Great Artesian Basin - understanding geology and hydrogeology

Objectives

A three dimensional cross-section through the GAB from West to east, cutting across the main Eromanga Basin depocentre. The image also shows deeper underlying basins.

Cross section of the Great
Artesian Basin and deeper
underlying basins.

Through a variety of programs and projects funded by various federal government departments, including the Department of Sustainability, Environment, Water, Populations and Communities External site link, Geoscience Australia has an ongoing commitment to investigate, assess and further our understanding of the hydrogeology of the Great Artesian Basin (GAB). One such project was the GAB Water Resource Assessment (GABWRA). Ongoing strategic investigations ensure Geoscience Australia continues to be at the forefront of GAB research and analysis. Our expertise in geology, hydrogeology, hydrogeophysics and hydrochemistry allow for a holistic, transdisciplinary approach to assessing the GAB.

Outputs

Geoscience Australia has been strategically developing and enhancing the value of existing data with rigorous re-assessments. Some recent innovative results include:

  • the connectivity of overlying Quaternary alluvium with GAB aquifers
  • an elevation map of the regional watertable in the GAB
  • the connectivity of underlying basins with GAB aquifers
  • mapping of Cenozoic cover and isopachs in the GAB and
  • groundwater use by riparian vegetation with the GAB.

The continued works of Geoscience Australia in the GAB incrementally advances our understanding of the hydrogeology of the system.

Outcomes

To date, studies of the hydrogeology of the GAB have paid much attention to the flowing artesian bores in the Hooray Sandstone and equivalent aquifers, but scant attention has been focused on the Early Cretaceous sub-artesian aquifers. The regional watertable elevation mapping relates to the Early Cretaceous Winton-Mackunda aquifers which host the watertable over large parts of the GAB. A key outcome of this study is the identification that the two largest rivers (Cooper Creek and the Diamantina River) in the Eromanga Basin are prominent watertable drains. Local-scale groundwater systems also operate in the unconfined aquifer, as the watertable contours are not smooth like the artesian pressure surface, but form local recharge mounds.

The GAB overlies basement that is a mosaic of crystalline rocks and older sedimentary basins. Many of these older underlying formations within each of these basins are in contact with the base of the GAB. The derived connectivity map shows the nature of the geological contact between the GAB and the underlying older basins and provides insights into the potential for groundwater interaction. With the recent exploration and proposed development of coal, gas and mineral abstraction from underlying basins - and even from within the GAB sequence - conceptual understanding and quantification of the potential hydraulic inter-connection is essential. This conceptual connectivity is a critical first step to the next phase of quantifying potential fluid movements.

This project investigates potential recharge zones to the GAB aquifers of the Surat Basin from downward leakage through the Quaternary alluvial aquifers, usually via rivers. It also investigates potential areas of upward leakage from the GAB to the Quaternary alluvium and river systems. Contours of recent water level measurements in the Quaternary alluvial aquifer in the Surat Basin demarcate areas where water levels in the Quaternary alluvium are higher than the regional watertable in the uppermost GAB aquifer, and areas where the GAB regional watertable is coplanar with or higher than the water levels in the alluvium.