundwater in the PDAs indicates recharge occurring in the aquifer outcrop at these locations. This is consistent with 3H concentrations in groundwater spatially across the PDAs, and with increasing depth ion the aquifer as described in Section 6.2.8. Concentrations of SF6 in groundwater collected from the PDAs ranged from 1.19 to 2.86 fMol/kg, while the one sample collected from the APV had an SF6 concentration of 2.72 fMol/kg (Figure 6-40). Three of the four samples collected in this study for SF6 across the PDAs and adjacent APV were greater than 1.7 fMol/kg indicating they are affected by excess air entrapped in the saturated zone. Only one sample collected from the Battle Creek Formation which is mostly comprised of siltstone appears to be unaffected by excess air. Excess air entrapment in the other samples can mostly be correlated with the influence of karstic features in the Bynoe and Skull Creek formations. Groundwater that contained low SF6 (<0.3 pMol/kg) also contained low 3H (<0.07 TU), see Section 6.2.8. Concentrations of H1301 in groundwater collected for the PDAs and adjacent APV range from 1.37 to 4.62 fMol/kg, with the sample from the APV exceeded the expected range for measurable H1301 in groundwater in equilibrium with the atmosphere (Figure 6-40). Similar to 3H and SF6 compositions in groundwater, H1301 exhibits a decreasing trend in concentration with increasing depth below the watertable (Figure 6-40). The highest concentrations of H1301 found in samples collected from the Bynoe Formation and APV. The lowest concentration of H1301 were found in sample collected from the Battle Creek Formation which is a low permeability siltstone (Figure 6-40). For more information on this figure please contact CSIRO on enquiries@csiro.au 02040601.010.0100.01000.0Depth below the water table (m) SF6 (fMol/kg) 02040600.00.51.01.52.02.53.03.54.04.55.0Depth below the water table (m) H1301 (fMol/kg) Battle Creek FormationBynoe FormationAntrim Plateau VolcanicsSkull Creek Formation(a) (b) Figure 6-40 Measured concentrations of sulfur hexafluoride (SF6) (b) bromotrifluoromethane (H1301) in groundwater and collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt Samples are symbolised by aquifers sampled. Green shading on (a) represents expected measurable range in SF6 concentrations in groundwater assuming samples are in equilibrium with atmospheric concentrations and are unaffected by excess air. Chlorofluorocarbons (CFC-11 and CFC-12) Cambrian Limestone Aquifer Concentrations of CFC-11 and CFC-12 in groundwater collected from aquifers hosted in the CLA (Montejinni Limestone) and aquifers hosted in adjacent hydrogeological units (APV) are summarised in Figure 6-41. Concentrations of CFC-11 ranged from 0.02 to 0.07 pMol/kg for CFC-11 and 0.04 to 0.29 pMol/kg for CFC-12 regardless of aquifer sampled (Figure 6-41). The sample with the highest CFC composition was for RN026444 which is installed in the aquifer outcrop for the CLA but is screened in both the Montejinni Limestone and the APV. CFC concentrations were low in the other two samples from the Montejinni Limestone and APV. The CFC composition, particularly the ratio of CFC-11 to CFC-12 in most samples collected except for RN026444 indicates CFC-11 has undergone varying degrees of degradation, which may be due in part to the mudstone composition of the CLA in places. Therefore CFC-12 in this case is a more reliable indicator of groundwater recharge. For more information on this figure please contact CSIRO on enquiries@csiro.au 0.000.010.020.030.040.050.060.070.000.050.100.150.200.250.300.35CFC-11 (pMol/kg) CFC-12 (pMol/kg) Montejinni LimestoneML/APVAntrim Plateau VolcanicsRN026444RN041173 Figure 6-41 Measured concentrations of CFC-11 and CFC-12 in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt Samples are symbolised by aquifers sampled. ML = Montejinni Limestone; APV = Antrim Plateau Volcanics. Proterozoic dolostone aquifers Concentrations of CFC-11 and CFC-12 in groundwater collected from the PDAs (Battle Creek, Bynoe and Skull Creek formations), and aquifers hosted in adjacent hydrogeological units (APV) are summarised in Figure 6-42. Concentrations of CFC-11 ranged from 0.04 to 1.12 pMol/kg for CFC-11 and 0.39 to 1.25 pMol/kg for CFC-12 regardless of aquifer sampled (Figure 6-42). The sample with the highest CFC composition (1.12 pMol/kg for CFC-11 and 1.25 pMol/kg for CFC- 12) was for the bore installed in a sandstone unit of the APV (Figure 6-42). Samples from the dolostone-rich Bynoe and Skull Creek formations also had moderate concentrations of CFCs in groundwater. The sample collected from the siltstone-rich Battle Creek formation had the lowest CFC composition. Unlike samples for CFCs from groundwater in the CLA, CFC-11 has undergone little degradation in the PDAs, with only the sample collected from the Skull Creek Formation exhibiting a small amount. For more information on this figure please contact CSIRO on enquiries@csiro.au 0.000.200.400.600.801.001.200.000.200.400.600.801.001.201.40CFC-11 (pMol/kg) CFC-12 (pMol/kg) Battle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau Volcanics Figure 6-42 Measured concentrations of CFC-11 and CFC-12 in groundwater collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt Samples are symbolised by aquifers sampled. 6.2.11 Carbon isotopes Cambrian Limestone Aquifer Figure 6-43 is a summary of the carbon isotopic composition in groundwater samples collected from the CLA (Montejinni Limestone) and adjacent hydrogeological units (APV). Carbon-14 (14C) is expressed as a percentage equivalent to percent modern carbon (pmC). Carbon-13 (13C) is expressed as a percentage equivalent to the standard, Vienna Pee Dee Belemnite (VPDB). The 14C composition for all groundwater samples regardless of aquifer span a large range from 14 to 90 pmC (Figure 6-43). The highest 14C compositions for samples collected from the CLA were at RN026444 and RN020019 both installed in the aquifer outcrop. The other sample collected from the CLA (RN041173) had a low 14C composition (14 pmC). This bore is installed in the CLA where it is overlain with Cretaceous and Cenozoic cover. One sample collected from the APV (RN021978) also had a high 14C composition (89 pmC). This bore is installed in the aquifer outcrop for the APV immediately adjacent the western margin of the CLA near Old Top Spring (see Figure 6-20). These results correlate well with 3H concentrations in groundwater. The samples from the CLA and APV with the highest 14C compositions, also had the highest 3H concentrations in groundwater indicating recent groundwater recharge (see Section 6.2.8). Samples with the lowest 14C composition also had the lowest 3H concentrations in groundwater. Carbon-13 compositions give an indication of the carbon sources and geochemical processes that can influence groundwater. The most depleted 13C compositions (–14 to –13‰ VPDB) measured in groundwater can sometimes be semi-indicative of a recharge endmember in equilibrium with CO2 in the unsaturated zone, particularly where the depth to watertable is shallow (> 20 m). The more enriched 13C compositions (–11 to –9‰ VPDB) measured in groundwater can sometimes be semi- indicative of an endmember for carbonate mineral weathering, in this case, carbonate dissolution. There are too few samples for 13C compositions for groundwater in this study to clearly define these types of endmembers. However, all three samples collected from the CLA have most of their total dissolved inorganic carbon in the form of aqueous HCO3 rather than aqueous CO2, and all three samples are oversaturated with respect to calcite and dolomite indicative of carbonate dissolution (see Section 6.2.5). This indicates that estimating MRTs for groundwater flow using 14C is problematic in the absence of more spatial data. For more information on this figure please contact CSIRO on enquiries@csiro.au 0102030405060708090100-14-12-10-814C (pmC) δ13C (‰VPDB) Montejinni LimestoneML/APVAntrim Plateua Volcanics(a) RN020019Potentialincreaseinresidence time010203040506070-14-12-10-8Depth below the watertable (m) δ13C (‰VPDB) Montejinni LimestoneML/APVAntrim Plateau Volcanics010203040506070020406080100Depth below the watertable (m) 14C (pmC) Montejinni LimestoneML/APVAntrim Plateau Volcanics(b)(c) RN026444RN026444RN021978RN021978RN020019RN026444RN041173RN041173RN041173RN020019Potentialincreaseinmineralweathering Figure 6-43 Measured concentrations of (a) 13C vs 14C, (b) 14C vs depth below the watertable, and (c) 13C vs depth below the watertable in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt Samples are symbolised by aquifers sampled. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Proterozoic dolostone aquifers Figure 6-44 is a summary of the carbon isotopic composition in groundwater samples collected from the PDAs (Battle Creek, Bynoe and Skull Creek formations) and adjacent hydrogeological units (APV). The 14C composition for all groundwater samples regardless of aquifer span a much narrow range than that for the CLA, ranging from 77 to 92 pmC (Figure 6-44). The highest 14C composition was for a sample collected from a bore drilled in a sandstone unit of the APV (92 pmC). Equally high was a sample collected from the Battle Creek Formation (89 pmC). Samples collected from both the Bynoe and Skull Creek formations exhibited similar 14C compositions at approximately 80 pmC (Figure 6-44). As found for groundwater in the CLA, the 14C results correlate well with results for 3H in groundwater. Samples from APV and PDAs with the highest 14C compositions, also had the highest 3H concentrations in groundwater, indicating recent groundwater recharge (see Section 6.2.8). The most depleted 13C composition (−13‰ VPDB) measured in groundwater was collected from the APV, while samples collected from the PDAs had a more enriched 13C composition (–12 to –11‰ VPDB). The 13C compositions for groundwater in this study indicate all three samples collected from the PDAs all of which are collected from bores installed in the aquifer outcrop have the addition of some dead-carbon from carbonate dissolution. This is consistent with all three samples having most of their total dissolved inorganic carbon in the form of aqueous HCO3 rather than aqueous CO2, and all three samples are oversaturated with respect to calcite and dolomite (see Section 6.2.5). Similar to the samples collected from the CLA, estimating MRTs for groundwater flow using 14C is problematic in the absence of more spatial data. For more information on this figure please contact CSIRO on enquiries@csiro.au 0102030405060708090100-14-12-10-814C (pmC) δ13C (‰VPDB) Battle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau Volcanics0102030405060-14-12-10-8Depth below the watertable (m) δ13C (‰VPDB) Battle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau Volcanics0102030405060707580859095100Depth below the watertable (m) 14C (pmC) Battle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau Volcanics(a)(b)(c) PotentialincreaseinmineralweatheringPotentialincreaseinresidence time Figure 6-44 Measured concentrations of (a) 13C vs 14C, (b) 14C vs depth below the watertable, and (c) 13C vs depth below the watertable in groundwater collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt Samples are symbolised by aquifers sampled. 6.2.12 Helium Cambrian Limestone Aquifer Figure 6-45 summarises measured dissolved helium and neon concentrations in groundwater collected from the CLA (Montejinni Limestone) and aquifers hosted in adjacent hydrogeological units (APV). Dissolved helium concentrations in groundwater varied from about atmospheric equilibrium (4.5E–08 cc/g) up to 1.4E–05 cc/g regardless of aquifer. However, samples collected from the CLA exhibit mostly a narrow range (7.0E–08 to 2.1E–07 cc/g); only the sample collected at RN041173 exhibited a helium concentration above atmospheric equilibrium. At this location (north-east of Top Springs, see Figure 6-20), the depth to watertable was 34 m at the time of sampling, and this site also had low 3H and low 14C in groundwater (see sections 6.2.8 and 6.2.11). This suggests that MRTs for groundwater flow in this part of the CLA are longer (decades or more) than in other parts of the CLA where it outcrops and helium samples in groundwater are equivalent to atmospheric such as at RN020019 (Figure 6-45). Figure 6-45 further highlights that most samples fall along a mixing line between solubility equilibrium (one sample from the CLA and most springs) and the accumulation of some terrigenic helium from radioactive decay of uranium and thorium in the limestone, dolostone and mudstone rocks. This is indicative of longer MRTs for groundwater flow for one sample in the CLA (RN041173) and both samples from the APV. The springs sampled (Lonely, Old Top, Palm and Winari springs) exhibited little water with dissolved helium above atmospheric equilibrium except a minor amount at Palm and Old Top springs. This suggests discharge occurs at Lonely and Winari springs from localised flow paths in the outcrop for the CLA with short residence times (years), while sources of discharge at Old Top and Palm springs are inferred to occur from slightly longer intermediate-scale flow paths with longer mean reside times for groundwater flow (decades). These results are consistent with chemistry and 3H concentrations found in groundwater (see sections 6.2.5 and 6.2.8). For more information on this figure please contact CSIRO on enquiries@csiro.au 0.0E+002.0E-074.0E-076.0E-078.0E-071.0E-061.2E-061.4E-061.6E-061.8E-062.0E-060.01.02.03.04.05.03He/4HeNe/HeMontejinni LimestoneAPV/MLAntrim Plateau VolcanicsSpringsSolEqTerrigenicHeTritiugenicHe(b) 0102030405060704.0E-084.0E-074.0E-064.0E-05Depth below the watertable (m) He (cc/g) Montejinni LimestoneML/APVAntrim Plateau VolcanicsSprings(a) RN021978RN026127RN020019RN041173RN041173RN026127RN020019Old Top SpringsPalm Spring Figure 6-45 Measured (a) helium and (b) helium-3 to helium-4 ratio versus neon to helium ratio in groundwater and spring samples collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt Plot includes trend lines for helium and neon compositions assuming accumulation of either helium-3 or helium-4. Samples are symbolised by aquifers or springs sampled. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Proterozoic dolostone aquifers Figure 6-466 summarises measured dissolved helium and neon concentrations in groundwater collected from the PDAs (Battle Creek, Bynoe and Skull Creek formations) and aquifers hosted in adjacent hydrogeological units (APV). Dissolved helium concentrations in groundwater varied from about atmospheric equilibrium (4.5E–08 cc/g) up to 2.2E–07 cc/g regardless of aquifer. Samples collected from the PDAs exhibit a very narrow range (1.2E–07 to 2.2E–07 cc/g), regardless of depth below the watertable (Figure 6-466). The accumulation of some dissolved helium in the PDAs above atmospheric equilibrium is consistent with lower 3H in groundwater (see Section 6.2.8), suggesting MRTs for groundwater flow are in the order of decades or longer. The sample collected from the APV exhibited a helium concentration equivalent to atmospheric equilibrium. This is again consistent with the highest 3H concentration in groundwater (see Section 6.2.8) being measured for this bore, which is installed in a sandstone unit of the APV where frequent recharge occurs most wet-seasons (Figure 6-466). Figure 6-466 further highlights that most samples fall along a mixing line between solubility equilibrium (the sample from the APV and some springs) and the accumulation of some terrigenic helium from radioactive decay of uranium and thorium in the dolostone, sandstone and siltstone rocks. This is indicative of MRTs for groundwater flow in the PDAs being decades or longer. However, two springs that were sampled (Crawford and Kidman springs) exhibited little water with dissolved helium above atmospheric equilibrium. This suggests discharge at Crawford and Kidman springs occur from localised flow paths in the outcrop for the PDAs (Skull Creek And Bynoe formations respectively) with short residence times (years), which is consistent with findings from sampling of chemistry and 3H in springs and groundwater (see sections 6.2.5 and 6.2.8). Dissolved helium in Dead and Waterbag springs was found to be slightly above atmospheric equilibrium, while the sample from Bulls Head Spring was slightly higher (Figure 6-466). These results suggest that the sources of discharge at these springs occur from a combination of localised and intermediate scale flow paths in the aquifer outcrop of the PDAs (Skull Creek Formation) at Waterbag Spring and APV at Dead Spring. These results are consistent with chemistry and 3H concentrations found in groundwater (see Section 6.2.5 and 6.2.8). The suggests discharge at Bulls Head Spring is inferred to occur from intermediate-scale flow paths in the aquifer outcrop of the PDAs (Bynoe Formation) with MRTs in the order of decades or longer. This is also consistent with findings from lower 3H concentrations in the spring and groundwater from the Bynoe Formation (see Section 6.2.8). For more information on this figure please contact CSIRO on enquiries@csiro.au 0.0E+002.0E-074.0E-076.0E-078.0E-071.0E-061.2E-061.4E-061.6E-061.8E-062.0E-060.01.02.03.04.05.03He/4HeNe/HeBattle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau VolcanicsSpringsSolEqTerrigenicHeTritiugenicHe01020304050604.0E-084.0E-074.0E-06Depth below the watertable (m) He (cc/g) Battle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau VolcanicsSprings(a) Bulls HeadSpringBulls HeadSpringWaterbagSpringDeadSpringWaterbagSpring Figure 6-466 Measured (a) helium and (b) helium-3 to helium-4 ratio versus neon to helium ratio in groundwater and spring samples collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt Plot includes trend lines for helium and neon compositions assuming accumulation of either helium-3 or helium-4. Samples are symbolised by aquifers or springs sampled. 6.2.13 Groundwater recharge rates and residence times Cambrian Limestone Aquifer Some of the anthropogenic gas tracers, particularly SF6 and H1301 in groundwater are affected by excess air entrapped in the saturated zone, CFC-11 appears to be degraded, and carbonate dissolution is affecting 14C. Therefore, 3H and CFC-12 were deemed the most suitable tracers for estimating recharge and MRTs for flow in shallow unconfined parts of the CLA. Using a combination of Equation 8 and the lumped parameter models (LPMs) described in Section 4.2.11 that assume two different types of advective groundwater flow in unconfined aquifers (piston- flow model (PFM) or exponential model (EMM)), semi-quantitative recharge rates and residence times for groundwater flow have been estimated. 3H concentrations sampled in groundwater were plotted against predicted transient distributions of 3H in precipitation versus depth below the watertable for both the PFM and EMM for a specified range in recharge rates (R) of between 15 to 65 mm and assumed effective porosities (ε) of 5 to 10% based on low storage properties for the karstic aquifer (Amery and Tickell, 2022). This resulted in a range in estimated recharge rates of between 15 to 60 mm/year using 3H sampled in groundwater (Figure 6-47). However, recharge rates of 15 to 20 mm/year appear more plausible as groundwater is close to tritium-free at a depth of about 35 m in the aquifer creating greater uncertainty deeper in the aquifer. These recharge rates are also applicable to the one bore in the APV which is installed in the APV outcrop along the western margin of the CLA where it discharges to Old Top Spring. Recharge rates of about 50 to 60 mm/year would be needed for recharge to deeper parts of the CLA (depth to watertable >30 m). For more information on this figure please contact CSIRO on enquiries@csiro.au 0102030405060700.00.51.01.52.02.5Depth below the watertable (mBGL) 3H (TU) PFM R=15 mm/yr, P=5%PFM R=55 mm/yr, P=10%EMM R=20 mm/yr, P=5% EMM R=65 mm/yr, P=10%Montejinni LimestoneML/APVAntrim Plateau VolcanicsRN041173RN026444RN021978RN020019 Figure 6-47 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt Plots include predicted curves for 3H compositions assuming different groundwater flow conditions. PFM = piston-flow model; EMM = exponential model and associated recharge (R) rates assuming porosity (P) ranges from 5% to 10%. Samples are symbolised by aquifers sampled. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Data source: mean annual tritium in precipitation used for predicted model curves were sourced from Tadros et al. (2014). Using CFC-12 concentrations in groundwater plotted along with the predicted transient distribution of CFC-12 versus depth below the watertable for both the PFM and EMM at two specified recharge rates using Equation 8, and assuming the same range in effective porosities, recharge rates of about 20 mm/year appear plausible for shallow unconfined parts of CLA (Figure 6-48). Estimated recharge rates although derived with only a few samples are also consistent with the range of estimates provided using 3H. These first-order estimates using environmental tracers are also consistent with recharge estimates from Tickell and Rajaratnam (1998) for aquifers across the Victoria River District. In addition, they are consistent with the range in recharge rates estimated using the CMB method (see Section 5.4.3). Furthermore, they are also consistent with a range of recharge estimates derived using multiple independent methods for other parts of the unconfined CLA (Bruwer and Tickell, 2015; Jolly et al., 2004; Taylor et al., 2023). For more information on this figure please contact CSIRO on enquiries@csiro.au 0102030405060700.00.51.01.5Depth below the watertable (m) CFC-12 (pmol/kg) PFM R=15 mm/yr, P=5%PFM R=70 mm/yr, P=10% EMM R=20 mm/yr, P=5%EMM R=85 mm/yr, P=10% Montejinni LimestoneML/APVAntrim Plateau VolcanicsRN026444RN021978RN020019 Figure 6-48 Concentrations of CFC-12 in groundwater collected from aquifers hosted in the Cambrian limestone and adjacent Cambrian basalt versus depth below the watertable Plots include predicted curves for CFC-12 assuming different groundwater flow conditions. PFM = piston flow model; EMM = exponential model. Assumed groundwater flow conditions for predicted curves include: (i) groundwater salinity = 1 g/L, (ii) mean annual recharge temperature = 30 °C. Samples are symbolised by aquifers sampled. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. Data source for atmospheric CFC-12 from Bullister (2015). Based on the: (i) the radioactive decay rate for 3H in groundwater, (ii) presence of CFC-12 in groundwater, (iii) absence of dissolved helium in groundwater above atmospheric equilibrium (see Section 6.2.12), and (iv) moderate to high 14C in groundwater (>75 pmC) though affected by carbonate dissolution, first-order estimates of MRTs for groundwater flow range from several years to about 10 years in shallow parts of the aquifer outcrop for the CLA (~20 m below the watertable). That is the area around and to the south of Top Springs where there is no Cretaceous and Cenozoic cover over the aquifer and flow paths are likely to be short (<15 km) (see Section 6.2.1). Elsewhere, where the CLA is overlain by Cretaceous and Cenozoic cover and groundwater contains some dissolved helium above atmospheric equilibrium, first-order estimates of MRTs range between a few decades to about a hundred years or so where flow paths can be about 25 km or longer. These are areas away from Top Springs around the groundwater flow divide near the catchment boundary, where some intermediate-scale flow occurs towards the discharge zone along the western margin of the CLA and some flow occurs to the east into the Wiso Basin (see Section 6.2.1). Although MRTs in this study have been derived from only a few samples, estimates of MRTs are consistent with estimates derived across the Victoria River District by Tickell and Rajaratnam (1998). They are also consistent with estimates of MRTs elsewhere in the same hydrogeological settings across the CLA by Taylor et al. (2023), as well as recent modelled groundwater flow rates for the CLA by ELA (2022), and Knapton (2024; 2023). Proterozoic dolostone aquifers Using the same approach to estimate recharge rates for the CLA, recharge estimates and MRTs for the PDAs have been derived using both 3H and CFC-12 concentrations in groundwater assuming two different types of advective groundwater flow in the unconfined part of the aquifers. A similar range in effective porosities for the dolostone aquifers of 5% to 10% has been assumed based on interpretation of pumping tests by Pearson (1985) at Timber Creek. Using 3H in groundwater sampled at different depths below the watertable, recharge rates ranging between 20 to 80 mm/year were estimated (Figure 6-49). Though similar to samples from deeper in the aquifer for the CLA, recharge rates of 20 to 25 mm/year appear more plausible as groundwater is almost tritium-free at a depth of 50 m in the aquifer. These first order estimates of recharge rates although derived with only a few samples are consistent with recharge estimates from Tickell and Rajaratnam (1998) and CSIRO (2009) for aquifers across the Victoria River District. They are also consistent with the range of recharge estimates derived using the CMB method (see Section 5.4.3). Using CFC-12 concentrations in groundwater and assuming the same range in effective porosities, recharge rates of about 40 mm/year appear plausible for shallow (depth to watertable ~25 m) unconfined parts of PDAs (Figure 6-50). Higher recharge rates in the order of about 50 to 70 mm/year are estimated for deeper parts (depth to watertable ~50 m) of the PDAs (Figure 6-50). These estimates of recharge rates although derived with only a few samples are consistent with the range of estimates provided using 3H. These first order estimates are also consistent with the range in recharge rates estimated using the CMB method (see Section 5.4.3). Based on the: (i) radioactive decay rate for 3H in groundwater, (ii) presence of CFC-12 in groundwater, (iii) presence of some dissolved helium in groundwater above atmospheric equilibrium (see Section 6.2.12), and (iv) moderate to high 14C in groundwater (>75 pmC) though affected by carbonate dissolution, first-order estimates of mean residence times for groundwater flow range from a few decades to a few hundred years in unconfined parts of the PDAs where depth to watertable exceeds 25 m. These first order estimates of MRTs for these aquifers are consistent with those estimated from Tickell and Rajaratnam (1998) for aquifers across the Victoria River District. However, they should be treated with a degree of uncertainty in the absence of further investigation. There is currently no information for deeper and confined parts of the aquifers where MRTs are likely to exceed hundreds of years. For more information on this figure please contact CSIRO on enquiries@csiro.au 0102030405060700.00.51.01.52.02.5Depth below the watertable (m) 3H (TU) PFM R=20 mm/yr, P=5%PFM R=80 mm/yr, P=10% EMM R=25 mm/yr, P=5%EMM R=110 mm/yr, P=10% Battle Creek FormationBynoe FormationSkull Creek Formation Figure 6-49 Tritium concentrations in groundwater versus depth below the watertable for aquifers hosted in the Proterozoic dolostone and Cambrian basalt Plots include predicted curves for 3H compositions assuming different groundwater flow conditions. PFM = piston flow model; EMM = exponential model and associated recharge (R) rates assuming porosity (P) ranges from 5% to 10%. Samples are symbolised by aquifers and springs sampled. Springs are shown for comparison with groundwater. Data source: mean annual tritium in precipitation used for predicted model curves were sourced from Tadros et al. (2014). For more information on this figure please contact CSIRO on enquiries@csiro.au 0102030405060700.00.51.01.5Depth below the watertable (m) CFC-12 (pmol/kg) PFM R=30 mm/yr, P=5%PFM R=55 mm/yr, P=5% EMM R=40 mm/yr, P=5%EMM R=75 mm/yr, P=5% Battle Creek FormationBynoe FormationSkull Creek FormationAntrim Plateau Volcanics Figure 6-50 Concentrations of CFC-12 in groundwater collected from aquifers hosted in the Proterozoic dolostone and adjacent Cambrian basalt versus depth below the watertable Plots include predicted curves for CFC-12 assuming different groundwater flow conditions. PFM = piston flow model; EMM = exponential model. Assumed groundwater flow conditions for predicted curves include: (i) groundwater salinity = 1 g/L, (ii) mean annual recharge temperature = 30 °C. Data source for atmospheric CFC-12 from Bullister (2015). 6.2.14 Groundwater–surface water interactions Water sampling at key spring complexes from either individual spring vents or lateral seepage zones, conducted in conjunction with the groundwater sampling has provided a good basis for better characterising their water sources. This is the first study in the Victoria catchment to apply environmental tracers to better understand the sources of discharge at the ecologically and culturally important sites. Results of the sampling are described below and more detail on each individual spring is provided in Appendix A.9. Cambrian Limestone Aquifer Figure 6-51 shows the location of springs sampled across the western margin of the CLA (Montejinni Limestone) and eastern part of the APV in the Victoria catchment. Topographic highs along the western edge of the Sturt Plateau where a groundwater divide exists (see Section 6.2.1), results in local to intermediate-scale flow in the CLA discharging along the western margin of the Montejinni Limestone. Discharge occurs via a combination of: (i) intermittent lateral outflow to streams (Armstrong River and Bullock, Cattle and Montejinni creeks) where they are incised into the aquifer outcrop, (ii) perennial localised discharge at discrete springs (Old Top, Lonely, Palm and Horse springs) in creek lines where it is in contact with various different lithologies of the APV (agglomerate, basalt and chert). Surface geology plays an important role in determining both the occurrence of, and water sources for, these karstic contact springs. In general, most springs (Lonely, Old Top, Palm and Winari springs) had a water-type reflective of groundwater across parts of the CLA, and a similar strontium isotopic composition, though the latter is based on fewer samples (see Table 6-3). Hydrogeological cross-sections (see Section 6.1.2), regional groundwater levels (see Section 6.2.1) and chemistry in groundwater and springs (see Section 6.2.5) provide some evidence for both vertical and horizontal interconnectivity between the Montejinni Limestone and the APV. The occurrence of more transmissive parts of the APV, particularly agglomerate and chert, is likely providing greater horizontal interconnectivity between the Montejinni Limestone and APV. It is inferred from these lines of evidence that the water sources for Lonely, Old Top, Palm and Winari springs is the CLA hosted in the Montejinni Limestone and its interconnectivity with the underlying APV and its occurrence adjacent the CLA outcrop along the western margin of the aquifer (Table 6-3). These findings are generally consistent with those from Tickell and Rajaratnam (1998) and those stated in the Springs of the Northern Territory database (Department of Environment Parks and Water Security, 2019b). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-51 Surface geology surrounding springs in the east of the Victoria catchment associated with the Cambrian limestone and Cambrian basalt APV = Antrim Plateau Volcanics. Data source: surface geology sourced from Raymond et al. (2012). Based on 3H in springs (Table 6-3) and groundwater (see Section 6.2.8), spring flows at Lonely and Winari springs appear to depend on localised recharge in the aquifer outcrop with short flow paths (<15 km) and short mean residence times (≤10 years)(see Section 6.2.13). Discharge gradually diminishes around these springs as wet-season groundwater levels in the aquifer outcrop decline. At Old Top and Palm springs, 3H concentrations in springs are lower and there has been minor accumulation of some dissolved helium above atmospheric equilibrium (Table 6-3). This indicates a mixture of local and intermediate-scale groundwater flow may be the source of discharge at these sites. Structural highs associated with the APV as shown in the hydrogeological cross-sections (see Section 6.1.2), may result in the CLA being thin (<20 m) and forcing groundwater upward at Palm Spring. That is, some flow paths in the CLA greater than 15 km, and potentially some mixing with groundwater in the APV that has longer MRTs (form a few decades to a few hundred years)(see Section 6.2.13). Note that while semi-quantitative estimates of MRTs have been derived from a few samples of 3H and dissolved helium, and they correlate well with modelled groundwater flow rates in the CLA by Knapton (2024), further investigations would help validate these results. Table 6-3 Summary of inferred spring water source for key springs sampled in the east of the Victoria catchment associated with the Cambrian limestone and Cambrian basalt SPRING SITE GEOLOGICAL BASIN/PROVINCE SURFACE GEOLOGY KEY HYDROGEOLOGICAL UNITS WATER- TYPE 87Sr/86Sr 3H (TU) He (cc/g) INFERRED SPRING WATER SOURCE Companion Spring Kalkarindji Igneous Province Antrim Plateau Volcanics Antrim Plateau Volcanics Na–HCO3 0.715593 1.1 – Antrim Plateau Volcanics Lonely Spring Wiso Basin Montejinni Limestone Montejinni Limestone Ca–HCO3 0.712343 1.0 4.4E-08 Montejinni Limestone Old Top Spring Wiso Basin Antrim Plateau Volcanics Montejinni Limestone and Antrim Plateau Volcanics Ca–HCO3 0.715202 0.75 6.0E-08 Montejinni Limestone and Antrim Plateau Volcanics Palm Spring Wiso Basin Montejinni Limestone Montejinni Limestone Ca–HCO3 0.714383 0.60 5.8E-08 Montejinni Limestone Winari Spring Kalkarindji Igneous Province Antrim Plateau Volcanics Montejinni Limestone and Antrim Plateau Volcanics Mg–HCO3 0.715202 1.1 4.8E-08 Montejinni Limestone and Antrim Plateau Volcanics Proterozoic dolostone aquifers Figure 6-522 summarises the location of springs sampled across the outcropping and subcropping parts of the PDAs in the centre of the Victoria catchment. As found in the CLA, surface geology appears to have an important influence on both the occurrence of, and water sources for, each spring. In general, most springs (Crawford, Bulls Head, Kidman and Waterbag springs) had a water- type reflective of groundwater across parts of the PDAs, and a similar strontium isotopic composition, though the latter is based on fewer samples (see Table 6-4). In addition, hydrogeological cross-sections shown in Section 6.1.2 indicate topography has an influence on groundwater levels with flow likely to occur towards margins of the outcropping and subcropping areas where springs occur at contact points with lower permeability sandstones and shales (see Section 6.1.2). Dead Spring appears to have a water-type and strontium isotopic composition more reflective of the that of the APV (Table 6-4). It is inferred from these limited lines of evidence that the water sources for Crawford, Bulls Head, Kidman and Waterbag springs is a combination of the PDAs (mostly the Bynoe and Skull Creek formations). Dead Spring is more likely to source its water from the APV. These findings are generally consistent with those from Tickell and Rajaratnam (1998) and in the Springs on the Northern Territory database (Department of Environment Parks and Water Security, 2019b). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-522 Surface geology surrounding springs in the centre of the Victoria catchment associated with the Proterozoic dolostone and adjacent hydrogeological units Data source: surface geology sourced from Raymond et al. (2012). Based on 3H in springs (Table 6-4) and groundwater (see Section 6.2.8) spring flows at Crawford, Dead, Kidman, and Waterbag springs appear to depend on localised recharge in the outcropping and subcropping parts of the PDAs with short flow paths and short mean residence times (≤10 years)(see Section 6.2.13). Bulls Head Spring has a much lower 3H concentration, and accumulation of some dissolved helium above atmospheric equilibrium (Table 6-4), suggesting the source of discharge is more from longer and perhaps deeper potentially confined flow paths in the PDAs with MRTs ranging from a few decades to a few hundred years (see Section 6.2.13). Outcropping and subcropping siltstone of the Bynoe Formation is prominent surrounding Bulls Head Spring providing an indication that the Bynoe Formation at this location is more like a partial aquifer with longer MRTs. As for the investigations of springs for the CLA, semi-quantitative estimates of MRTs have been derived from only a few samples of 3H and dissolved helium, and the PDAs are generally data sparse. Therefore, further investigations would help validate these results. Table 6-4 Summary of inferred spring water source for key springs sampled in the centre of the Victoria catchment associated with the Proterozoic dolostone and adjacent hydrogeological units SPRING SITE GEOLOGICAL BASIN/PROVINCE SURFACE GEOLOGY KEY HYDROGEOLOGICAL UNITS WATER- TYPE 87Sr/86Sr 3H (TU) He (cc/g) INFERRED SPRING WATER SOURCE Crawford Spring Birrindudu Basin Skull Creek Formation Antrim Plateau Volcanics Ca–HCO3 0.735133 1.1 4.4E-08 Skull Creek Formation Bulls Head Spring Birrindudu Basin Quaternary alluvium Bynoe Formation and Skull Creek Formation Ca–HCO3 0.733649 0.32 9.4E-08 Bynoe Formation and Skull Creek Formation Kidman Springs Birrindudu Basin Bynoe Formation Bynoe Formation and Skull Creek Formation Ca–HCO3 0.731822 1.4 4.9E-08 Bynoe Formation and Skull Creek Formation Dead Spring Kalkarindji Igneous Province Antrim Plateau Volcanics Antrim Plateau Volcanics Mg–HCO3 0.713340 1.1 5.9E-08 Antrim Plateau Volcanics Waterbag Spring Birrindudu Basin Battle Creek Formation Antrim Plateau Volcanics and Battle Creek Formation Mg–HCO3 0.716993 1.0 5.8E-08 Battle Creek Formation 6.3 Numerical groundwater flow modelling This section provides a high-level summary of the results of indicative modelling of future climate and future hypothetical groundwater developments in the CLA using the DR2 FEFLOW model. The complete details of the results are documented in the companion technical report on groundwater modelling (Knapton et al., 2024). In this report, the results of the hypothetical groundwater development scenarios are presented as modelled groundwater-level drawdown at receptors such as groundwater-dependent environmental assets or existing groundwater users. No judgement is made about whether these impacts are acceptable. The CLA is mostly a regional-scale groundwater flow system, so across most parts of the aquifer it may take several hundreds of years, for the system to re-establish a quasi-equilibrium state; that is, for the groundwater flow patterns to stabilise following a change in state, such as climate or groundwater development (Knapton et al., 2023). In the Victoria catchment, only a small, thin portion of the western part of the CLA occurs along the eastern edge of the Victoria catchment (12,000 km2) where flow paths are much shorter (<30 km). Consequently, the impacts of hypothetical future groundwater development or future climate tested in this study appear to occur over timescales of a few years to tens of years due to the moderate spatial extent of the aquifer. Different scenarios were modelled to examine how different levels of groundwater development and/or a change in projected climate might affect water resources in the CLA over a specific period in the future (2055 to 2065), using historical and future climate data. The approximately 40-year time period (2060) was considered a pragmatic time period over which to consider the impacts of changes in climate and groundwater extraction because: (i) it is equivalent to more than twice the length the investment period of a typical agricultural enterprise; (ii) it is about the service life of an appropriately constructed groundwater production borefield; (iii) it is about five times the length of the current period over which NT water licences are assigned; and (iv) it is consistent with the time period over which future climate projections have been evaluated. 6.3.1 Modelled annual recharge Figure 6-53 summarises the modelled annual recharge estimated: (i) across the spatial extent of the CLA in the Victoria catchment occurring within the model domain (approximately 70% of the aquifer within the catchment), and (ii) the spatial extent of the CLA across the entire model domain. Modelled annual recharge for the spatial extent of the CLA in the Victoria catchment occurring within the model domain ranges from 0.2 to 670 GL/year with a mean and median of 93 and 29 GL/year, respectively (Figure 6-53a). Modelled annual recharge for the entire spatial extent of the CLA within the model domain ranges from 2 to 23,743 GL/year with a mean and median of 995 and 469 GL/year, respectively (Figure 6-53b). For more information on this figure please contact CSIRO on enquiries@csiro.au 0110100100010000100000191019201930194019501960197019801990200020102020Modelled annual recharge (GL) Date110100100010000100000191019201930194019501960197019801990200020102020Modelled annual recharge (GL) DateModelled mean annual recharge10-year moving average Figure 6-53 Modelled annual recharge (a) for the spatial extent of the CLA in the Victoria catchment and (b) for the entire spatial extent of the CLA within the DR2 model domain The dashed black line is the 10-year moving mean. Modelled estimates are presented on a logarithmic scale. 6.3.2 Changes in water balance under future climate and hypothetical groundwater development Across the model domain within the Victoria catchment under Scenario A (historical climate and current groundwater extraction) at 2060, modelled recharge of 36.3 GL/year is balanced by 11.1 GL/year discharging to springs, 15.6 GL/year adding to groundwater storage, and 9.7 GL/year flowing further east outside of the Victoria catchment into the Wiso Basin. The modelled extraction under Scenario B sees a reduction in all forms of outflow with the change in storage having the greatest reduction in percentage terms. For the modelled future climate scenarios, the modelled recharge relative to Scenario A is lower for scenarios Cdry and Cmid and higher for Scenario Cwet (Cdry = -34%, Cmid = -15%, and Cwet = +42% relative to Scenario A). These changes in modelled recharge are reflected in the outflows with discharge to the springs, changes in storage and flow to the east decreasing under Cdry and Cmid and increasing under Cwet. The D scenarios reflect the additive responses of the scenarios B and C. Even under increased modelled extraction, the increased recharge under the three Dwet scenarios results in an increase in spring discharge and flow to the east. The 3 Ddry and Dmid scenarios result in less discharge of all forms compared to Scenario A except for the additional extraction. Modelled groundwater inflows to the Wiso Water Management Zone under scenario A is about 9.7 GL/year. Under all scenarios with either current or hypothetical future groundwater development (i.e., Scenarios C and D), the regime does not change, although the inflows are reduced under Cdry (8.6 GL/year) by about 11% and Cmid (9.3 GL/year) by about 4% and increased under Cwet (10.8 GL/year) by about 11% relative to Scenario A. Note that there are no flows to/from rivers or to from areal evapotranspiration in the Wiso Water Management Zone region because the depth of the water is greater than the depth at which these processes operate. In the Flora Tindal Water Management Zone, modelled recharge relative to Scenario A (134.6 GL/year) decreases in the scenarios Cdry and Cmid and increases in the Scenario Cwet. Specifically, recharge decreases to 89.4 GL/year (-34%) in Cdry and to 115.7 GL/year (-14%) in Cmid, while it increases to 182.3 GL/year (+36%) in Cwet, compared to Scenario A. The outflow components of the water balance components do not change under Scenario B as none of the hypothetical groundwater developments are within the Flora Tindall Water Management Zone. Discharge to the Flora River is lowest in the Scenario Cdry (142.1 GL/year) and highest in the scenarios Cwet and Dwet (195.1 GL/year). This represents a decrease to 142.2 GL/year (-9%) in Cdry and to 154.4 GL/year (-1%) in Cmid, and an increase to 195.1 GL/year (+25%) in Cwet, relative to Scenario A. Because none of the hypothetical future development sites are in the Flora Tindal Water Management Zone, scenarios B, D9, D12, and D15 have the same extraction as scenarios A and C (i.e., 0 GL/year). The changes in discharge to the Flora River are due to changes in climate, not changes in extraction within the Victoria catchment. 6.3.3 Cumulative spatial drawdown under future climate and hypothetical groundwater development The drawdown contours at 2060 under scenarios B12, Ddry12, Dmid12 and Dwet12 relative to Scenario A are presented in Figure 6-54a, Figure 6-54b, Figure 6-54c and Figure 6-54d respectively. The drawdown contours for Scenario B12 show that the maximum modelled drawdown (<20 m) is centred on each of the three hypothetical extraction sites and the 1m drawdown contour extends 15 to 20 km to the south and east of these sites. The pattern of modelled drawdown is similar for Dmid12 with greater drawdown shown under the drier future climate (Ddry12). Under the wetter future climate (Dwet12) there is less modelled drawdown than under the historical climate around the hypothetical extraction sites with modelled increases to groundwater levels occurring to the north and west of these sites. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 6-54 Mean modelled drawdown relative to Scenario A for scenarios (a) B12, (b) Ddry12, (c) Dmid12 and (d) Dwet12 representative of climate conditions at the year 2060 Dashed lines indicate increased groundwater level relative to Scenario A. 6.3.4 Cumulative drawdown at specified reporting locations Table 6-5 presents the mean modelled groundwater-level at the ten reporting sites for the ten- year period (2055 to 2065) under all scenarios. Each table row corresponds to a specific scenario, and the columns represent the mean groundwater-level for each reporting site. Under Scenario B, all reporting sites show a reduction in modelled groundwater level except for RN005578, which is outside the influence of the increased extraction after 40 years. The reduction in groundwater level is proportional to the extraction with Scenario B15 having the greatest reduction in groundwater level and Scenario B9 having the least. Compared to Scenario A, mean modelled groundwater levels are lower at all reporting sites under Scenario Cdry, generally comparable to or slightly lower under Scenario Cmid, and generally higher under Scenario Cwet. Two sites show no change under Cdry (RN026490 and RN035496), whereas under Cmid three sites show no change (RN026490, RN035496, and RN026441). The mean modelled groundwater-levels for Scenario Ddry9 are 0.6 to 7.9 m lower than under Scenario A, and for Scenario Ddry12 are 0.8 to 8.7 m lower. The mean modelled groundwater- levels for Scenario Ddry15 are 0.8 to 9.4 m lower than under Scenario A, and for Scenario Dmid9 are 0.4 to 3.5 m lower. The mean modelled groundwater-levels for Scenario Dmid12 are 0.4 to 4.2 m lower than under Scenario A, and for Scenario Dmid15 are 0.5 to 4.9 m lower. The mean modelled groundwater-levels for Scenario Dwet9 are between 2.0 m lower and 5.5 m higher than under Scenario A, and for Scenario Dwet12 are 3.5 m lower and 4.8 m higher. The mean groundwater-levels for Scenario Dwet15 are 4.9 m lower and 4.1 m higher. Reporting sites RN026109, RN026490 and RN026552 are the bore locations most sensitive to changed conditions and show the largest modelled decreases especially under increased hypothetical extraction and dry conditions. Table 6-5 Mean modelled groundwater levels (mAHD) at six locations within the Cambrian Limestone Aquifer (CLA) under scenarios A and B. Locations shown in Figure 4-13. Values in parentheses are the differences in modelled groundwater level in metres relative to scenario A. See Knapton et al. (2024) for more information. RN000594 (~13 KM EAST OF LONELY SPRING) RN005578 (~45 KM NORTH-EAST OF OLD TOP SPRING) RN020020 (~15 KM EAST OF OLD TOP SPRING) RN026109 (~20 KM SOUTH-EAST OF PALM SPRING) RN026490 (~56 KM SOUTH OF PALM SPRING) RN035496 (~58 KM SOUTH-EAST OF PALM SPRING) RN026441 (~26 KM SOUTH OF PALM SPRING) RN026552 (~15 KM SOUTH-EAST OF LONELY SPRING) RN037936 (~26 KM EAST OF OLD TOP SPRING) RN042219 (~27 KM SOUTH-EAST OF OLD TOP SPRING) Scenario (mAHD) A 190 (–) 169.7 (–) 160.5 (–) 166.4 (–) 150.1 (–) 151.4 (–) 158.5 (–) 181.8 (–) 159.6 (–) 158.6 (–) B9 187.8 (-2.2) 169.7 (–) 160.3 (-0.2) 162.2 (-4.3) 148.9 (-1.3) 150.6 (-0.8) 158 (-0.5) 179.9 (-1.9) 159.3 (-0.2) 157.7 (-0.9) B12 187.1 (-2.9) 169.7 (–) 160.2 (-0.3) 160.8 (-5.7) 148.4 (-1.7) 150.3 (-1.1) 157.9 (-0.7) 179.3 (-2.5) 159.3 (-0.3) 157.4 (-1.2) B15 186.4 (-3.6) 169.7 (–) 160.1 (-0.3) 159.4 (-7.1) 148 (-2.1) 150 (1.4) 157.7 (-0.8) 178.6 (-3.2) 159.2 (-0.4) 157.1 (-1.5) Cdry 184.3 (-5.7) 167.4 (-2.3) 160.0 (-0.5) 164.8 (-1.6) 150.1 (0) 151.4 (0) 158.4 (-0.1) 177.8 (-4) 158.3 (-1.3) 157.9 (-0.7) Cmid 188.7 (-1.3) 168.7 (-1.0) 160.4 (-0.1) 166.0 (-0.4) 150.1 (0) 151.4 (0) 158.5 (0) 180.9 (-0.9) 159.2 (-0.4) 158.4 (-0.2) Cwet 197.6 (7.6) 171.9 (2.2) 161.1 (0.6) 168.6 (2.2) 150.1 (0) 151.4 (0) 158.7 (0.2) 187.1 (5.3) 161.3 (1.7) 159.6 (1) Ddry9 182.1 (-7.9) 167.4 (-2.3) 159.8 (-0.7) 160.7 (-5.7) 148.9 (-1.2) 150.6 (-0.8) 157.9 (-0.6) 175.9 (-5.9) 158.1 (-1.5) 157.0 (-1.6) Dmid9 186.5 (-3.5) 168.7 (-1.0) 160.1 (-0.4) 161.8 (-4.6) 148.9 (-1.2) 150.6 (-0.8) 158.0 (-0.5) 179.0 (-2.8) 159.0 (-0.6) 157.5 (-1.1) Dwet9 195.5 (5.5) 171.9 (2.2) 160.9 (0.4) 164.4 (-2.0) 148.9 (-1.12) 150.6 (-0.8) 158.2 (-0.3) 185.2 (3.4) 161.1 (1.5) 158.7 (0.1) Ddry12 181.3 (-8.7) 167.4 (-2.0) 159.7 (-0.8) 159.3 (-7.1) 148.4 (-1.7) 150.3 (-1.1) 157.7 (-0.8) 175.3 (-6.5) 158.0 (-1.6) 156.7 (-1.9) Dmid12 185.8 (-4.2) 168.7 (-1.0) 160.1 (-0.4) 160.4 (-6) 148.4 (-1.7) 150.3 (-1.1) 157.8 (-0.7) 178.3 (-3.5) 159.0 (-0.6) 157.2 (-1.4) Dwet12 194.8 (4.8) 171.9 (2.2) 160.8 (0.3) 162.9 (-3.5) 148.4 (-1.7) 150.3 (-1.1) 158.0 (-0.5) 184.6 (2.8) 161.0 (1.4) 158.4 (-0.2) Ddry15 180.6 (-9.4) 167.4 (-2.3) 159.7 (-0.8) 157.9 (-8.5) 148.0 (-2.1) 150 (-1.4) 157.6 (-0.9) 174.6 (-7.2) 157.9 (-1.7) 156.4 (-2.2) Dmid15 185.1 (-4.9) 168.7 (-1.0) 160 (-0.5) 159.1 (-7.3) 148.0 (-2.1) 150 (-1.4) 157.7 (-0.8) 177.7 (-4.1) 158.9 (-0.7) 156.9 (-1.7) Dwet15 194.1 (4.1) 171.9 (2.2) 160.8 (0.3) 161.5 (-4.9) 148.0 (-2.1) 150 (-1.4) 157.9 (-0.6) 184.0 (2.2) 160.9 (1.3) 158.1 (-0.5) 6.3.5 Changes in groundwater discharge under future climate and hypothetical groundwater development The results of the modelled groundwater discharge are summarised in Table 6-6, which shows the mean groundwater discharge (in GL/year) for the ten-year period 2055 to 2065. Each row corresponds to a specific scenario, and the columns represent the mean groundwater discharge and the percentage change from Scenario A. Under Scenario B9, the modelled mean groundwater discharge in the form of combined evapotranspiration and localised spring discharge from the CLA is 9.9 GL/year, a reduction in modelled discharge of 11% compared to under Scenario A (11.1 GL/year). Under Scenario B15, the modelled mean groundwater discharge from the CLA via evapotranspiration and springs is 9.1 GL/year. This is 2 GL/year less (18% reduction) than the mean modelled groundwater discharge under Scenario A (Table 6-6). The reductions in mean modelled groundwater discharge under groundwater extraction scenarios B9, B12 and B15 are due to the small spatial extent of the CLA in the Victoria catchment (12,000 km2), and the short distance (about 15 km) between the closest hypothetical groundwater extraction site and the spring complexes around Top Springs (Figure 4-13). The three hypothetical extraction sites occur between about 15 and 80 km from the discharge areas of the aquifer. These results highlight that changes in an aquifer’s water balance depends on a range of factors, including the hydrogeological conceptual model, the location, magnitude and duration of extraction, and the hydrogeological properties of the aquifer (saturated aquifer thickness, aquifer hydraulic properties,) on spatial and temporal changes in groundwater flow in an aquifer. The results for the mean modelled groundwater discharge via evapotranspiration and springs from the CLA at spring complexes along the western margin of the CLA in the Victoria catchment under a projected future climate (Scenarios C and D), illustrate the impacts that climate variability may have on the aquifer’s water balance. Under Scenario Cdry (projected future dry climate with current groundwater development), the reduction in modelled groundwater recharge to the aquifer is projected to have a larger impact on groundwater discharge via ET and localised spring discharge than the current or hypothetical groundwater extraction rates tested in this study (scenarios A and B). This is because the CLA outcrops nearby Top Springs, which receives localised recharge, and has relatively short groundwater flow paths to the spring complexes, so inter-annual variations in climate are evident in inter-annual variations in discharge. Table 6-6 Mean annual modelled groundwater discharge at springs and via evapotranspiration for the 2060 representative conditions (10-year period from 2055 to 2065) For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au +na = not applicable A = Historical climate and current development; A = Historical climate and current development; B9 = Historical climate sequences with current development and additional future development of 9GL/year; B12 = Historical climate sequences with current development and additional future development of 12GL/year; B15 = Historical climate sequences with current development and additional future development of 15GL/year; ; Cdry = Future dry climate sequences current development; Cmid = Future mid climate sequences current development; Cwet = Future wet climate sequences current development; Ddry9 = Future dry climate sequences current development and future development of 9GL/year; Ddry12 = Future dry climate sequences current development and future development of 12GL/year; Ddry15 = Future dry climate sequences current development and future development of 15GL/year; Dmid9 = Future mid climate sequences current development and future development of 9GL/year; Dmid12 = Future mid climate sequences current development and future development of 12GL/year; Dmid15 = Future mid climate sequences current development and future development of 15GL/year); Dwet9 = Future wet climate sequences current development and future development of 9GL/year; Dwet12 = Future wet climate sequences current development and future development of 12GL/year; Dwet15 = Future wet climate sequences current development and future development of 15GL/year. 7 Discussion 7.1 Summary This groundwater study involved several key components: (i) reviewing literature and data from all previous hydrogeological investigations in the catchment, (ii) regional-scale desktop data collating and analysing, including reviewing digital geology datasets to map aquifers, digitising data contained in hand-written and typed drilling records to attribute aquifers to groundwater bore data, and evaluating spatial trends in groundwater levels, groundwater salinity, and bore yields, (iii) regional-scale recharge modelling to assess all aquifers in the catchment, (iv) identifying and mapping potential groundwater discharge areas using remote sensing, and (v) investigating, via targeted field, desktop, and modelling of the Cambrian Limestone Aquifer (CLA) in the east of the catchment and the Proterozoic dolostone aquifers (PDAs) in the centre of the catchment. The literature review provided good insight into all aquifers in the catchment and their current knowledge gaps. Prior to the 1990s, water resources of the Victoria catchment were poorly understood in comparison to the regional geology. Throughout the 1990s, hydrogeological investigations were prominent at various scales, securing both community and domestic water supplies (Britten, 1986; Karp, 1987; Pearson, 1985). In the mid to late 1990s, numerous water resource surveys were conducted and used for the first time to describe and map water resources at a much larger scale (1:250,000 scale map sheets) across parts of the catchment (Karp, 1995; Matthews, 1997; Tickell and Rajaratnam, 1998). These surveys highlighted that the fractured and karstic CLA and PDAs hosted the most promising groundwater systems across the catchment, rather than those hosted in the highly heterogenous but spatially extensive Cambrian basalt or Proterozoic shale and sandstone. More recently, water resource studies as part of CSIRO’s Northern Australia Sustainable Yields project, the Strategic Regional Environmental and Baseline Assessment (SREBA) project, and development of the Georgina Wiso Water Allocation Plan have further highlighted the prospectivity of the CLA (CSIRO, 2009; Department of Environment Parks and Water Security, 2022; ELA, 2022). The regional-scale desktop assessment proved useful as a screening tool for: (i) identifying and mapping the spatial extent of hydrogeological units across the catchment, (ii) reviewing spatial trends in important groundwater attributes (groundwater levels, groundwater salinity, and bore yields), including the type of groundwater systems they host (local, intermediate or regional- scale), their productivity and the suitability of their water for different purposes, and (iii) collating the chloride concentrations in groundwater for input into the regional-scale recharge modelling using the chloride mass balance (CMB) method. It also helped identify the most promising aquifers for targeted investigations (CLA and PDAs) and provided important baseline datasets for use in detailed desktop analyses and modelling. These included aquifer-specific bores for use in: (i) interpreting airborne geophysical data, (ii) developing hydrogeological cross-sections, (iii) interpolating groundwater-level surfaces, (iv) strategically selecting appropriate groundwater bores and springs for general chemistry and environmental tracer sampling, and (v) identifying bores for reporting modelled changes in groundwater levels. 7.1.1 Literature review and regional scale data assessment The literature review and regional-scale desktop assessment clearly identified the CLA and PDAs as the most promising aquifers with potential for future groundwater resource development. These aquifers were therefore the focus of more detailed and targeted desktop, field and modelling investigations. The outcomes also provided useful insights into other aquifers offering potential for future development but requiring further investigation to confirm the nature and scale of opportunities and risks. This includes two key hydrogeological units hosting aquifers in the PDAs that were not subject to targeted investigations as they mostly outcrop and subcrop outside of the southern part of the Victoria catchment: Campbell Springs and Pear Tree dolostones. Only sparse hydrogeological information (lithology, water quality and bore yield) exists for these units, but despite this, the limited information for them highlights their potential suitability for future development. Additional drilling and pumping test investigations would be required to confirm those aquifers’ spatial extent, saturated thickness, water quality and potential to yield sufficient water to support irrigation without depleting the aquifer or affecting the reliability of access to water for existing users and groundwater-dependent-ecosystems (such as springs and streams). Other aquifers in the catchment – including those hosted in the Proterozoic sandstone and shale, and Cambrian basalt host localised groundwater systems of variable water quality and bore yields and so only offer potential as localised or conjunctive water resources. Aquifers hosted in the Devonian to Carboniferous sandstone are situated close to the coast and are likely to be vulnerable to saltwater intrusion. The alluvium of the Angalarri and West Baines rivers are the most extensive in the catchment and may offer some opportunities for future development but are also data sparse and require further investigation. 7.1.2 Regional scale recharge modelling Regional-scale recharge modelling produced in this assessment was useful for providing groundwater-recharge estimates that could be used with: (i) to compare with previous recharge estimates across the CLA derived using CMB or other techniques, as well as initial estimates of recharge for the PDAs, (ii) to deriving arithmetic estimates of recharge fluxes to outcropping and subcropping areas of the CLA and PDAs, and (iii) as input into numerical modelling. The recharge estimation technique highlighted two key findings: (i) the upscaled CMB is a useful technique that estimates net recharge at a catchment or regional scale (noting that it cannot estimate recharge from overbank flooding, or to deep semi-confined and confined parts of aquifers), and (ii) while the upscaled CMB is useful, it remains challenging to better account for spatial changes in both the physical properties of the underlying geology and runoff in areas of high rainfall and steep terrain which constrain recharge. Furthermore, the recharge estimation technique does not account for aquifer storage, so it is not always clear whether the aquifers can accept these rates of recharge. The methods also do not account for preferential recharge from flooding or through localised features in aquifer outcrops such as sinkholes. Therefore, the key features of an aquifer must be carefully conceptualised before deriving recharge fluxes based on the surface area of an aquifer outcrop and/or subcrop and an estimated recharge rate. Nevertheless, the recharge estimates by CMB are a good starting point for deriving a water balance for target aquifers where there is appropriate level of knowledge, either arithmetically or through use in a groundwater model. 7.1.3 Regional-scale remote sensing analyses Regional-scale remote sensing analyses used to identify and map different classes of potential groundwater discharge areas proved useful for: (i) assisting with confirming the hydrogeological conceptual model for groundwater discharge across the catchment, and (ii) assisting with the co- design of field water sampling program for general chemistry and environmental tracers. Coastal discharge was identified as the largest mapped area of potential groundwater discharge (~13,000 ha), followed by seasonally varying (~3,900 ha) and perennial groundwater discharge (~1,500 ha). Coastal discharge occurs along the estuarine part of the Victoria River and coastline of Joseph Bonaparte Gulf. Coastal discharge from swamps and mangroves includes groundwater discharge and evapotranspiration of surface water and sea water in the vicinity of the freshwater–saltwater interface. Seasonally varying discharge was mostly associated with minor alluvial aquifers of the Victoria River and its major tributaries. These are conceptualised as being recharged by surface water (bank recharge) during the wet-season and then discharging back to streams via bank discharge and also evapotranspiration from riparian vegetation. As the hydrogeology of the catchment is dominated by local-scale groundwater systems hosted in Cambrian basalt and Proterozoic shale and sandstone, there may also be some localised discharge from these systems to the alluvium and streams. However, because these sources are localised, most streams in the Victoria catchment dry up to disconnected pools by the end of the dry-season. Perennial groundwater discharge occurs predominantly via springs and seeps related to geological contacts between aquifers hosted in different hydrogeological units. This is where the fractured and karstic CLA and PDAs, or localised fractured Cambrian basalt aquifers make contact with shales and sandstones. 7.1.4 Targeted desktop, field and modelling investigations The targeted investigations of the CLA and PDAs have provided new information for karstic aquifers and further validated and refined previous hydrogeological conceptualisations of both systems. For the CLA, this includes but is not limited to the following historical investigations: (i) groundwater in the Wiso Basin by Randal (1973), (ii) the water resource survey of the Victoria River District by Tickell and Rajaratnam (1998), (iii) CSIRO’s Northern Australia Sustainable Yields project (CSIRO, 2009), (iv) components of the SREBA water level, quality and quantity studies by Amery and Tickell (2022) and ELA (2022), and (v) the most recent study for the Wiso Basin water management area by the Department of Environment Parks and Water Security (2022). The PDAs have been the subject of fewer investigations including: (i) those characterising the aquifers at specified locations for community water supplies (Britten, 1986; Moser, 1993; Pearson, 1985), (ii) the water resource survey of the Victoria River District by Tickell and Rajaratnam (1998), and (iii) CSIRO’s Northern Australia Sustainable Yields project (CSIRO, 2009). Investigations of the CLA were particularly useful for characterising: • the spatial variability in groundwater-recharge rates and fluxes, using both the chloride mass balance (CMB) method and environmental tracers • the spatial variability in the direction, scale and mean residence times (MRTs) for groundwater flow processes across different parts of the aquifer • the spatial changes in both depth to the top of the CLA and depth to groundwater • to a better degree the spatial occurrence, geological controls and sources of groundwater– surface water connectivity along the western margin of the CLA around Top Springs • the nature and scale of potential opportunities for future groundwater resource development as well as the key constraints, including the potential influences of climate variability and hypothetical groundwater extraction on the CLA’s water balance. Investigations of the PDAs were particularly useful for characterising: • the spatial variability in groundwater recharge rates and fluxes, using both the CMB method and environmental tracers • some of the key controls on groundwater flow including deriving initial estimates of MRTs for groundwater flow processes across different parts of the aquifers • identifying the spatial occurrence, geological controls and sources of groundwater–surface water connectivity along the outcropping and subcropping parts of the PDAs in the centre of the catchment • the nature and scale of potential opportunities for future groundwater resource development. 7.2 Cambrian Limestone Aquifer 7.2.1 Regional hydrogeological context The CLA occurs in in the east of the Victoria catchment and is hosted almost exclusively in the Montejinni Limestone, though it occurs in minor parts of the overlying Cretaceous and Cenozoic strata, and parts of the underlying Antrim Plateau Volcanics (APV). It occurs across about 12,000 km2 (about 15% of the total catchment area). The CLA is composed of weathered and karstic limestone and dolostone and some mudstone, which is reflected in the Ca–HCO3 to Mg–HCO3 chemical composition of the groundwater (see Section 6.2.5). Groundwater is generally fresh (<1000 mg/L TDS), though because of the high HCO3 composition of groundwater the water can be hard causing scale build-up on water infrastructure. The aquifer outcrops around and to the south of Top Springs but is also unconfined beneath Cretaceous and Cenozoic strata elsewhere. The thickness of the CLA varies spatially beneath the catchment, influenced by historical weathering of the limestone and dolostone in places and by changes in the topography of the underlying APV (see Section 6.1.2). It is generally about 50 to 120 m thick beneath the Victoria catchment. Groundwater flow in the CLA is complex due to the karstic nature of the aquifer exhibiting a high degree of heterogeneity in physical properties. Its complexity arises from the variability and interconnectivity between fractures, fissures and karsts, which influences groundwater flow processes across the aquifer extent. The saturated thickness of the aquifer also varies spatially and is an important characteristic along with aquifer hydraulic properties, in determining groundwater storage and flow. Across some parts of the CLA in the catchment, the saturated thickness can be thin (i.e. <20 m), or unsaturated, as demonstrated by historical drilling having mixed success (i.e. finding dry holes or bores with little water). Across other parts of the CLA, the saturated thickness is variable, ranging from about 10 and 100 m (see Section 6.1.2). The CLA beneath the Victoria catchment is generally flat. Depth to the top of the CLA in the subsurface is generally shallow (<50 mBGL). To the north-east of Top Springs, depth to the top of the CLA increases to about 120 mBGL where overlying Cretaceous rocks are more extensive (see Section 6.1.3). Changes in depth to groundwater across the CLA exhibit similar spatial patterns as the depth to the top of the aquifer. For example, groundwater is shallow (<10 mBGL) along the western margin of the aquifer around and to the south of Top Springs where groundwater discharge occurs. For this reason, GDEs associated with the CLA in the Victoria catchment are largely limited to the western margin of the aquifer around Top Springs. Depth to groundwater then increases gradually to depths ranging from 40 to 50 mBGL in a somewhat radial pattern north-east, east and south-east from the western aquifer boundary toward the eastern margin of the Victoria catchment (see Section 6.2.2). 7.2.2 Conceptual model Figure 7-1 is a three-dimensional representation of the hydrogeological conceptual model for the CLA within the Victoria catchment. It highlights the key geological controls on groundwater flow processes, the sources and directions of groundwater flow, and the spatial occurrence of key discharge processes in the groundwater discharge zone around Top Springs, which were further characterised from targeted field investigations in this study. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 7-1 Simplified conceptual block model of part of the Cambrian Limestone Aquifer near Top Springs along the eastern margin of the Victoria catchment Block model not drawn to scale. Large blue arrows represent groundwater flow directions. Larger blue sections associated with streams represent perennial reaches where groundwater discharge supports surface water flow. Texture in the hydrogeological units represent fractured and/or karstic rocks. Recharge Recharge to the CLA occurs either directly in the aquifer outcrop or where it is unconfined beneath overlying Cretaceous and Cenozoic strata (Figure 7-1). Recharge processes include a combination of: (i) localised preferential infiltration of rainfall and streamflow via sinkholes directly in the aquifer outcrop, and (ii) broad diffuse infiltration of rainfall through overlying Cretaceous and Cenozoic strata which vertically leaks to the underlying CLA in these areas. However, recharge processes and rates, are spatially and temporally variable depending on the spatial occurrence, thickness and lithology of the overlying Cretaceous and Cenozoic strata. Where the aquifer outcrops or where the overlying Cretaceous and Cenozoic strata is less than 30 m thick and the lithology is composed of some sandstone (see Section 6.1.2), groundwater is generally fresh (<500 mg/L TDS) (see Section 5.2 and 6.2.5), has a depleted isotopic composition with little sign of evaporation prior to recharge (see Section 6.2.6), and has reasonable concentrations of 3H and CFC-12 (see sections 6.2.8 and 6.2.10). Hydrological monitoring while only conducted at one location in the outcrop, exhibited reasonably quick (days to weeks) rainfall- recharge responses in the hydrograph over the 2023/24 wet-season (see Section 6.2.3). Where the overlying cover is thicker (>30 m) and the lithology is mostly composed of claystone and siltstone, groundwater is generally of a slightly higher salinity (500 to 1000 mg/L TDS) and has a lower concentration of 3H in groundwater (though few samples have been collected for 3H in these areas). These findings are consistent with recharge processes characterised across other parts of the unconfined CLA by Bruwer and Tickell (2015), in the SREBA project by ELA (2022) and most recently by Taylor et al. (2023). Mean annual recharge rates estimated by both upscaled CMB (see Section 5.4.3) and 3H and CFC- 12 concentrations in groundwater (see Section 6.2.13) for the CLA range between 3 and 20 mm/year. This range is consistent with previous estimates derived using multiple methods for other parts of the unconfined CLA (Bruwer and Tickell, 2015; Jolly et al., 2004; Taylor et al., 2023). In addition, numerical modelling of the annual recharge to the CLA within the DR2 FEFLOW model domain further highlights the temporal variability in recharge across the aquifer. The modelled annual recharge flux for the spatial extent of the CLA in the Victoria catchment occurring within the DR2 FEFLOW model domain (approximately 70% of the aquifer within the catchment) ranges from 0.2 to 670 GL/year with a mean and median of 93 and 29 GL/year, respectively. The modelled annual recharge flux for the entire spatial extent of the CLA within the DR2 FEFLOW model domain ranges from 2 to 23,743 GL/year with a mean and median of 995 and 469 GL/year, respectively (see Section 6.3.1). As shown in recent modelling by Knapton et al. (2023), the modelling conducted in this study has further highlighted the sensitivity of the CLA’s water balance to climate variability (for more information, see Knapton et al. (2024). Groundwater flow and residence times Local to intermediate-scale flow (i.e. flow paths of up to about 25 km) in the CLA beneath the Victoria catchment is generally from east to west following a subdued form of the topographic gradient, with some flow heading east outside of the catchment (see Section 6.2.1 and Figure 7-1). A combination of a slightly higher elevation along the western edge of the Sturt Plateau and a shift from areas with and without Cretaceous and Cenozoic cover of the Carpentaria Basin inside the catchment influences the position of the groundwater flow divide. Westerly flowing groundwater discharges via seeps and springs at lower elevations along the western margin of the CLA around Top Springs, while easterly flowing groundwater adjoins the Flora flow path, eventually discharging well north of the catchment at the Flora River. Mean residence times (MRTs) for localised groundwater flow range from several years to a few decades for short flow paths (<15 km) discharging at springs and range from a few decades up to about a hundred years for longer and deeper flow paths near the groundwater flow divide (see Section 6.2.13). This conceptualisation is consistent with historical investigations by Randal (1973) and those for the SREBA project (Amery and Tickell, 2022; ELA, 2022) and for the Wiso Basin water management zone (Department of Environment Parks and Water Security, 2022). Discharge Discharge from the CLA occurs via a combination of: (i) intermittent lateral outflow to streams (Armstrong River and Bullock, Cattle and Montejinni creeks) where they are incised into the aquifer outcrop, (ii) perennial localised spring discharge (Old Top, Lonely, Palm and Horse springs), (iii) evapotranspiration via riparian and spring-fed vegetation (pandanus are prominent around springs) (see Figure 7-2), (iv) vertical leakage to the underlying APV, and (v) groundwater extraction for stock and domestic use, including community water supply at Top Springs. Discharge at streams is intermittent and relies on localised recharge in the aquifer outcrop and short flow paths (<15 km), but streams dry up and cease to flow as wet-season groundwater levels decline below stream level. Perennial localised discharge occurs mostly at contact springs along the western margin of the CLA where the karstic aquifer contacts the highly heterogenous APV (see Section 6.2.14). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 7-2 Pandanus lining the edge of Old Top Spring Most spring flow is reliant on localised recharge in the aquifer outcrop that discharges from short flow paths with short MRTs (years to a decade) as shown by high concentrations of 3H in springs and nearby groundwater (see Section 6.2.8). Springs such as Old Top Spring and Palm Spring receive discharge from longer path paths (>15 km) with longer MRTs as shown by lower concentrations of 3H in springs and groundwater (see Section 6.2.8). Old Top Spring for example, is situated along the western margin of the CLA where lateral outflow adjoins agglomerate and chert units of the APV which create greater horizontal interconnectivity between the Montejinni Limestone and APV (see Section 6.2.14). Palm Spring, occurrence may be associated with the combination of a structural high in the underlying APV and a topographic low resulting in the CLA being thin (<20 m) at this location and groundwater being forced upward discharging at the spring. Changes in the topography of the underlying APV as shown in the hydrogeological cross-section (see Section 6.1.2) has a large influence on the saturated thickness of the CLA. 7.2.3 Scenario-based numerical groundwater flow modelling of hypothetical future changes in climate and groundwater extraction Scenario-based numerical groundwater flow modelling of the CLA within the Victoria catchment was conducted to evaluate: (i) the current water balance, and (ii) the potential impacts of changes in rainfall and potential evaporation and/or potential increased groundwater resource development on groundwater levels and spring discharge in the vicinity of the western margin of the CLA around the Top Springs region. Mean annual recharge modelling using the historical climate record for the DR2 FEFLOW model domain (much larger than the Victoria catchment) indicates that recent climate conditions have led to higher recharge than the long-term mean. Using the historical climate to simulate the future climate to 2060 suggests that a hypothetical increase in groundwater extraction of between 9 and 15 GL/year will result in a modelled reduction in spring discharge along the western aquifer margin of between 11% and 18% compared to the current level of groundwater extraction (stock and domestic use mostly, other than the water supply at Top Springs). Cumulative modelled drawdown in groundwater levels ranges from about 1 m at a distance of 20 km to the south and east of the hypothetical extraction sites near the springs to about 25 m at the hypothetical extraction sites. The modelled reductions in groundwater levels are proportional to the hypothetical extraction rate with Scenario B15 having the greatest reduction in groundwater level and Scenario B9 having the least. The reductions in mean modelled groundwater discharge and groundwater levels under groundwater extraction scenarios B9, B12 and B15 over the 40-year time period, are due to the small spatial extent of the CLA in the Victoria catchment (12,000 km2), and the short distance (about 15 km) between the closest hypothetical groundwater extraction site relative to the spring complexes around Top Springs (see Section 6.3). The three hypothetical extraction sites occur between 15 and 80 km from the discharge areas of the aquifer. This highlights that changes in the aquifer’s water balance depends on a range of factors, including the hydrogeological conceptual model, the location, magnitude and duration of extraction, and the impacts of hydrogeological properties of the aquifer (saturated aquifer thickness, aquifer hydraulic properties) on spatial and temporal changes in groundwater flow in an aquifer. Simulating a future drier climate, Scenario C (10% reduction in rainfall) to 2060 and current groundwater extraction compared to the historical climate and current groundwater extraction (Scenario A), results in a 34% reduction in spring discharge along the western aquifer margin. Modelled groundwater levels are also lower across most parts of the aquifer for Scenario C than to Scenario A. Simulating a future drier climate (Scenario D) to 2060 and hypothetical extraction compared to Scenario A (current climate and current groundwater extraction) results in larger reductions in modelled spring discharge and groundwater drawdown, except for the Dwet scenarios (Dwet9, Dwet12 and Dwet15). The results for the mean modelled groundwater discharge via evapotranspiration and springs from the CLA at spring complexes along the western margin of the CLA in the Victoria catchment under a projected future climate (scenarios C and D) illustrate the impacts climate variability may have on the aquifer’s water balance. Under Scenario Cdry the reduction in groundwater recharge to the aquifer will have a larger impact on groundwater discharge via evapotranspiration and localised spring discharge than will the hypothetical groundwater extraction rates tested at specified locations in this study. This is because the CLA outcrops near Top Springs, which receives localised recharge and has relatively short groundwater flow paths to the spring complexes. Consequently, inter-annual variations in climate are evident in inter-annual variations in discharge. However, these results are indicative and specific to the projected future climate and locations and rates of hypothetical groundwater extraction tested in this study. The nature and scale of hydrological impacts from any future groundwater extraction will always be location, magnitude and duration specific in terms of causing changes to the reliability of access to water by GDEs and existing groundwater users. 7.3 Proterozoic dolostone aquifers 7.3.1 Regional hydrogeological context The PDAs across the centre and south of the catchment are composed of a number of dolostone- rich hydrogeological units. The most significant of these are the dolostone and sandstone-rich Skull Creek Formation and to a lesser extent the dolostone and siltstone-rich Bynoe and Timber Creek formations found between Timber Creek and Yarralin. In the south of the catchment, are the dolostone-rich Campbell Springs and Pear Tree dolostones between Daguragu and Limbunya, though these occur mostly outside the catchment boundary. Collectively, the spatial extent of their outcropping and subcropping area beneath overlying Cenozoic strata is about 7000 km2 (about 9% of the total catchment area). The PDAs are mostly composed of weathered and karstic dolostone, sandstone and siltstone, with minor shale and mudstone. The mineral composition of these rocks is reflected in the Ca–HCO3 to Mg–HCO3 chemical composition of the groundwater (see Section 6.2.5). Groundwater is generally quite fresh (<500 mg/L TDS), though for groundwater in the CLA, because of the high HCO3 composition of groundwater the water can be hard causing scale build-up on water infrastructure. The aquifer outcrops and subcrops are associated with structural anticline features that are folded and faulted between Timber Creek and Yarralin in the centre of the catchment, and in the south of the catchment between Daguragu and Limbunya. The aquifers dip gently to steeply in the subsurface away from the outcropping and subcropping areas and become confined by adjacent Cambrian basalt and Proterozoic sandstone and shale (see Section 6.1.2). Groundwater flow in the PDAs, as in the CLA, is complex due to the karstic nature of the aquifer exhibiting a high degree of heterogeneity in its physical properties. Its complexity arises from the variability and interconnectivity between fractures, fissures and karsts which influence groundwater flow processes across the aquifer extent. Drilling information for the aquifers is limited to the outcropping and subcropping areas where the aquifers can be intersected at relatively shallow depths (mostly <100 mBGL) in areas where elevation is about 100 mAHD (see Section 6.1.2). Depth to groundwater is generally less than 50 mBGL across most areas in the centre of the catchment, except for in the far south where it is deeper (>75 mBGL), west of Daguragu and Kalkarindji (see 6.2.2). The shallowest groundwater (<10 mBGL) occurs along the margins of the outcropping and subcropping areas in the centre of the catchment where karstic springs occur at contact points with highly heterogenous Proterozoic sandstone and shale and the Cambrian basalt (see Section 6.1.3). These spring complexes where localised discharge occur are the main GDEs identified for the PDAs thus far. 7.3.2 Conceptual model Recharge Recharge to the PDAs is spatially variable and occurs via a combination of: (i) localised preferential infiltration of rainfall or streamflow via sinkholes, fractures and faults where streams traverse the outcrop, and (ii) broad diffuse infiltration of rainfall through the overlying Cenozoic strata which vertically leaks to the underlying aquifers. Because of the dynamic topography associated with the PDAs and their structural complexity, some recharge occurs in fractures and faults across elevated areas, though most occurs in topographic lows where the accumulation of runoff is locally recharged. Elsewhere, dolostone aquifers are confined by overlying Proterozoic sandstones and shales of the Auvergne and Tijunna groups respectively, or the APV of the Kalkarindji Province. Both topography and the presence or absence of these overlying units influence the spatial variability in recharge to, and discharge from the aquifers. Across the outcropping and subcropping areas of the PDAs, groundwater is quite fresh (<500 mg/L) (see sections 5.2 and 6.2.5), has a depleted isotopic composition and little sign of evaporation prior to recharge (see Section 6.2.6), and has reasonable concentrations of 3H and CFC-12 (see sections 6.2.8 and 6.2.10). Historical hydrological monitoring for shallow bores near Timber Creek in the outcropping areas of the Skull Creek and Timber Creek formations, exhibit reasonably quick (weeks) moderate magnitude recharge responses in hydrographs to wet-season rainfall events (see Section 5.1.2). Mean annual recharge rates estimated by both upscaled CMB (see Section 5.4.3) and 3H and CFC- 12 concentrations in groundwater indicate a range in recharge of between 15 to 50 mm/year. These rates are higher than for the CLA, but the PDAs occur in a higher rainfall zone (700 to 900 mm/year) than the CLA (500 to 600 mm/year). This range in recharge rates is consistent with previous estimates reported by Tickell and Rajaratnam (1998) and CSIRO (2009). Groundwater flow and residence times Information related to groundwater flow is limited for the PDAs because of the paucity of data. However, it is inferred from static groundwater level data (see Section 6.2.2) and hydrogeological cross-sections (see Section 6.1.2) that groundwater flow within the outcropping and subcropping areas follows a subdued form of topography. Groundwater flows from higher elevation areas such as Mount Dempsey and the Fitzgerald Range to areas of lower elevations around streams and springs. Mean residence times (MRTs) for local to intermediate-scale groundwater flow ranges from several years to a few decades for short flow paths (<15 km) discharging at most springs, and from a few decades up to a few hundred years or so for slightly longer and flow paths (~25 km) such as found at Bulls Head Spring. There is currently an absence of further information for groundwater flow in deeper and confined parts of the PDAs, though it is anticipated that MRTs for more intermediate to regional-scale flow would exceed hundreds of years. This conceptualisation is consistent with that inferred by Tickell and Rajaratnam (1998). Discharge Discharge from the PDAs occurs via a combination of: (i) intermittent lateral outflow to streams (East Baines River and Crawford, Giles and Middle creeks) where they are incised into the aquifer outcrop, (ii) perennial localised spring discharge at Bulls Head, Kidman and Crawford springs across the dolostone units of the Bullita Group in the centre of the catchment, and Depot, Farquharson and Wickham springs across the dolostone units of the Limbunya Group in the south of the catchment, (iii) evapotranspiration via riparian and spring-fed vegetation (see Figure 7-3), and (iv) groundwater extraction for stock and domestic use, including community water supply at Timber Creek. Discharge at streams is intermittent and reliant on localised recharge in the aquifer outcrop and short flow paths (<15 km) but streams dry up and cease to flow as wet-season groundwater levels decline below stream level. Perennial localised discharge occurs mostly at contact springs in topographic lows along the margins of the outcropping and subcropping areas of the PDAs where fractures and karsts in the aquifers contact adjacent highly heterogenous Proterozoic sandstone and shale, as well as the basalt of the APV (see Section 6.2.14). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 7-3 Spring-fed and riparian vegetation lining Kidman Creek immediately downstream of Kidman Springs Photo source: CSIRO Figure 7-4 is a simplified representation of the type of discrete contact springs that occur along the margin of the outcropping and subcropping areas of the PDAs. In this case, Kidman Springs occurs at the base of Lounger Hill on the edge of the Fitzgerald Range. Topography drives flow towards lower elevation areas at the base of the hill where groundwater hosted in fractures and karstic features in both the interconnected Bynoe and Skull Creek formations intersects the land surface near contact points with Proterozoic sandstone and shale. A series of small vents occur upstream in slightly elevated areas prior to the main vent of the springs complex where discharge supports lush riparian and spring-fed vegetation as well as dry-season flow in Kidman Creek (see Figure 7-3 and Figure 7-4). Nearly all spring flow is reliant on localised recharge in the aquifer outcrop and discharge from short flow paths with short MRTs (years to a decade) as shown by high concentrations of 3H in springs and nearby groundwater (see Section 6.2.8). Bulls Head Spring, however, receives discharge from longer paths (>15 km) with longer MRTs, as shown by the lower concentration of 3H in the spring and some groundwater (see sections 6.2.8 and 6.2.14). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 7-4 Simplified conceptual block model of the springs complex at Kidman Springs associated with the Proterozoic dolostone aquifers in the centre of the Victoria catchment Block model not drawn to scale. Groundwater discharge supports streamflow and spring-fed vegetation. Texture in the hydrogeological units represent fractured and/or karstic rocks. Bulls Head Spring occurs at the base of Burt Hill, which is associated with regional faulting of the Bynoe and Skull Creek formations and adjacent Proterozoic sandstone (Jasper Gorge Sandstone). Outcropping and subcropping siltstone of the Bynoe Formation is prominent at the spring providing an indication that the Bynoe Formation at this location may be only a partial aquifer exhibiting longer mean residence times for groundwater flow (Figure 7-5). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 7-5 Outcropping siltstone of the Bynoe Formation at Bulls Head Spring Photo source: CSIRO 7.4 Opportunities for future groundwater resource development Planning future groundwater resource developments and authorising licensed groundwater entitlements require value judgments of what is an acceptable impact to receptors such as environmental assets or existing users at a given location. These decisions can be complex, and they typically require considerable input from a wide range of stakeholders, particularly government regulators and communities. Scientific information to help inform these decisions includes: (i) identifying aquifers that may be potentially suitable for future groundwater resource development; (ii) characterising their depth, spatial extent, saturated thickness, hydraulic properties and water quality; (iii) conceptualising the nature of their flow systems; (iv) estimating aquifer water balances; and (v) providing initial estimates of potential extractable volumes and associated drawdown in groundwater level over time and distance relative to existing groundwater users and groundwater-dependent ecosystems. The changes in groundwater levels over time at different locations provide information on the potential risks of changes in aquifer storage and therefore reliability of access to water for existing users or the environment. The hydrogeological units of the Victoria catchment contain a variety of local-, intermediate- and regional-scale aquifers that host localised to regional-scale groundwater flow systems (Figure 7-6). The intermediate- to regional-scale fractured and karstic limestone and dolostone aquifers (CLA and PDAs) are present in the subsurface across moderate areas, collectively occurring beneath about 24% of the catchment. Given their moderate spatial extent, they underlie and partially coincide with areas of soil suitable for irrigated agriculture (Thomas et al., 2024). They contain mostly fresh groundwater (<1000 mg/L TDS) and have potential to yield water at a sufficient rate to support irrigation development (>10 L/second) with appropriately constructed and sited bores. Compared to local-scale aquifers, these intermediate- to regional-scale aquifers contain larger volumes of groundwater in storage (tens to hundreds of gigalitres) and their storage and discharge characteristics across some parts of their extent can be less affected by short-term (yearly) variations in recharge rates caused by inter-annual variability in rainfall. Furthermore, their moderate spatial extent provides greater opportunities for groundwater resource development away from existing groundwater users and GDEs at the land surface, such as springs, spring-fed vegetation and surface water, which can be ecologically and culturally significant. In contrast, local-scale aquifers in the Victoria catchment, such as fractured and weathered sandstone, shale and basalt rock and alluvial aquifers, host local-scale groundwater systems that are highly variable in composition, salinity and yield. They also have a small and variable spatial extent and less storage than the larger aquifers, limiting groundwater resource development to localised opportunities such as stock and domestic use, community water supply, or in some instances as a conjunctive water resource (i.e. combined use of groundwater with surface water or rainwater). This is equally applicable to sandstone aquifers closer to the coast that are vulnerable to saltwater intrusion. This study identified six hydrogeological units hosting aquifers that may have potential for future groundwater resource development in the Victoria catchment (see Table 7-1 for details): • Cambrian limestone • Proterozoic dolostone • Cambrian basalt • Devonian–Carboniferous sandstone • Proterozoic sandstone • Quaternary alluvium. Figure 7-6 Hydrogeological units with potential for future groundwater resource development The spatial extent of the outcropping and subcropping component of each hydrogeological unit is presented with the majority of overlying Cretaceous and Cenozoic cover removed (except the alluvium). Right lower inset shows the spatial extent of the Cambrian limestone and Proterozoic dolostone that extend outside the Victoria catchment. Geology data sources: Adapted from Department of Industry, Tourism and Trade (2024) and Department of Environment, Parks and Water Security (2008) Geological faults data from Department of Industry, Tourism and Trade (2010) Spring data from Department of Environment Parks and Water Security (2019b) Table 7-1 Opportunity-level estimates of the potential scale of groundwater resource development opportunities in the Victoria catchment Indicative scale of the resource based on a combination of numerical modelling, estimates of mean annual recharge, and conceptualisation of the aquifers hosted in different hydrogeological units. The actual scale will depend upon government and community acceptance of potential impacts to groundwater-dependent ecosystems and existing groundwater users. For more information on this table please contact CSIRO on enquiries@csiro.au For more information on this table please contact CSIRO on enquiries@csiro.au †Actual scale will depend upon government and community acceptance of impacts to GDEs and existing water users. 7.4.1 Cambrian Limestone Aquifer Based on the findings of the numerical modelling (Section 6.3), with appropriately sited borefields, it is estimated that about 10 GL/year could potentially be extracted from the CLA in areas to the south-east of Top Springs (i.e. groundwater extraction occurring 20 to 80 km to the south-east), depending upon community and government acceptance of potential impacts on groundwater- dependent ecosystems and existing groundwater users (Table 7-1). The CLA in areas to the north of Top Springs can have a thin saturated thickness (<20 m), or be unsaturated, and is less promising for future development. In contrast, in the area from Top Springs south-east towards Cattle Creek, has in places a saturated thickness of >50 m (see Section 6.1.2) and occurs at relatively shallow depths (~50 m BGL) (see Section 6.2.2). Due to time lags associated with groundwater flow (tens of years), additional hypothetical extraction in this area may result in 11% to 14% reduction in modelled groundwater discharge to spring complexes and groundwater-fed vegetation near Top Springs. It also may result in modelled reductions in groundwater levels of about 15 m at the centre of the hypothetical developments and of about 1 m up to 20 km away. The hypothetical scenario-based modelling in this study has demonstrated that potential impacts from any future development will depend on a range of factors including the location, magnitude and duration of extraction, and the impacts of hydrogeological properties (saturated aquifer thickness, aquifer hydraulic properties, hydrogeological conceptual model) on spatial and temporal changes in groundwater flow in an aquifer. In addition, modelled changes in the water balance from a projected drier future climate are an important consideration to factor into adaptive management of the groundwater resources hosted in the CLA in the future. Furthermore, any proponent seeking a groundwater license will most likely be required to undertake a hydrogeological assessment to assess aquifer properties and bore performance, and ensure their proposed extraction meets licensing conditions in relation to changes in groundwater storage (groundwater drawdown) and flow. 7.4.2 Proterozoic dolostone aquifers Insufficient information exists to develop geological models and water balance models for the PDAs. However, an indicative scale of the resource can be derived by applying the estimated recharge rates for the aquifers (sections 5.4.3 and 6.2.13) to their outcropping and subcropping areas to assess the potential recharge flux of the water balance for these aquifers. Given the likelihood that the water balance for the PDAs will be sensitive to climate variability similar to that of the CLA, a conservative approach of using the 5th percentile (95th percentile exceedance) of the estimated range in annual recharge rates to the outcropping and subcropping areas of the PDAs results in a conservative estimate of the annual recharge flux of 105 GL/year. Assuming 20% of the conservative recharge flux may potentially be available for future groundwater resource development, an indicative scale of the groundwater resource in the PDAs was estimated to be less than or equal to 20 GL/year (Table 7-1). Further hydrogeological investigations (drilling and pump testing) and hydrological risk assessment modelling are required to evaluate groundwater extraction and climate variability impacts on existing groundwater users and GDEs for the PDAs. Similar to the CLA, the actual scale of future development of the PDAs will depend upon community and government acceptance of potential impacts on groundwater-dependent ecosystems and existing groundwater users, as well as approval of licenses to extract groundwater. 7.4.3 Opportunities from other aquifers Opportunities for future development of groundwater resources from aquifers hosted in other hydrogeological units (Cambrian basalt, Devonian–Carboniferous sandstone and Proterozoic sandstone) across the Victoria catchment are most likely to be limited to use for stock and domestic purposes and occasional community water supply. Productive local-scale aquifers hosted in the Quaternary alluvium occurring in patches associated with the streambed, stream channel and floodplain of major streams and their tributaries may offer some opportunities, these will require local investigation. The largest occurrences of the alluvium are in the north of the catchment along the lower reaches of the Angalarri, Victoria and West Baines rivers (Figure 7-6). Indicative bore yield data suggest bore yields can be as high as 11 L/second (see Section 5.3), but the aquifer is currently sparsely tested. Water quality can vary from fresh to brackish, but it is also sparsely tested (see Section 5.2). However, in places the aquifers may offer potential for small- scale (<1.0 GL/year) localised developments or use as a conjunctive water resource. Opportunities are likely to be limited where the alluvium is: (i) storage limited (thin saturated thickness <15 m), (ii) composed mostly of fine-textured sediments (clay lenses), (iii) regularly flooded, and (iv) highly connected to perennial reaches of streams such that development may limit the reliability of access to water by GDEs. 7.5 Constraints on future groundwater resource development 7.5.1 Cambrian Limestone Aquifer Potential constraints for small-to intermediate-scale (1 to 3 GL/year) future groundwater-based irrigation opportunities from the CLA in the Victoria catchment include: • being constrained to areas across the CLA occurring from Top Springs south-east towards Cattle Creek, where the saturated thickness of the aquifer is sufficient (i.e. >20 m) to support the installation and operation of purpose-built production bores • in siting and installing successful high-yielding (i.e. bore yields >10 L/second) production bores, the possible need to drill multiple investigation holes to identify productive parts of the aquifer (fractured and karstic limestone and dolostone rather than less productive mudstone), which can create uncertainty in the cost of drilling programs • the high hardness of the groundwater, which could result in scale build-up on water infrastructure and the foliage of broad-leaf crops • the need to ensure that cumulative extraction from development does not adversely affect the reliability of access to water for existing groundwater users for stock and domestic water across numerous pastoral stations, and the water supply to the community at Top Springs • the need to ensure that cumulative extraction does not adversely affect the reliability of access to water for numerous culturally and ecologically important GDEs along the western margin of the CLA • in maintaining the reliability of access to water for all groundwater users and GDEs, the impact of climate variability, which has a larger influence on the unconfined parts of the aquifer’s water balance than does current groundwater extraction. The latter reflected by natural variations in recharge to the aquifer under the current and historical climate which is an important consideration for adaptive management of groundwater in the CLA into the future. 7.5.2 Proterozoic dolostone aquifers Potential constraints for small-to intermediate-scale (1–3 GL/year) future groundwater-based irrigation opportunities from the PDAs in the Victoria catchment include: • being constrained to lower elevation areas (~100 mAHD) across the PDAs between Timber Creek and Yarralin where topography is relatively flat, but development could occur at sufficient distances away karstic contact springs along the margin of the outcropping and subcropping areas • the need to better characterise the water balance, saturated thickness, groundwater levels and hydraulic properties of the aquifers, as the PDAs are currently data sparse across large areas. This is particularly relevant to aquifers hosted in the Campbell Springs and Pear Tree dolostone in the south of the catchment which were not the subject of targeted investigations in this study • in siting and installing successful high-yielding (i.e. bore yields >10 L/second) production bores, the possible need to drill multiple investigation holes to identify productive parts of the aquifer (fractured and karstic limestone and dolostone rather than less productive mudstone), which can create uncertainty in the cost of drilling programs • ensuring that cumulative extraction from development does not adversely affect the reliability of access to water for existing groundwater users for stock and domestic water across numerous pastoral stations, and the water supply to the community at Timber Creek • the need to ensure that cumulative extraction does not adversely affect the reliability of access to water for numerous culturally and ecologically important GDEs along the margin of the outcropping and subcropping areas • in maintaining the reliability of access to water for all groundwater users and GDEs, the impact of climate variability, which as in the CLA, is likely to have an influence on the unconfined parts of the aquifers water balance. 7.5.3 Other aquifers A key constraint on future groundwater resource development opportunities for groundwater- based irrigation from the less extensive but potentially productive alluvial aquifers include is the need to better characterise the nature of these aquifers along the Angalarri and West Baines rivers in the northern parts of the catchment. While existing geological mapping and sparse hydrogeological data indicate their potential as a promising groundwater resource (i.e. they host some fresh groundwater and, indicative bore yields can be >10 L/second) further drilling and pumping test investigations would be required. These investigations would confirm the aquifers’ spatial extent, saturated thickness, water quality and potential to yield sufficient water to support irrigation without depleting aquifer storage or compromising the reliability of access to water by groundwater-fed streams. 8 Summary and conclusions 8.1 Opportunities and constraints for future groundwater development The literature review and regional desktop assessment of existing hydrogeological data served as an effective screening tool for identifying potential groundwater resource development opportunities within the catchment. Insights gained from targeted field investigations, desktop analyses and modelling have improved the understanding of unconfined parts of the Cambrian Limestone Aquifer (CLA) Proterozoic dolostone aquifers (PDAs) that offer the most promising opportunities for future development. These areas included: (i) the unconfined parts of the CLA from Top Springs south toward Cattle Creek and east to the catchment boundary, and (ii) the unconfined parts of the PDAs where topography is relatively flat at elevations approximately 100 mAHD situated between Timber Creek and Yarralin. However, the nature and scale of these opportunities will ultimately depend upon community and government acceptance of impacts on groundwater-dependent ecosystems and existing groundwater users. Cambrian Limestone Aquifer Key findings from targeted investigations have demonstrated that the identified parts of the CLA: • exhibit a moderate spatial extent that aligns with areas suitable for agricultural intensification, potentially supporting a few small- to intermediate-scale (1 to 3 GL/y) groundwater-based irrigation opportunities • have depths to groundwater of less than or equal to 50 mBGL across moderate-size areas, suggesting relatively economical costs for extracting the resource for consumptive use • can be accessed by drilling at economically viable depths (≤100 mBGL) across most parts of the aquifer • predominantly contain fresh groundwater suitable for a variety of uses, although scale build-up may occur due to the water’s high hardness. Furthermore, this study has further refined the hydrogeological conceptual model for the CLA by: • better characterising the key vertical and horizontal geological controls influencing groundwater discharge from the CLA, including: (i) vertical leakage to the underlying Antrim Plateau Volcanics (APV), (ii) the influence spatial changes in the topography of the underlying APV have on the saturated thickness of the overlying CLA, and (iii) enhanced horizontal interconnectivity between the CLA with agglomerate and chert lithologies of the APV along the western margin of the CLA near Old Top and Winari springs • clarifying key controls on groundwater flow directions for the CLA, adding value to recent investigations by the Strategic Regional Environmental and Baseline Assessment (SREBA) project (Amery and Tickell, 2022; ELA, 2022) and for the Wiso Basin water management zone (Department of Environment Parks and Water Security, 2022) • better characterising the various spatial sources of discharge from the CLA along the western aquifer margin, where karstic springs occur at contact points with the highly heterogenous APV, thereby adding value to historical investigations by Tickell and Rajaratnam (1998) • deriving new estimates of groundwater recharge using two independent methods that align with previous estimates from Tickell and Rajaratnam (1998) and CSIRO (2009) • deriving initial or first-order estimates of mean residence times (MRTs) for groundwater flow which appear generally consistent with previous estimates by Tickell and Rajaratnam (1998) • further characterising, that spatial changes in the presence, thickness and lithology of the overlying Cretaceous and Cenozoic strata, combined with spatial changes in the thickness and hydraulic properties of the CLA, influence the time lags for hydrological impacts due to climate variability and groundwater extraction to propagate through the aquifer • identifying that the potential hydrological impacts of both climate variability and groundwater extraction are equally important considerations for adaptive management of the CLA’s groundwater resources into the future. The nature and scale of hydrological impacts from any future groundwater extraction will always be location, magnitude and duration specific in terms of causing changes to the reliability of access to water by GDEs and existing groundwater users. Proterozoic dolostone aquifers Key findings from targeted investigations have indicated that the identified unconfined parts of the PDAs: • have a moderate spatial extent that coincides with portions of land suitable for agricultural intensification, potentially supporting multiple small- to intermediate-scale (1 to 3 GL/y) groundwater-based irrigation opportunities • exhibit depths to groundwater of ≤50 mBGL across moderate sized areas, suggesting potentially economical costs to pump the resource to the surface for consumptive use • can be accesses by drilling at economically viable depths (≤100 mBGL) across most parts of the aquifer • contain fresh groundwater suitable for a wide range of purposes, although scale build-up may occur due to the water’s high hardness Furthermore, this study has further refined the hydrogeological conceptual model for the PDAs by: • better characterising the key vertical and horizontal geological controls on groundwater in the PDAs, including: (i) the influence spatial changes in the topography of major anticline features, such as Fitzgerald Range and Longer Hill on recharge and groundwater flow, and (ii) the occurrence of karstic contact springs where discharge occurs, highlighting the interconnectivity between the Bynoe and Skull Creek formations • better characterising various spatial sources of discharge from the PDAs along the margin of the outcropping and subcropping areas where karstic springs occur at contact points with highly heterogeneous Proterozoic sandstone and shale, as well as Cambrian basalt, thereby adding value to historical investigations by Tickell and Rajaratnam (1998) • deriving new groundwater recharge estimates using two independent methods that align with previous estimates by Tickell and Rajaratnam (1998) and CSIRO (2009) • deriving initial or first-order estimates of mean residence times for groundwater flow that generally align with historical estimates by Tickell and Rajaratnam (1998). 8.2 Knowledge gaps and uncertainty Cambrian Limestone Aquifer The CLA has been the subject of multiple hydrogeological investigations over the past two decades, particularly in the past decade. However, few studies have focused on its western margin in the Victoria catchment where little groundwater development exists. While the targeted investigations in this study have improved the characterisation of the CLA and the quantification of groundwater flow processes, they have also highlighted several knowledge gaps and areas of uncertainty: (i) there is a lack of spatial groundwater-level data across the southern parts of the aquifer between Cattle Creek and Lajamanu, (ii) baseline observations of spring flows, and stream flows, and their water quality along the western margin of the aquifer are sparse, and (iii) little is known about phreatophytes associated with springs and groundwater-fed reaches of streams along the western margin of the aquifer. The evaluation and interpolation of available spatial static groundwater-level data for the CLA has further revealed that little data exists just east of the Camfield River between Cattle Creek and Lajamanu. There is a lack of confidence in the interpolated surface for this area, and currently, there is little evidence of groundwater discharge features (perennial springs or stream reaches) along the western aquifer margin in this area from which to infer the nature of any flow processes. This part of the aquifer is not included in the existing DR2 FEFLOW model domain due to the scarcity of data. The perennial springs and groundwater-fed reaches of streams along the western aquifer margin are culturally and ecologically important. However, the current understanding of their flow and spatial extent is poor. In addition, areas of springs and diffuse evapotranspiration were represented using Dirichlet or seepage face boundary conditions in the DR2 FEFLOW model in this study to obtain order of magnitude estimates of the effects of climate variability and groundwater extraction on these discharge areas. Obtaining baseline estimates of temporal changes in perennial spring flow and intermittently groundwater-fed stream reaches would help to better constrain the water balance of the CLA in the Victoria catchment. Furthermore, having a baseline understanding of the phreatophytes supported by spring flow would assist in mapping the spatial extent of the discharge area for the aquifer. This additional spatial data would provide further insights into the nature of the CLA in the Victoria catchment and better constrain the aquifer’s water balance, as well as the opportunities and risks for future groundwater resource development. Proterozoic dolostone aquifers The PDAs have only been the subject of a few hydrogeological investigations in the past, resulting in a lack of data. While the targeted investigations in this study have improved the characterisation of the PDAs and semi-quantified groundwater flow processes, they have also highlighted several knowledge gaps and areas of uncertainty: (i) little information currently exists for the PDAs hosted in Campbell Springs and Pear Tree dolostones in the southern part of the catchment, (ii) baseline observations of spring flows, stream flows and their water quality along the margins of the outcropping and subcropping areas of the PDAs are sparse, and (iii) little is known about phreatophytes associated with springs and groundwater-fed reaches of streams along the margins of the outcropping and subcropping areas of the PDAs. The perennial springs and groundwater-fed reaches of streams along the margins of the outcropping and subcropping areas of the PDAs are culturally and ecologically important. However, the current understanding of their flow and spatial extent is poor. Obtaining baseline estimates of temporal changes in perennial spring flow and intermittently groundwater-fed stream reaches would improve the current understanding of groundwater discharge from the PDAs. Furthermore, having a baseline understanding of the phreatophytes supported by spring flow would assist in mapping the spatial extent of the discharge area for the aquifer. This additional spatial data would provide further insights into the nature of the PDAs in the Victoria catchment, as well as the opportunities and risks for future groundwater resource development. Alluvial aquifers Very little is known about the alluvial aquifers in the Victoria catchment due to a lack of data. Key knowledge gaps and uncertainties include their saturated thickness, lithology, water quality and aquifer properties. Along the Angalarri and West Baines rivers in the northern part of the catchment, where the aquifers appear to be most extensive, near-surface geophysics, targeted drilling and pumping test investigations would improve current information and reduce uncertainty on the aquifers’ spatial extent, saturated thickness, water quality and potential to yield sufficient water for consumptive use without depleting aquifer storage or compromising the reliability of access to water by groundwater-fed streams. References Ahmad M and Munson TJ (2013) Chapter 36: Bonaparte Basin: in Ahmad M and Munson TJ (compilers). ‘Geology and mineral resources of the Northern Territory’. Northern Territory Geological Survey, Special Publication 5., https://geoscience.nt.gov.au/gemis/ntgsjspui/bitstream/1/81516/1/GNT_Ch36_Bon.pdf. 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Part IV Appendices A.1 Details of groundwater bores sampled Apx Table A-1 Details of all bores sampled in the study BORE RN DATE SAMPLED EASTING NORTHING HYDROGEOLOGICAL UNIT GROUNDWATER LEVEL (mBTOC) TOP SCREEN (mBGL) BOTTOM SCREEN (mBGL) BORE DEPT (mBGL) RN026127 8/08/2023 788516 8196102 Antrim Plateau Volcanics 13.1 a 84 90 100 RN026444 9/08/2023 789843 8137978 ML & APV na 17 23 69 RN021978 9/08/2023 805331 8163563 Antrim Plateau Volcanics 8.70 a 45.7 53.7 75 RN020019 10/08/2023 812831 8157863 Montejinni Limestone 13.7 a 32 38 38 RN031740 11/08/2023 748131 8173362 Antrim Plateau Volcanics 4.20 b 54 72 72 RN041173 12/08/2023 813976 8185912 Montejinni Limestone 34.0 b 62 74 74 RN033280 12/08/2023 732953 8163423 Battle Creek Formation 2.40 b 46 58 64 RN036194 13/08/2023 713307 8216005 Bynoe Formation 19.0 42 47 48 RN007403 13/08/2023 701854 8217969 Skull Creek Formation 18.3 b 54.9 79.2 79 a SWL on day of sample collection; b SWL from bore database. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. A.2 Details of field parameters measured at each groundwater bore Apx Table A-2 Field parameters measured at all groundwater sampling sites during field sampling BORE RN DATE SAMPLED EASTING NORTHING HYDROGEOLOGICAL UNIT TEMP (OC) pH EC (mS/cm) DO (mg/L) TOTAL ALKALINITY (mg/L CaCO3) RN026127 8/08/2023 788516 8196102 Antrim Plateau Volcanics 30.0 9.30 0.38 0.6 22 RN026444 9/08/2023 789843 8137978 ML & APV 31.1 6.80 0.97 0.2 407 RN021978 9/08/2023 805331 8163563 Antrim Plateau Volcanics 31.9 7.04 1.07 0.5 404 RN020019 10/08/2023 812831 8157863 Montejinni Limestone 32.0 6.83 0.98 0.2 410 RN031740 11/08/2023 748131 8173362 Antrim Plateau Volcanics 30.2 7.69 1.29 0.9 392 RN041173 12/08/2023 813976 8185912 Montejinni Limestone 30.8 7.30 0.92 0.8 316 RN033280 12/08/2023 732953 8163423 Battle Creek Formation 30.9 7.54 0.87 1.3 363 RN036194 13/08/2023 713307 8216005 Bynoe Formation 34.0 6.94 0.72 0.8 288 RN007403 13/08/2023 701854 8217969 Skull Creek Formation 33.2 7.05 1.08 1.1 434 ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. A.3 General chemistry results for groundwater sampling sites Apx Table A-3 Laboratory chemical analyses for groundwater samples collected at each bore site BORE RN DATE SAMPLED HYDROGEOLOGICAL UNIT EC (mS/cm) Ca (mg/L) Mg (mg/L) Na (mg/L) K (mg/L) SO4= (mg/L) Cl– (mg/L) TOTAL ALKALINITY (mg/L AS CaCO3) Br– (mg/L) NO3– (mg/L) RN026127 8/08/2023 Antrim Plateau Volcanics 0.26 1.39 <0.1 61.9 <0.2 5.3 13.0 117 <0.5 <0.5 RN026444 9/08/2023 ML & APV 0.69 128 30.9 7.42 1.69 1.5 5.7 417 <0.5 0.4 RN021978 9/08/2023 Antrim Plateau Volcanics 0.75 73.7 45.4 35.2 2 9.4 25.2 409 <0.5 <0.5 RN020019 10/08/2023 Montejinni Limestone 0.68 93.2 38.9 10.8 2.87 1.5 5.8 410 <0.5 0.1 RN031740 11/08/2023 Antrim Plateau Volcanics 0.92 62.9 37.7 103 6.81 16.9 58.5 401 <0.5 39.4 RN041173 12/08/2023 Montejinni Limestone 0.66 69.4 31.2 42.9 6.08 44.6 18.5 306 <0.5 <0.5 RN033280 12/08/2023 Battle Creek Formation 0.64 41 38.6 52.9 2.63 1.6 7.1 376 <0.5 5.2 RN036194 13/08/2023 Bynoe Formation 0.51 71.3 26.2 4.01 1.38 1.2 4.5 300 <0.5 1.8 RN007403 13/08/2023 Skull Creek Formation 0.76 97.8 43.9 8.11 3.47 2.6 8.2 455 <0.5 0.8 ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. A.4 Environmental tracer results for groundwater sampling sites Apx Table A-4 Measured environmental tracer data from groundwater samples collected at each bore site BORE RN DATE SAMPLED HYDROGEOLOGICAL UNIT δ2H (‰ VSMOW) δ18O (‰ VSMOW) 87Sr/86Sr 3H (TU) CFC-11 (pMol/kg) CFC-12 (pMol/kg) 14C (pmC) δ13C (‰ PDB) SF6 (fMol/kg) RN026127 8/08/2023 Antrim Plateau Volcanics -47 -6.1 0.711 -0.014 0.015 0.045 24.45 -12.64 0.041 RN026444 9/08/2023 ML & APV -49 -7.7 0.715 0.634 0.065 0.290 89.75 -9.37 100.704 a RN021978 9/08/2023 Antrim Plateau Volcanics -58 -8.4 0.713 0.059 0.020 0.070 88.72 -10.66 0.320 RN020019 10/08/2023 Montejinni Limestone -52 -8.2 0.716 0.245 0.020 0.060 73.80 -10.10 2.048 RN031740 11/08/2023 Antrim Plateau Volcanics -44 -6.5 0.718 1.274 1.115 1.245 92.09 -13.35 2.723 RN041173 12/08/2023 Montejinni Limestone -55 -7.9 0.711 0.067 0.050 0.040 14.25 -13.57 0.125 RN033280 12/08/2023 Battle Creek Formation -42 -6.0 0.715 0.145 0.435 0.385 89.39 -11.29 1.187 RN036194 13/08/2023 Bynoe Formation -54 -7.9 0.733 0.496 0.900 0.915 76.98 -12.00 2.863 RN007403 13/08/2023 Skull Creek Formation -55 -8.3 0.737 0.181 0.455 0.645 81.67 -11.98 2.222 a RN026444 contains SF6 far above atmospheric equilibrium levels. ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. A.5 Noble gas results for groundwater sampling sites Apx Table A-5 Measured noble gas data from groundwater sampled at each bore site BORE RN DATE SAMPLED HYDROGEOLOGICAL UNIT He (ccSTP/g) Ne (ccSTP/g) Ar (ccSTP/g) Kr (ccSTP/g) Xe (ccSTP/g) RN026127 8/08/2023 Antrim Plateau Volcanics 1.4E-06 3.4E-07 3.6E-04 7.1E-08 9.1E-09 RN026444 9/08/2023 ML & APV 7.0E-08 2.5E-07 3.3E-04 6.1E-08 7.5E-09 RN021978 9/08/2023 Antrim Plateau Volcanics 1.4E-05 2.4E-07 3.0E-04 5.8E-08 7.2E-09 RN020019 10/08/2023 Montejinni Limestone 9.1E-08 2.5E-07 3.2E-04 5.9E-08 7.3E-09 RN031740 11/08/2023 Antrim Plateau Volcanics 6.1E-08 2.4E-07 3.2E-04 6.5E-08 8.2E-09 RN041173 12/08/2023 Montejinni Limestone 2.1E-07 3.0E-07 3.3E-04 6.3E-08 7.9E-09 RN033280 12/08/2023 Battle Creek Formation 1.5E-07 2.3E-07 3.0E-04 5.9E-08 7.7E-09 RN036194 13/08/2023 Bynoe Formation 2.2E-07 2.3E-07 2.9E-04 5.6E-08 7.1E-09 RN007403 13/08/2023 Skull Creek Formation 1.2E-07 2.6E-07 3.3E-04 5.8E-08 7.2E-09 ML = Montejinni Limestone. APV = Antrim Plateau Volcanics. A.6 General chemistry results for spring sampling sites Apx Table A-6 Chemical analyses from water samples collected at springs SITE NAME PH EC (mS/CM) TOTAL ALKALINITY (mg/L AS CaCO3) F– (mg/L) Cl– (mg/L) Br– (mg/L) NO3– (mg/L) SO4= (mg/L) Ca (mg/L) K (mg/L) Mg (mg/L) Na (mg/L) Companion Spring 8.4 0.88 489 0.51 22.1 <0.5 <0.5 3.5 44.7 4.62 46.6 89.5 Crawford Spring 7.8 0.77 439 0.24 7.0 <0.5 <0.5 0.6 116 3.37 42.2 5.9 Bulls Head Spring 7.6 0.56 318 0.14 4.4 <0.5 <0.5 2.4 93.8 1.92 24.5 3.16 Kidman Springs 8.0 0.31 172 < 0.1 4.0 <0.5 <0.5 <0.5 42.1 1.48 17.9 3.12 Dead Spring 8.0 0.96 553 0.29 10.9 <0.5 <0.5 1.4 59.7 2.37 66.2 86 Waterbag Spring 8.6 1.07 602 0.46 35.6 <0.5 <0.5 3.3 33.1 6.2 79.8 126 Palm Spring 7.5 0.66 359 0.40 11.0 <0.5 <0.5 1.8 132 2.45 40.1 10.1 Lonely Spring 7.7 0.65 380 0.42 2.4 <0.5 <0.5 <0.5 125 1.7 24 5.56 Winari Spring 8.3 0.35 185 0.34 11.4 <0.5 <0.5 1.1 36.8 2.29 22.9 8.7 Old Top Spring 7.8 0.75 439 0.39 8.3 <0.5 <0.5 1.0 90.3 2 53.7 13.9 A.7 Environmental tracer results for spring sampling sites Apx Table A-7 Measured environmental tracer data from water samples collected at springs SITE NAME EASTING NORTHING DATE δ2H (‰ VSMOW) δ18O (‰ VSMOW) 87Sr/86Sr 3H (TU) Companion Spring 788899 8174106 5/10/2023 -25 -2.8 0.716 1.128 Crawford Spring 705500 8199607 19/10/2023 -51 -6.9 0.735 1.148 Bulls Head Spring 697895 8210584 19/10/2023 -51 -7.6 0.734 0.317 Kidman Springs 710799 8215906 19/10/2023 -21 0.0 0.732 1.399 Dead Spring 743949 8168096 20/10/2023 -38 -4.9 0.713 1.058 Waterbag Spring 756176 8179814 20/20/23 -6 3.0 0.717 1.000 Palm Spring 780799 8134106 21/10/2023 -54 -7.4 0.714 0.600 Lonely Spring 787899 8151306 21/10/2023 -47 -6.7 0.712 1.037 Winari Spring 778641 8138567 21/10/2023 -26 -1.5 0.714 1.145 Old Top Spring 801199 8161306 22/10/2023 -45 -6.4 0.715 0.745 Comment: No 14C or 13C as lab unavailable A.8 Noble gas results for spring sampling sites Apx Table A-8 Measured noble gas data from water samples collected at springs APX TABLE B8 MEASURED NOBLE GAS DATA FROM SURFACE WATER SAMPLES COLLECTED SITE NAME DATE SAMPLED He (ccSTP/g) Ne (ccSTP/g) Ar (ccSTP/g) Kr (ccSTP/g) Xe (ccSTP/g) Companion Spring 5/10/2023a na na na na na Crawford Spring 19/10/2023 4.4E-08 1.8E-07 2.8E-04 5.9E-08 7.6E-09 Bulls Head Spring 19/10/2023 9.4E-08 2.3E-07 2.9E-04 6.0E-08 7.6E-09 Kidman Springs 19/10/2023 4.9E-08 1.9E-07 2.7E-04 6.0E-08 7.6E-09 Dead Spring 20/10/2023 5.9E-08 2.2E-07 3.0E-04 6.2E-08 8.6E-09 Waterbag Spring 20/20/23 5.8E-08 2.1E-07 2.8E-04 5.8E-08 7.7E-09 Palm Spring 21/10/2023 5.8E-08 2.1E-07 2.7E-04 6.0E-08 7.6E-09 Lonely Spring 21/10/2023 4.4E-08 1.9E-07 2.8E-04 6.3E-08 8.1E-09 Winari Spring 21/10/2023 4.8E-08 1.9E-07 2.7E-04 6.2E-08 7.8E-09 Old Top Spring 22/10/2023 6.0E-08 2.2E-07 2.9E-04 6.0E-08 7.8E-09 Footnote: aCompanion Spring not sampled for noble gases A.9 Summary of springs characterised in this study For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-1 Overview of Companion Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-2 Overview of Crawford Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-3 Overview of Bulls Head Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-4 Overview of Kidman Springs For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-5 Overview of Dead Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-6 Overview of Waterbag Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-7 Overview of Palm Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-8 Overview of Lonely Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-9 Overview of Winari Spring For more information on this figure please contact CSIRO on enquiries@csiro.au Apx Figure A-10 Overview of Old Top Spring AsAustralia’s national science agency and innovation catalyst, CSIRO is solving the greatestchallenges through innovativescience and technology. CSIRO. Unlocking a better futurefor everyone. Contact us 1300 363 400+61 3 9545 2176csiroenquiries@csiro.aucsiro.au For further informationEnvironment Dr Chris Chilcott+61 8 8944 8422chris.chilcott@csiro.au Environment Dr Cuan Petheram+61 3 6237 5669cuan.petheram@csiro.au Agriculture andFood Dr Ian Watson+61 7 4753 8606 Ian.watson@csiro.au