2.6.2.9 Limitations and conclusions

Summary

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 due to the additional coal resource development, and the year of maximum change, as well as to 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).

Model results indicate a 100% probability of exceeding a 0.2 m drawdown within the mine footprint areas, but this reduces with increasing distance from the mines. The contour of 5% probability of exceeding 0.2 m drawdown is generally within 20 km of the mine footprint boundary.

The year of maximum change varies throughout the Hunter subregion. It is most likely to be during the decades after mining activity ceases, and it increases with increasing distance from mine tenements.

The Hunter subregion groundwater model is a stochastic regional-scale model: it has a large modelling domain and a relatively coarse model resolution. As such, it does not provide a deterministic result and it does not represent the level of lithological and hydrogeological information that is represented in local-scale groundwater models that have been built for small areas (i.e. individual coal mines) within the Hunter subregion.

Opportunities to improve the model can be directed to better constraining the assumptions that have the most influence on model results. 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; and somewhat sensitive to the mine pumping rates, the decay of hydraulic conductivity with depth, and the riverbed conductance. Generally, the change in surface water – groundwater flux is most sensitive to riverbed elevation (and thus river stage height), followed by the hydraulic conductivity, and the riverbed conductance. Neither is very sensitive to the recharge, the decay of porosity with depth, the ratio of vertical to horizontal conductivity, and the conductivity-enhancement ramp function.