This web service provides access to geological, hydrogeological and hydrochemical digital datasets that have been published by Geoscience Australia for the Great Artesian Basin (GAB).

This report presents the results of an assessment of geoscience data and tools applied in the eastern Eromanga Basin to improve the hydrogeological conceptualisations. The assessment is one component of the Australian Government funded project ‘Assessing the Status of Groundwater in the Great Artesian Basin’. The results demonstrate that the application of existing and new geoscientific data and technologies has the potential to further improve our understanding of the Great Artesian Basin (GAB) hydrogeological system thus supporting the responsible management of basin water resources. Hydrogeological synthesis using airborne electromagnetic data, in conjunction with hydro- and chrono-stratigraphic data and well geological information, are effective at mapping the three dimensional distribution of the aquifers and aquitards. The results lead to an improved understanding of groundwater intake bed geometry, potential connectivity between aquifers, possible structural controls on groundwater flow paths, and plausible source of groundwater discharging as springs. In the southern part of the study area, the dominantly shale-rich Evergreen Formation is electrically conductive, but is locally resistive in places due to sand-rich facies. These areas allow hydraulic connectivity between the overlying and underlying Hutton and Precipice sandstone aquifers. Anticlinal folds and juxtaposed strata are observed on AEM traverses along the strike of the aquifer units, and includes the Hutton, Adori and Cadna-owie – Hooray sandstones. Abrupt folding and juxtaposed strata were interpreted as fault zones. Both structural features have the potential of controlling groundwater flow directions or groundwater storage compartmentalisation. The northern limits of Precipice Sandstone and Evergreen Formation are at Blackall and south of Barcaldine towns respectively. This zone also coincides with the southern edge of the east-west trending sub-surface Barcaldine Ridge where the basal Jurassic sequence abut against. On and north of the Barcaldine Ridge, the Cadna-owie – Hooray, Adori and Hutton sandstones are present. Mapping using AEM conductivity sections affirm that the Hutton Sandstone is the major aquifer in the northern part of the study area. The Poolowanna Formation, an age equivalent to the Evergreen Formation and Precipice Sandstone, is laterally extensive towards the northern part of the study area. This formation crops-out west of Lake Buchanan in the Great Dividing Range, but forms sub-crops elsewhere along the groundwater recharge areas. Numerous groundwater springs and spring clusters are present along the east and west of the outcropping sandstone hills in the Great Dividing Range. In the northern parts of the study area, source of groundwater for the springs are mainly derived from the Hutton Sandstone aquifers through either gravity-feed or lateral groundwater flow process. Polygonal faults mainly occur on conductive and fined-grained sedimentary units of the Rolling Downs Group. There is lack of observable evidence from AEM conductivity sections on the presence of polygonal faults to suggest preferential groundwater flows along these potential hydraulic conduits. Further investigation using ground based methods are needed to establish the presence of the faults and their hydraulic properties.

This web service provides access to geological, hydrogeological and hydrochemical digital datasets that have been published by Geoscience Australia for the Great Artesian Basin (GAB).

This report presents palynological data compiled and analysed as part of Geoscience Australia’s ‘Assessing the Status of Groundwater in the Great Artesian Basin’ project, commissioned by the Australian Government through the National Water Infrastructure Fund – Expansion. Diverse historic nomenclature within the Great Artesian Basin (GAB) Jurassic‒Cretaceous succession in different parts of the GAB makes it difficult to map consistently GAB resources across borders, at a basin-wide scale, in order to provide a geological and hydrogeological framework to underpin effective long-term management of GAB water resources. The study undertaken by MGPalaeo, in collaboration with Geoscience Australia, examined 706 wells across the GAB and compiled 407 wells, having Jurassic‒Cretaceous succession, with reviewed palynology data (down to total depth). This initial palynology data review allowed identification of new data samples from 20 wells (within the 407 wells) in Queensland and South Australia to fill data and knowledge gaps within the Jurassic‒Cretaceous GAB succession. This study resulted in: 1) a summary compilation of existing palynology data on 407 wells selected to create a regional framework between the Surat, eastern Eromanga and western Eromanga basins, to help regional correlations across the GAB, 2) a review of several different palynology zonation schemes and adaptation to a single consistent scheme, applying the scheme of Price (1997) for the spore pollen zonation and Partridge (2006) for the marine zonation, 3) updated stratigraphic charts across the Surat, Eromanga and Carpentaria basins, 4) identification of data and knowledge gaps, and 5) sampling of new palynology data to help fill some data and knowledge gaps identified in 13 key wells in the Surat Basin and 10 key wells in the Eromanga Basin. In the Surat Basin the new sampling program has targeted units within: the Evergreen Formation, Hutton Sandstone, Springbok Sandstone, Gubberamunda Sandstone, Orallo Formation, Mooga Sandstone, Bungil Formation. In the Eromanga Basin the sampling program targeted units within: the Poolowanna Formation, Hutton Sandstone, Adori Sandstone, Algebuckina Sandstone, Namur Sandstone and Hooray Sandstone. The study undertaken by MGPalaeo, in collaboration with Geoscience Australia, provides updated biostratigraphic information compiled in a standardised chronostratigraphic framework across the Surat, Eromanga and Carpentaria basins that mostly comprise the GAB. This work allows comparison of various geological, lithological, hydrogeological schemes. It provides links between various lithostratigraphic units, with different nomenclature, across jurisdictions. It also links these units to some key regional chronostratigraphic markers that can be used to generate consistent surfaces that correlate to aquifer and aquitard boundaries. The compilation of legacy and newly sampled and analysed palynology data allows refinement of a regional chronostratigraphic framework that can be used to map a common Mesozoic play interval scheme across all the resource types, for basin-scale assessments of groundwater, hydrocarbons, carbon capture and storage, and mineral potential. From this correlation of time equivalent geological units deposited in different environments, it is then possible to map internal lithological variations in stratigraphic facies within sequences that influence hydraulic properties and connectivity within and between aquifers across the GAB. The updated geometry and variability mapping within and between aquifers will help refine the conceptual hydrogeological model, to assess how aquifers and aquitards are connected within the GAB. The revised conceptual hydrogeological model can facilitate an improved understanding of potential impacts from exploitation of sub-surface resources in the basin, providing a basis for more robust water balance estimates.

This web service provides access to geological, hydrogeological and hydrochemical digital datasets that have been published by Geoscience Australia for the Great Artesian Basin (GAB).

Groundwater from the Great Artesian Basin (GAB) is a vital resource for pastoral, agricultural and extractive industries, as well as for town water supplies, supporting at least $12.8 billion in economic activity annually (Frontier Economics, 2016). It is an essential resource that supports Indigenous cultural values and sustains a range of groundwater-dependent ecosystems. The complex nature and large size of the GAB, in conjunction with increasing and competing demands for water to support new or expanding industries, communities and the environment, present complex challenges for the long-term management of the basin’s groundwater resources. Although considerable research has previously been undertaken to improve our understanding of the GAB groundwater systems, in large part by the individual jurisdictions, current knowledge varies across the basin, and there remain major hydrogeological knowledge gaps that limit management of groundwater resources in the GAB. A key challenge is to manage the groundwater resource in a way that protects existing values, accounting for inflows and outflows for the basin and ensures long-term access to artesian groundwater. The project ‘Assessing the Status of Groundwater in the Great Artesian Basin’ (referred to as the ‘Project’), was funded by the Australian Government through offsets from the former National Water Infrastructure Development Fund. Work under the Project is informing the Science Program of the National Water Grid Authority. The Project assessed existing and new geoscientific data and technologies, including satellite data, to improve our understanding of the groundwater system and water balance in the GAB, with focus areas in the northern Surat Basin (Queensland) and western Eromanga Basin (South Australia). This Project has revised and updated fundamental aspects of the GAB hydrogeological system understanding to underpin ongoing groundwater assessments and to guide water policy and resource planning in the basin. - Hydrogeological framework An updated classification of GAB aquifers and aquitards was produced, linking the hydrostratigraphic classification used in Queensland (Surat Basin) with that used in South Australia (western Eromanga Basin). This updated hydrogeological framework was produced at the whole-of-GAB scale, through the development and application of an integrated basin analysis workflow, resulting in an updated whole-of-GAB stratigraphic interpretation, consistent across jurisdictional boundaries. A total of 19 updated geological and hydrogeological surfaces were generated and used to produce a new three-dimensional hydrogeological model of the GAB. - Groundwater recharge estimation Regional groundwater recharge volumes were revised for aquifer outcrop areas along the entire eastern GAB recharge area using an improved groundwater recharge rate mapping method, integrating chloride concentration in groundwater, rainfall, soil clay content, vegetation type and surficial geology. The modelled 50th percentile (‘median’) of new groundwater recharge rate estimates, for the eastern GAB intake beds, range spatially from 0.79 mm/yr to 458.64 mm/yr, with a mean of 15 mm/yr. The modelled groundwater recharge rate output maps are subject to uncertainty. Using 1000 model replicates, the 5th percentile groundwater recharge rate map ranges from 0.06 mm/yr to 349.73 mm/yr (mean of 9 mm/yr) and the 95th percentile groundwater recharge rate map ranges from 1.19 mm/yr to 678.48 mm/yr (mean of 26 mm/yr). Groundwater recharge flow pathways into the main GAB aquifers were assessed using groundwater sample hydrochemical and environmental tracer analyses. Significant revisions to the mapped geometry and heterogeneity of the groundwater recharge beds were made using regional scale airborne electromagnetic (AEM) geophysical data, which identified the geometry of and potential connectivity between aquifers, possible structural controls on groundwater flow paths and plausible groundwater sources of spring discharge. - Groundwater system conceptualisation Revised groundwater system conceptual models of groundwater recharge processes and groundwater flow were developed. These revised groundwater system conceptualisations illustrate aquifer architecture and potential stratigraphic and/or structural variability which has the potential to affect groundwater flow paths. - Water balance estimates The water balance presented here has been undertaken to test incorporating and communicating uncertainty estimates as part of a whole-of-GAB water balance. The water balance incorporates new work, where available, from the jurisdictions and from this project (specifically groundwater recharge estimates). This project has produced quantified uncertainty for a single component of the water balance - groundwater recharge, the largest input component of the water balance. The water balance, as presented in this report, is not intended to represent a comprehensive critical appraisal of the techniques used by previous workers to estimate each element of the water balance, nor was it intended to develop new techniques for the estimation of the water balance elements other than groundwater recharge. The incorporation of a component of the uncertainty in the water balance of the GAB is new. The uncertainty in the water balance has always been there, in all past iterations, though it has not been quantified. This estimation of uncertainty is important in the communication of the water balance, as it highlights key issues such as: 1) a whole-of-basin water balance for the GAB using current information is not sufficiently detailed to be of use to water managers; and 2) local monitoring of groundwater levels and pressures remains the primary management tool for monitoring groundwater resources in the GAB. The Project produced a point-in-time assessment of the water balance of the GAB, comparing inflows (including long-term average groundwater recharge) and outflows to the main regional aquifers, for the year 2019 (The year 2019 was the latest year for which data was available at the start of the Project). The whole-of-GAB water balance, calculated using the 5th, 50th and 95th percentiles of modelled groundwater recharge rates, estimates a range of storage change volumes of -859 GL, -29 GL and +1,212 GL respectively, in 2019. The large variation in estimated storage volumes, ranging from a decreasing to increasing groundwater storage change, highlights the large uncertainty associated with the water balance when considering the groundwater recharge rate uncertainty. GAB sub-basin water balances also show a range of groundwater storage volume changes based on modelled groundwater recharge rates for the Eromanga Basin (-229 GL 5th percentile; 51 GL 50th percentile and 424 GL 95th percentile), Carpentaria Basin (-413 GL 5th percentile; -72 GL 50th percentile and 511 GL 95th percentile) and Surat Basin (-217 GL 5th percentile; -9 GL 50th percentile and 277GL 95th percentile). Using 50th percentile modelled groundwater recharge rates for major aquifer groups across the basin, water balance estimates for the Cadna-owie Aquifer Group and Precipice Aquifer Group suggest negative change in storage volumes, while water balance estimates for the Hutton - Injune Creek Aquifer Group and the Rolling Downs Aquifer Group suggest an increasing change in storage volumes. While the whole-of-GAB, sub-basin and major aquifer water balances provide basin-wide perspectives of the groundwater resource components, they also highlight the uncertainties associated with estimating groundwater recharge at a regional scale. The large range in groundwater storage values calculated for the water balance presented here, are too great to confidently provide a whole-of-GAB scale assessment of groundwater resources. - Assessment of new techniques for whole-of-GAB groundwater evaluation Assessments of new techniques, including spatial and temporal satellite data, show promising results for remote monitoring of groundwater levels at a whole-of-GAB scale. The new monitoring techniques are not currently operationalised and require further work to allow them to be integrated into current monitoring programs. The new techniques rely on ongoing groundwater level and pressure data, making existing on-ground groundwater monitoring networks essential for managing groundwater resources in the GAB. Gravity Recovery and Climate Experiment (GRACE) Gravity Recovery and Climate Experiment (GRACE) satellite derived groundwater storage change estimates were found to be largely consistent with calculated water balance groundwater storage change estimates, for GAB aquifers at the whole-of-GAB and sub-basin scale. The accuracy of GAB GRACE groundwater storage estimates is largely dependent on the accuracy of supplementary datasets required to account for gravity signals not associated with the GAB groundwater (e.g. surface water, soil moisture and shallow groundwater). The assessments undertaken for the Project indicate GRACE satellite observations are a powerful tool to remotely monitor confined groundwater storage change trends, at the whole-of-GAB and sub-basin scale over monthly to decadal time-scales. Interferometric Synthetic Aperture Radar (InSAR) Based on Interferometric Synthetic Aperture Radar (InSAR) satellite data, downward ground surface motion (subsidence) was shown to be associated with decreases in groundwater levels (drawdown) in aquifers of the northern Surat Basin focus area. In the western Eromanga Basin focus area, InSAR derived ground surface measurements were stable, consistent with stable groundwater levels over time. The Project has delivered the largest consistent mosaic of InSAR derived ground surface movement in Australia, with InSAR data processed for the majority of the GAB (~90% coverage). This assessment indicates that, where appropriate datasets for local scale corrections and interpretation are available, InSAR is a useful tool to remotely sense groundwater level changes over time. Groundwater flow model An assessment of whole-of-GAB scale groundwater flow modelling was undertaken using newly developed open-source geodynamic modelling code ‘Underworld’. The ‘Underworld’ code utilises high-performance computing capabilities and has the potential to produce groundwater flow simulations at unprecedented scale and resolution. A Bayesian-approach was applied to model simulations to characterise uncertainties with model results. The model outputs fit the hydraulic head input data acceptably and the results were consistent with the current groundwater system conceptualisation of the basin. The model has been developed as a proof-of-concept steady state model that currently has limitations. However, with further development the technique has potential to reconstruct past GAB groundwater flow regimes, testing revisions to GAB-scale hydrogeological conceptualisations and could be a useful tool to simulate basin-scale groundwater movement to complement and provide a broader context for local-scale groundwater flow models within the basin. - Data and knowledge gaps and recommendations for future work Remaining data and knowledge gaps were identified through the Project. Recommendations for future work are listed by theme below and include data acquisition, data integration and data processing at local, regional and whole-of-GAB scales to better constrain key groundwater system processes in order to improve sustainable resource management. Hydrogeological framework • Further updates to the geological framework, in particular New South Wales and Northern Territory, may be necessary to reduce lithostratigraphic interpretation uncertainty in areas where scarce palynological data combined with the presence of sandy units makes it difficult to interpret and to maintain consistency across jurisdiction boundaries. • Expand mapping of aquifer sand/shale ratios to quantify variability and connectivity within and between aquifers in targeted areas, particularly across jurisdiction boundaries. • Refine three-dimensional hydrogeological model in areas identified as having high uncertainty, such as the Carpentaria Basin. Groundwater recharge evaluation • Quantify the effects that aquifer geometry, lithological heterogeneity and structural influences have on groundwater recharge rates and pathways within the main GAB aquifers through acquisition of targeted complimentary geophysical, geological and hydrogeological data. • Rainwater sample acquisition and chemical analysis to reduce uncertainty of calculated regional groundwater recharge rates. • Acquisition of groundwater hydrochemistry and environmental tracer data to quantify key groundwater processes, including groundwater recharge rates and connectivity within and between aquifers. Groundwater system conceptualisation • Improved conceptualisation of saprolite, including hydraulic properties, extent, thickness, erosional variation, and structural disruption, to quantify the effect of saprolite on groundwater recharge processes. • Measure vertical pressure gradients and groundwater pressure-elevation profiles between GAB aquifers to conceptualise groundwater leakage, and in which direction. Water balance estimates • Standardise the methods used to estimate water balance components across the GAB and report uncertainty to increase confidence in the water balance values. • Estimation of uncertainty for all of the inflow and outflow parameters. • More exploration of how to incorporate uncertainty in the water balance calculations. Assessment of new techniques for whole-of-GAB groundwater evaluation • Develop large scale whole-of-GAB scale datasets to effectively remove non-groundwater components of GRACE signal and independently assess the GRACE estimated groundwater storage change. • Integrate InSAR with complementary datasets to correct for non-groundwater effects and identify groundwater signals at local and regional scales. • Apply the large scale groundwater flow model ensemble to estimate groundwater residence times and fluxes between aquifers as well as assess the role of hydraulic conductivity variations on groundwater flow paths with GAB aquifers. Science knowledge sharing and communication activities • Dedicated workshops with each State and Territory to share findings and identify major changes related to the new work that may have implications for water bore aquifer attribution, water level and pressure monitoring and water management. Groundwater science data and knowledge supporting the Great Artesian Basin Strategic Management Plan Australian and GAB state/territory governments have developed the Great Artesian Basin Strategic Management Plan (GAB SMP) in 2020 (DAWE, 2020). This Strategic Management Plan takes a principles-based approach to guiding governments, industry and the community in managing this important resource together to achieve economic, environmental, cultural and social outcomes for the GAB and its users. The outputs of the current project will contribute to the Great Artesian Basin Strategic Management Plan Principle 6 - Information and knowledge generation ensures that accurate, timely and readily accessible information supports good management of the Great Artesian Basin. This summary report is one of a number of products released as part of the GAB groundwater project. In addition to the key findings and outcomes presented here, companion technical reports, datasets and associated value-added products for the project are available for public use.

The Great Artesian Basin (GAB), a hydrogeological entity that contains predominantly the Jurassic-Cretaceous Eromanga, Surat and Carpentaria geological basins, is the largest groundwater basin in Australia. It underlies one fifth of the continent, including parts of Queensland, New South Wales, South Australia and the Northern Territory. Groundwater from the GAB is a vital resource for agricultural and extractive industries, as well as for community water supply. It supports cultural values and sustains a range of groundwater-dependent ecosystems. Water managers from each jurisdiction regulate GAB resources using hydrogeological conceptualisations based on a diverse historical geoscientific nomenclature that is often unique to a jurisdiction. However, the basin and its resources are continuous across borders, and recent studies have shown high spatial variability in the hydrostratigraphic units across the basin. There is, therefore, a clear need to map the geological complexity consistently at a basin-wide scale in order to provide a hydrogeological framework to underpin effective long-term management of GAB water resources. The present study is part of the Australian Government funded project ‘Assessing the Status of Groundwater in the Great Artesian Basin’ to refine the basin conceptualisation and water balance estimates (Figure 1.1). This study focuses on an updated GAB hydrogeological architecture by compiling and standardising existing and newly interpreted biostratigraphic and well formation picks from geological logs, 2D seismic and airborne electromagnetic data in a consistent chronostratigraphic framework. This framework is used to correlate geological units across the GAB. The basin-wide correlation identifies age-equivalent sediments in different depositional settings encompassing transgressive and regressive phases. Biostratigraphic control using a common unified zonation scheme is used to identify lithological correlations. Rock properties are attributed based on sediment facies deposited during similar geological events. The approach provides a consistent way of mapping the distribution and properties of aquifers and aquitards across the GAB. In particular, the refined correlation of Jurassic and Cretaceous units between the Surat and Eromanga basins improves the resolution of hydrogeological unit geometry and lithological variation that may influence groundwater flow within and between aquifers. The 3D hydrogeological architecture developed provides a model for refining hydrogeological conceptualisations and assists in revising GAB water balance estimates. Key findings are: • The new 3D model of the GAB extends the connectivity of aquifers across the entire GAB, with potential implications for jurisdictional groundwater management. For example, the Adori Sandstone, which was previously mapped largely in the central and eastern Eromanga Basin, potentially has connectivity with the time-equivalent Springbok Sandstone in the Surat Basin across the boundary between the two basins (the Nebine Ridge). Coincident with the Nebine Ridge is a groundwater divide that tends to segregate groundwater flow between the two basins. However, cumulative impacts from excessive pumping could cause the groundwater divide to migrate due to the continuation of sandstone unit (and connectivity) across the Nebine Ridge. In addition, the Adori Sandstone is connected with the time-equivalent Algebuckina Sandstone found towards the western margin of the Eromanga Basin, which suggests there is potential for connectivity from basin margin to basin centre. This key finding improves estimates of volume and distribution of sandstone of this aquifer across all GAB jurisdictions. • The extent of other hydrogeological units have also been refined. For instance the Cadna-owie-Hooray aquifer of Ransley et al. (2015) is now separated into two units 1. Murta Formation/Hooray–Namur–Mooga sandstones aquifer and the 2. Cadna-owie–Bungil formation and equivalents aquifer. The updated mapping highlights that the upper Cadna-owie‒Bungil‒Wyandra aquifer extends across the whole GAB, and is potentially confined by the underlying Murta and lower Cadna-owie‒Bungil aquitards and overlying Rolling Downs aquitard. Higher resolution mapping of sub-units within the Cadna-owie–Bungil–Hooray and equivalents aquifer provides an improved understanding of lithological variability and the potential compartmentalisation of groundwater that may be isolated from from regional flow paths (i.e. ‘dead ends’). The lithological variability mapping within hydrogeological units highlights zones of potential connectivity where leakage may occur between the deeper and shallower aquifers, affecting upward loss of groundwater from GAB aquifers in areas distal to the outcropping recharge beds. • The new lithology mapping also highlights that the Birkhead and Westbourne formations, classified as interbedded aquitard and tight aquitard, respectively, in the Eromanga Basin, correlate laterally with time-equivalent intervals within the Algebuckina Sandstone aquifer, suggesting connection between the Hutton, Adori and Namur‒Hooray aquifers across the central and western Eromanga Basin. • The new 3D model updates hydrogeological conceptualisations in the GAB and improves groundwater balance estimates for the GAB (Ransley et al., 2022.). It is also used to constrain a regional-scale groundwater flow dynamics model for the region, including uncertainty analysis within a Bayesian framework (Knight et al., 2022). This aspect of the study is assessing a powerful approach for solving non-unique inverse problems in terms of quantifying model uncertainty. This is crucial in providing a context for, and awareness of, uncertainties in system conceptualisation that need to be accounted for, or at least acknowledged up front. • This study compiles, collates and integrates existing and newly acquired geoscientific data characterising Jurassic Cretaceous geological units that represent the hydrostratigraphy of the GAB. The updated stratigraphy improved correlations between the Eromanga, Surat and Carpentaria basins leading to better hydrogeological interpretations at the whole of GAB scale. The work draws upon the results of other recent studies to gain new insights into the geological architecture and depositional history, which have implications for groundwater occurrence and flow within and between key GAB aquifers. This updated understanding has basin-wide implications for water management, and plays a key role in revising water balance estimates for the whole GAB. The chronostratigraphic approach used here can be applied at a national scale to correlate consistently hydrostratigraphic units, providing a broader context for groundwater systems assessments.

The GAB covers parts of New South Wales, Northern Territory, Queensland and South Australia, each with their own water management regimes. Regular assessment of the groundwater resources of the GAB at a whole-of-basin scale is important for reviewing the effectiveness and interaction of the different management approaches. However, historically this type of assessment has been undertaken on an ad hoc basis, by different agencies, for different reporting units, using different methodologies and input datasets, meaning it is difficult to compare assessments. The water balance presented here has been undertaken to test incorporating and communicating uncertainty estimates as part of a whole-of-GAB water balance. The water balance incorporates new work, where available, from the jurisdictions and from this project (specifically groundwater recharge estimates). This project has produced quantified uncertainty for a single component of the water balance - groundwater recharge, the largest input component of the water balance. The water balance, as presented in this report, is not intended to represent a comprehensive critical appraisal of the techniques used by previous workers to estimate each element of the water balance, nor was it intended to develop new techniques for the estimation of the water balance elements other than groundwater recharge. The incorporation of a component of the uncertainty in the water balance of the GAB is new. The uncertainty in the water balance has always been there, in all past iterations, though it has not been quantified. This estimation of uncertainty is important in the communication of the water balance, as it highlights several key issues: • using current information, a whole-of-basin water balance for the GAB is not sufficiently detailed to be of use to water managers • local monitoring of groundwater levels and pressures remains the primary management tool for monitoring groundwater resources in the GAB. The Project produced a point-in-time assessment of the water balance of the GAB, comparing inflows (including long-term average groundwater recharge) and outflows to the main regional aquifers, for the year 2019 (The year 2019 was the latest year for which data was available at the start of the Project). Most components of the water balance cannot be directly measured and in some cases reported values are long-term averages (e.g. groundwater recharge) and estimated values for the reporting time period. As such, the water balance relies on indirect measurements, long term averages and approximations, which have a level of inherent uncertainty resulting from the underlying assumptions used in their estimation. To calculate a whole-of GAB water balance, the project divided all the formations in the GAB into four different ‘aquifer groups’: Rolling Downs Aquifer Group; Cadna-owie Aquifer Group; Hutton – Injune Creek Aquifer Group; and Precipice Aquifer Group. This approach was developed by KCB (KCB 2016b,c,d) in Queensland and has been adopted for this Project, as it ensures all inflows and outflows from the GAB were included in the water balance. The whole-of-GAB water balance has been calculated, based on water balances estimates calculated for each constituent sub-basin – the Eromanga, Carpentaria and Surat basins. This was done to provide a picture of variations in the water balance across different parts of the GAB. The whole-of-GAB water balance, calculated using the 5th, 50th and 95th percentiles of modelled long term average groundwater recharge rates, estimates a range of storage change volumes of -859 GL, -29 GL and +1,212 GL respectively, in 2019. The large variation in estimated storage volumes, ranging from a negative to positive groundwater storage change, highlights the large uncertainty associated with the water balance. Using 50th percentile modelled groundwater recharge rates, sub-basin water balances shows an increase in groundwater storage for the Eromanga Basin (-229 GL 5th percentile; 51 GL 50th percentile and 424 GL 95th percentile) and a decrease in groundwater storage of the Carpentaria Basin (-413 GL 5th percentile; -72 GL 50th percentile and 511 GL 95th percentile) and Surat Basin (-217 GL 5th percentile; -9 GL 50th percentile and 277GL 95th percentile). Using 50th percentile modelled groundwater recharge rates for major aquifer groups across the basin, water balance estimates for the Cadna-owie Aquifer Group and Precipice Aquifer Group suggest a decrease in storage volumes, while water balance estimates for the Hutton - Injune Creek Aquifer Group and the Rolling Downs Aquifer Group suggest an increase in storage volumes. While the whole-of-GAB, sub-basin and major aquifer water balances provide basin-wide perspectives of the groundwater resource components, they also highlight the high uncertainties associated with estimating groundwater recharge at a regional scale. The large range in groundwater storage values calculated for the water balance presented here, are too great to confidently provide a whole-of-GAB scale assessment of groundwater resources. The techniques used to estimate some water balance components have improved, for example Queensland has developed a repeatable methodology for estimating unmetered groundwater extraction. This project shows that our ability to confidently model groundwater recharge to the GAB is still evolving, and will continue to improve as further investigations are undertaken. Limited hydrograph analysis showed that areas where formerly free-flowing artesian bores have been rehabilitated, through the Great Artesian Basin Sustainability Initiative and predecessor programs, are seeing stabilisation or even recovery of water levels. Monitoring bores and hydrograph analysis continue to be important elements of any water resource assessment. Undertaking the water balance for the GAB and sub-basins has been useful for highlighting components of the water balance that should be considered carefully by groundwater resource managers and where necessary, targeted for future research eg. groundwater recharge, consistent GAB wide bore discharge estimates and evapotranspiration.

The Great Artesian Basin Research Priorities Workshop, organised by Geoscience Australia (GA), was held in Canberra on 27 and 28 April 2016. Workshop attendees represented a spectrum of stakeholders including government, policy, management, scientific and technical representatives interested in GAB-related water management. This workshop was aimed at identifying and documenting key science issues and strategies to fill hydrogeological knowledge gaps that will assist federal and state/territory governments in addressing groundwater management issues within the GAB, such as influencing the development of the next Strategic Management Plan for the GAB. This report summarises the findings out of the workshop.

As part of the Great Artesian Basin (GAB) Project a pilot study was conducted in the northern Surat Basin, Queensland, to test the ability of existing and new geoscientific data and technologies to further improve our understanding of hydrogeological systems within the GAB, in order to support responsible management of basin water resources. This report presents selected examples from the preliminary interpretation of modelled airborne electromagnetic (AEM) data acquired as part of this pilot study. The examples are selected to highlight key observations from the AEM with potential relevance to groundwater recharge and connectivity. Previous investigations in the northern Surat Basin have suggested that diffuse groundwater recharge rates are generally low (in the order of only a few millimetres per year) across large areas of the GAB intake beds—outcropping geological units which represent a pathway for rainfall to enter the aquifers—and that, within key aquifer units, recharge rates and volumes can be heterogeneous. Spatial variability in AEM conductivity responses is identified across different parts of the northern Surat Basin, including within the key Hutton Sandstone aquifer. Consistent with findings from other studies, this variability is interpreted as potential lithological heterogeneity, which may contribute to reduced volumes of groundwater entering the deeper aquifer. The influence of geological structure on aquifer geometry is also examined. Larger structural zones are seen to influence both pre- and post-depositional architecture, including the presence, thickness and dip of hydrogeological units (or parts thereof). Folds and faults within the Surat Basin sequences are, in places, seen as potential groundwater divides which may contribute to compartmentalisation of aquifers. Discrete faults have the potential to influence inter-aquifer connectivity. The examples presented here demonstrate the utility of AEM models, in conjunction with other appropriate geophysical and geological data, for characterising potential recharge areas and pathways within the main GAB aquifer units, by helping to better define aquifer geometry, lithological heterogeneity and possible structural controls. Such assessments have the potential to further improve our understanding of groundwater recharge and flow path variability at local to regional scales. Acquisition of broader AEM data coverage across groundwater recharge areas, along with complementary geophysical, geological and hydrogeological data, would further assist in quantifying recharge variability, facilitating revised water balance estimates for the basin and thereby supporting GAB water resource management and policy decision-making.