2.6.2.3 Model development

Summary

The numerical groundwater model aims at evaluating the direct and indirect impacts of additional coal resource development on water resources in the Richmond river basin of the Clarence-Moreton bioregion. The hydrogeological conceptualisation of the groundwater model is underpinned by a three-dimensional geological model of the Clarence-Moreton Basin. The numerical groundwater MODFLOW model comprises six numerical layers that represent the major types of hydrostratigraphic units in the Richmond river basin within the Clarence-Moreton bioregion. A small area of the Richmond river basin close to the Mount Barney intrusion was excluded from the northern part of the model domain. The western part of the groundwater model domain was extended to include the entire extent of the Walloon Coal Measures. The outcrop or subcrop areas of the older sedimentary bedrock units (Koukandowie Formation and the Gatton Sandstone) were excluded from the eastern and western parts of the groundwater model domain. Faults were not included in the current implementation of the Clarence-Moreton groundwater model due to the lack of information.

Groundwater recharge in the Clarence-Moreton bioregion occurs via different mechanisms, which include diffuse rainfall recharge and surface water recharge. The entire extent of the Lamington Volcanics forms a preferential recharge area with maximal hydraulic connection between the streams, and the alluvial and volcanic aquifers in the headwaters of the Richmond river basin. Diffuse recharge from precipitation seems to be the more dominant recharge process in the lower catchment. In the eastern part of the Richmond river basin, upwelling of groundwater from the sedimentary bedrock into the shallow alluvial aquifers likely occurs. The three-dimensional geological model for the Clarence-Moreton Basin suggests that groundwater flows towards the lowest point in the eastern part of the Richmond river basin, where it discharges to the alluvium and streams.

The transient stage of the groundwater model spanned a total simulation period of 120 years from 1983 to 2102. Available observations were used to constrain the model parameters during the historical period from 1983 to 2012. During the prediction period that spanned a period of 90 years from 2013 to 2102, the 95 coal seam gas (CSG) wells were gradually activated from 2018 to 2036. Monthly stress periods were adopted throughout the simulation, which led to a total of 1441 stress periods including the first steady-state simulation. The model domain was discretised into 248 rows and 184 columns. Grid spacing varied from 200 to 800 m. MODFLOW-NWT was adopted to execute the numerical model.