2.6.2.9.3 Conclusions

For the Bioregional Assessment Programme, a GW AEM is developed to probabilistically estimate the hydrological change due to coal resource development in the Galilee subregion. The groundwater model provides the change in surface water – groundwater flux that is integrated in the surface water model, AWRA-L, which is reported in companion product 2.6.1 for the Galilee subregion (Karim et al., 2018).

The simulations indicate that the maximum drawdown in the upper Permian coal measures extends far into the Galilee Basin, with probabilities of exceeding 5 m drawdown in excess of 30% 50 km west of the coal mines. The Rewan Group aquitard acts as a regional seal that impedes upwards propagation of drawdown from upper Permian coal measures into the overlying Clematis Group aquifer and Eromanga Basin.

The Cenozoic cover and alluvial sediments are modelled as laterally infinite and continuous. As illustrated by the sensitivity analysis, drawdowns in this hydrostratigraphic unit are controlled by the direct extraction of water from this unit and to a much lesser extent by the extraction of water from the upper Permian coal measures. Drawdowns are specified as smaller or equal to 10 m. The probability of exceeding 0.2 m drawdown generally drops below 5% from about 25 km from the edge of the mine footprints.

Simulated drawdowns in the Clematis Group are the result of propagation of drawdown from the Cenozoic aquifer layer, which requires the unconfined aquifer system to be laterally continuous between the mines and the Clematis Group outcrop area.

The change in surface water – groundwater flux includes the change in baseflow to the Belyando River and the evapotranspiration by riparian vegetation. The maximum change is less than 1% of the baseflow estimated for the Belyando River, estimated in companion product 2.1-2.2 for the Galilee subregion (Evans et al., 2018). In companion product 2.6.1 for the Galilee subregion (Karim et al., 2018) this change in surface water – groundwater flux is integrated with the total streamflow to estimate the change in selected aspects of the hydrograph, summarised in hydrological response variables.

Companion product 3-4 (Lewis et al., 2018) for the Galilee subregion reports impacts on, and risk to, landscape classes and water-dependent assets arising from the simulated changes in groundwater and surface water reported in this product and in companion product 2.6.1 for the Galilee subregion (Karim et al., 2018).

The greatest potential in reducing the predictive uncertainty lies in improved characterisation of the surface water – groundwater interaction and the Cenozoic and alluvial aquifer systems. Further development of the numerical MODFLOW model (as outlined in Turvey et al., 2015), including its integration into a probabilistic framework, has great potential to improve the current predictions.