2.6.2.5.1 Open cut mines

Table 5 Open-cut coal mines modelled in the Maranoa-Balonne-Condamine subregion

In this table, results are presented for only that part of the coal resource development pathway (CRDP) that can be modelled.

ACRD = additional coal resource development, BGL = below ground level, ML = mining lease, MLA = mining lease application, * = estimated

A conservative approach is used to simulate mine pit dewatering to account for the relatively coarse regional-scale model grid (1.5 km × 1.5 km grid cells). The MODFLOW Drain package is used to set the boundary conditions in the model cells corresponding to the mine pit footprints. For each mine, OGIA and the Assessment team used publicly available information to delineate the extent of operational and proposed pits, referred to as mine footprints, to represent coal mines in the revised OGIA model (Table 5). All model grid cells intersected by operational and proposed pit extents are assumed to be part of the modelled open-cut coal mine. The model grid cells for each of the modelled operational and proposed mine pits are shown in the insets in Figure 9.

It is assumed that mine pits are dewatered to the depth of the coal seams to enable coal mining operations. This is represented in the groundwater model by activating and deactivating the Drain cells to replicate the modelled development timeline shown in Figure 10. The Drain cell elevation in the Walloon Coal Measures model layer (Layer 10) is set to either the mean coal seam depth or, if the mean coal seam depth is below the bottom elevation of the model cell, then the Drain cell elevation is set to the bottom of the model grid cell. This means that the groundwater model draws the groundwater level down over the entire model grid cell where a pit is present and does not adjust for the proportion of the grid cell covered by the actual pit area. This approach may tend to overestimate groundwater drawdown associated with open-cut coal mine pit dewatering.