Great Artesian Basin aquifers

The entire Maranoa-Balonne-Condamine subregion falls within the boundary of the geological Surat Basin, which forms part of the wider GAB. The variably confined layers of complex sandstone aquifers in the GAB are separated and confined by fine-grained mudstone and siltstone aquitards (Figure 9) (Ransley and Smerdon, 2012). The intake beds for the GAB outcrop areas cover much of the subregion in the north and the east where the alluvial cover is absent. North of Mitchell, in the northernmost extension of the subregion, there are outcrops of the entire Mesozoic sequence of the GAB. The Walloon Coal Measures in the Surat Basin are laterally continuous with the Birkhead Formation of the Eromanga Basin.

Groundwater gradients between the Surat Basin and the overlying alluvium indicate the potential for upwards leakage to alluvial deposits in parts of the subregion. The intense weathering of exposed GAB rocks prior to deposition of overlying alluvium resulted in a basin-wide saprolite layer of low permeability in the basal portion (Kellett and Stewart, 2013). This is considered to reduce connectivity with overlying systems except in some places where the saprolite has been removed by erosion. Kellett and Stewart (2013) identifies those areas of potential interaction between the alluvium and the GAB aquifers.

Regional conceptualisation

The OGIA model has simplified the hydrostratigraphy of the subregion by grouping stratigraphic units into major aquifers, aquitards and productive coal measures (Figure 11). These simplified hydrostratigraphic layers are based on similarities in aquifer properties that are appropriate for the regional scale groundwater model.

The Walloon Coal Measures comprises a varied sequence of sedimentary rocks, with permeability ranging from high to low, where the coal seams are the main water-bearing layers in a sequence of predominantly low permeability mudstones, siltstones and fine-grained sandstones (QWC, 2012). The OGIA model is conceptualised as 19 layers to represent all major aquifers, aquitards and productive coal measures. QWC (2012) state that ‘The Walloon Coal Measures is represented in the model by three layers:

  • an upper layer representing generally low permeability mudstone (Layer 9)
  • a composite middle layer representing all coal seams from the top of the uppermost productive seam to the base of the lowermost productive seam and the inter-bedded sediments (Layer 10); and
  • a lower layer representing a low permeability formation of dominantly mudstone (Layer 11).

Figure 11

Figure 11 Stratigraphy of the Surat, Bowen and Clarence-Moreton basins and the corresponding model layers in the Office of Groundwater Impact Assessment (OGIA) model

Fm = Formation, Mem = Member, Mst = Mudstone, Sst = Sandstone

Source: GHD (2012)

Regional groundwater flow

The potentiometric surface of the main confined aquifers (Gubberamunda and Mooga sandstones) shows that regional groundwater flow in the Maranoa-Balonne-Condamine subregion is from the intake beds in the northern and eastern areas of the subregion, flowing to the south-west (Figure 12) (Ransley and Smerdon, 2012). Groundwater flow within the Kumbarilla beds immediately south-west of The Range and west of New Acland Coal Mine is generally down the regional formation dip to the south-west towards the Taroom Trough. However, there is some debate surrounding groundwater flow directions in parts of the Hutton Sandstone in the Surat Basin in Queensland. Based on drill stem pressure test data of petroleum wells combined with water bore head measurements, Hodgkinson and Grigorescu (2012) indicated the potential for a significant component of northward groundwater flow from Chinchilla toward Taroom. This finding is counter intuitive, as expected groundwater flow directions are down dip, toward the south-west, and away from the Hutton Sandstone intake beds.

Ransley and Smerdon (2012) found that the watertable in the north of the Surat Basin lies in the GAB aquifer outcrop areas (intake beds) and aquitards along the western slopes of the Great Dividing Range. Detailed mapping reveals an area of flow loss to river baseflow from GAB aquifer outcrop areas where the Dawson River forms a deeply incised valley that cuts into the Hutton and Precipice sandstones (Figure 13). The interchange between aquifers and the Dawson River potentially accounts for some but not all of the northward flow directions proposed by Hodgkinson and Grigorescu (2012). It is likely that a localised component of northward groundwater flow occurs within the Hutton Sandstone near the Dawson River. The estimated groundwater flow loss from the GAB intake beds (mainly Hutton Sandstone) to the Dawson River is 8623 ML/year (Ransley and Smerdon, 2012).

Figure 12

Figure 12 Potentiometric surface of the main confined aquifers, showing regional groundwater flow directions in the Maranoa-Balonne-Condamine subregion

GAB = Great Artesian Basin, Main confined aquifers = Gubberamunda and Mooga sandstones.

Data: Bioregional Assessment Programme (Dataset 5); Geoscience Australia (Dataset 6, Dataset 7)

Figure 13

Figure 13 Watertable elevation and mapped GAB aquifer outcrops relative to the Dawson River

Data: Geoscience Australia (Dataset 8, Dataset 9, Dataset 10)

Last updated:
16 October 2018
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