2.3.2 Summary of key system components, processes and interactions


The protracted geological history of deposition, deformation, uplift and erosion that has occurred in the Galilee subregion has implications for the degree of hydraulic connectivity that may exist between various geological units, coal resource development and water-dependent assets. This geological overview includes a brief review of the geological history. Specific aspects of the geological framework that are likely to have an important bearing on hydraulic connectivity include: variations in the thickness and extent of Moolayember Formation; areas where the Hutton Sandstone is in direct contact with Clematis Group or upper Permian coal measures; thickness of sediments and sedimentary rocks that overlie the upper Permian coal measures; and location and extent of faulting, in particular around the margin of the geological Galilee Basin.

The hydrogeological conceptualisation outlines three major groundwater systems hosted in different geological basins: (i) Cenozoic aquifers, (ii) Great Artesian Basin (GAB) aquifers (Eromanga Basin), where the main aquifers are the regional watertable aquifer developed in Winton-Mackunda formations and the Wallumbilla Formation and the GAB regional artesian aquifer sequence (Hutton Sandstone to Cadna-owie—Hooray Sandstone), and (iii) Galilee Basin, which comprises the Clematis Group aquifer, and aquifers in the upper Permian coal measures and Joe Joe Group.

Groundwater flow as inferred from potentiometric surface mapping of the regional watertable in the Eromanga Basin is controlled by local and regional topography and focuses towards major drainage lines. The GAB regional artesian aquifers outcrop as a prominent ridge along the eastern margin of the Eromanga Basin. Much of the groundwater flow is inferred to head in a westerly direction away from outcrop areas. West of outcrop areas, GAB regional artesian aquifer is confined by the Rolling Downs Group aquitard. In GAB outcrop areas, some east-directed flow towards the GAB aquifer margin forms a largely unconfined local groundwater system on the eastern side of the ridge. A line of springs has formed in the vicinity where GAB regional artesian aquifers become confined by the overlying Rolling Downs Group aquitard. Another line of springs occurs as discharge areas at base of ranges for local east-directed groundwater systems.

Groundwater flow in the Galilee Basin (Clematis Group aquifer, upper Permian coal measures and Joe Joe Group) is more complex than in overlying GAB aquifers. Features that are unique to the hydrodynamics of the Galilee Basin aquifers are: (i) a north–south trending groundwater divide, located to the north of the Barcaldine Ridge. This feature has been found to be common to all Galilee Basin groundwater systems for which groundwater mapping has been undertaken as part of the BA for the Galilee subregion and (ii) trends in coal seam gas (CSG) content, its relationship to groundwater systems and structure.

The major groundwater divide segregates the Galilee Basin groundwater systems into easterly and westerly flow components, with potential discharge occurring towards the margins of the Galilee Basin. One hypothesis is that regional-scale landscape features and processes may be influencing the hydrodynamics of the Galilee Basin aquifers and the location and formation of this regional groundwater divide. Some examples of landscape features and processes include broad topographically elevated areas situated between the Great Dividing Range and the Eromanga Basin margin that encompasses lakes Galilee and Buchanan, and headward erosion and incision of the Carmichael River offsetting the location of the Great Dividing Range.

CSG pressure in coal seams may be less of an influencing factor, as trends in CSG cross-cut the groundwater divide. Furthermore, the groundwater divide feature is common to all aquifers in the Galilee Basin; it is not a feature that only occurs in the upper Permian coal measures. Distribution of CSG resources is inferred to be influenced by east trending geological structures in the upper Permian coal measures and inferred groundwater flow direction that occurs in the vicinity of identified CSG resources.

Surface water – groundwater interactions in the Galilee subregion take the following forms: baseflow from shallow groundwater systems to rivers, losing streams (surface water recharging shallow aquifers), spring discharge to outflow pools, discharge to lakes (e.g. Lake Galilee) and shallow groundwater utilised by deep-rooted plants. The Galilee subregion encompasses the headwaters of seven major river basins with almost all proposed coal mine developments situated in the headwaters of the Burdekin river basin. Surface water flow is strongly seasonal and, from year to year, can vary greatly from almost none to significant floods. Mean annual potential evaporation far exceeds rainfall, particularly in the summer months when rainfall is highly variable for most of the subregion. The lack of continuous surface water flow throughout the year shows that groundwater-controlled baseflow to rivers is often not sufficient to keep rivers continuously flowing during prolonged low rainfall periods.

Due to variability of surface water resources there is a strong dependence on groundwater supplies. Groundwater is utilised for agricultural purposes, town water supplies and industry. Most groundwater in the Galilee subregion is extracted from GAB aquifer systems, the Hutton Precipice and the Wyandra-Hooray aquifers. In the Galilee Basin, most extraction occurs from the Clematis Group aquifer.

Last updated:
17 December 2018