2.6.2.9.3 Conclusions

The Hunter subregion groundwater model is developed with the finite element, multiphase groundwater flow simulator MOOSE (Multiphysics Object-Oriented Simulation Environment) to probabilistically assess the drawdown (dmax) due to the additional coal resource development, and year of maximum change (tmax), as well as provide the change in surface water – groundwater flux as a boundary condition for the surface water modelling reported in companion product 2.6.1 for the Hunter subregion (Zhang et al., 2018).

For more than three-quarters of the model output nodes, the median value of the simulated dmax is less than 0.2 m. The probability of exceeding this threshold is 100% within the immediate vicinity of the mine footprint area and decreases rapidly with increasing distance from the mine. The contour of 5% probability of exceeding 0.2 m drawdown is generally within 20 km of the mine footprint boundary. This also means that the zone of hydrological change of two mines only overlaps when they are within 20 km of each other.

Generally, the magnitude of drawdown due to the additional coal resource development, which is the difference in drawdown between CRDP and baseline, is most sensitive to the porosity and hydraulic conductivity; somewhat sensitive to the mine pumping rates, the decay of hydraulic conductivity with depth and the riverbed conductance; and much less sensitive to the river stage height, the rainfall recharge, the decay of porosity with depth, the ratio of vertical to horizontal conductivity, and the conductivity-enhancement ramp function.

The tmax varies between 2012 and 2102 and thus spans the entire simulation period. It indicates that while dmax can be achieved during mining operations, it is very likely that dmax is attained in the decades after mining ceases. The tmax increases with increasing distance from mine tenements. The largest drawdowns due to additional coal resource development occur in close vicinity of the mines, within or shortly after the peak mining period and within the simulation period. Further away from the mines, the drawdown due to additional coal resource development takes longer to reach a maximum, potentially beyond the simulation period, but as the drawdowns are not likely to be significant and are increasingly uncertain, there is little to be gained through extending the simulation period to provide a more precise estimate of tmax.

The simulated changes in surface water – groundwater flux are integrated into the surface water modelling, reported in companion product 2.6.1 for the Hunter subregion (Zhang et al., 2018). The additional coal resource development can sometimes lead to an increase in baseflow for some parameter combinations. The possibility of baseflow increases is consistent with the understanding of the dynamics of the groundwater system during and after mining as well as with the conceptualisation of the groundwater model, although observations of mine-induced baseflow increases have not been reported in the literature.

The probabilistic estimates of dmax are constrained by a distance-based weighting of groundwater level observations and by estimates of total streamflow. The groundwater level and streamflow observations mostly constrained the drainage level assigned to river nodes in the model and to a lesser extent the hydraulic properties of the groundwater model. The predictions of dmax were not sensitive to the drainage level, but to the hydraulic properties and, to a lesser extent, the mine pumping rates and the implementation of the hydraulic enhancement after longwall mine collapse. The latter two parameters are not strongly constrained by the observations.

The probabilistic hydrological changes presented in this product will form the basis of the further receptor impact modelling reported in companion product 2.7 for the Hunter subregion (as listed in Table 2) and the impact and risk analysis reported in companion product 3-4 for the Hunter subregion (as listed in Table 2). The significance of the groundwater changes due to additional coal resource development relative to groundwater changes under the baseline are also considered in the 3-4 product.

Input data, model files (including pre- and post-processing scripts and executables) and results are available at www.bioregionalassessments.gov.au.