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.