2.6.2.3 Model development

Summary

The objective of groundwater modelling undertaken as part of bioregional assessments (BAs) is to assess the cumulative impacts of coal resource development on water-dependent assets.

The groundwater analytic element model (GW AEM) is designed to predict changes in groundwater level at specific model nodes, resulting from the cumulative impact of pumping for mine dewatering within the upper Permian coal measures of the Galilee subregion.

The Galilee subregion is host to three groundwater systems, which generally operate independently but in some places are hydraulically connected. The three systems in order from youngest to oldest are:

1. Cenozoic aquifers
2. layered Mesozoic aquifers and aquitards of the Eromanga Basin (Great Artesian Basin)
3. layered Permo-Triassic aquifers and aquitards of the Galilee Basin.

The GW AEM simulates a simplified hydrostratigraphic model representing the upper Galilee Basin sequence and overlying Cenozoic cover along the eastern margin of the subregion, in the vicinity of coal resource development pathway (CRDP) mines. The model excludes the early Jurassic to late Cretaceous Eromanga Basin sequence comprising the Hutton Sandstone and Hooray Sandstone and aquifers in the Winton-Mackunda formations. Inclusion of these formations in the model is only warranted if sizeable drawdowns are predicted in the underlying Clematis Group.

For this groundwater model the analytic element methodology is selected (Bakker, 2013) using the open-source implementation available in TTim (Bakker, 2015). This grid-independent, flexible groundwater methodology allows a comprehensive uncertainty analysis for a fraction of the computational cost of such an analysis with a finite-difference model.

The groundwater system is implemented as an alternating sequence of aquifers and aquitards, represented by seven model layers. Changes in groundwater level and flow are only simulated for the aquifers. The model layers are horizontal, with constant thickness. The top of the model is set to an arbitrary level of zero metres and the total nominal thickness of the simulated sedimentary column is 600 m.

The model calculates a time series of the change in groundwater level within the upper Permian coal measures and aquifers above, resulting from groundwater discharge associated with coal resource developments identified in the CRDP.