2.6.2.3.4 Representation of groundwater processes

Most of the rock strata within the model are fully saturated and Darcy’s equation governs the flow of groundwater. However, many of the water-dependent assets identified in the asset register for the Hunter subregion (see companion product 1.3 for the Hunter subregion (Macfarlane et al., 2016)) lie close to the surface, and are either within the unsaturated zone or are directly affected by it. Therefore, single-phase unsaturated Darcy-Richards physics is used in the model.

The groundwater model assumes water has a constant bulk modulus of 2 GPa, and that the Biot coefficient is 1, which is standard practice in poroelasticity (the study of fluid flows in elastic media, see Detournay and Cheng, 1993). Therefore, the specific storage does not include any effects from rock elasticity. If the Biot coefficient is less than 1 and the rock bulk modulus is not too large then the specific storage is increased because the rock grains can be squashed to accommodate more water. For example, if porosity is 0.05 and rock bulk modulus is 10 GPa, then the maximum specific storage (at Biot coefficient 0.525) is almost twice that used by the groundwater model. This causes the response time of any drawdown effect to be increased (but not linearly with respect to storage increase since the main driver of the timescales involved is the mine pumping rates) and reduces the magnitude of the drawdown. Therefore, by assuming the Biot coefficient is unity or that the rock is incompressible, the Hunter groundwater model will slightly overestimate the magnitude of drawdown. Since porosity is varied by more than an order of magnitude in the uncertainty analysis, it more than compensates for any changes in specific storage from varying the Biot coefficient. To assess the effect of altering the Biot coefficient or the rock bulk modulus, the emulators can be biased towards slightly higher porosity.

To represent the hydrological changes due to longwall mining on groundwater fluxes, hydraulic conductivities are enhanced in mesh elements above the longwall mines. Section 2.6.2.6 details the relevant parameters for defining magnitude and depth of enhancement.