Assessing the status of groundwater in the Great Artesian Basin – Summary report

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.

Tags: EARTH SCIENCES, GAB, Great Artesian Basin, Published_External, groundwater
Organization: Geoscience Australia Data