2.6.2.2.7 Suitability of existing groundwater models

The mine-scale numerical groundwater models developed by proponents in the Galilee subregion are deterministic models developed for the purpose of assessing the impact of a single mining development. While the relatively small scale of these models allows for higher resolution in cell size and parameter variability, treating each coal resource development in isolation will not account for the possibility of cumulative impacts on the groundwater system.

Several of the mine-scale models have attempted to account for cumulative impacts using the principal of superposition. That is, where the drawdown contours of two models overlap, the total drawdown in that area is calculated by summing the drawdown predicted by the overlapping models. This is a straightforward way of accounting for cumulative impacts, but is relatively simple, and does not account for the more complex impacts that multiple coal projects may have on local to regional groundwater systems, such as local changes in groundwater flow direction, reductions in subsurface flow volumes, or changes to recharge volumes and distribution. The way mine-scale models clump the Galilee Basin and Eromanga stratigraphic sequences varies from model to model. Ideally, all coal resource development should be represented in a single numerical groundwater model to accurately simulate the combined impact they will have on both local and regional-scale groundwater flow systems.

While the mine-scale models outlined in previous sections show good correlation between observed and predicted groundwater levels, in many cases the data available for calibration only covers a short time period, for many bores this is less than five years. In predicting water levels decades or centuries into the future this places a great deal of significance on data representing a small part of a complex system.

The groundwater models built by the mine proponents (Alpha and Kevin’s Corner, Carmichael, China First, China Stone and South Galilee coal projects) can provide some degree of guidance at a semi-regional scale on cumulative impacts but for reasons stated above, as well as issues surrounding the availability of models for use in the Bioregional Assessment Programme, these models are not suitable to use in this analysis of the Galilee subregion.

The Galilee Basin hydrogeological model (Section 2.6.2.2.6) encompasses the whole Galilee subregion and the second predictive run of the transient model (Section 2.6.2.2.6.3) incorporates the Galilee subregion’s modelled CRDP. This model does take into account regional complexities as well as changes to hydraulic properties as mining progresses in mine areas. However, model limitations (Section 2.6.2.2.6.4) suggest that further refinements would improve the model’s outputs and its predictive capacity. Also, as outlined in Section 2.6.2.1, BA modelling methodology takes a probabilistic approach in order to provide an estimate of uncertainty for modelling results. Due to long model runtimes of the GBH model (around 20 hours for a single evaluation) and concerns regarding the numerical stability, it was not considered suitable for BA use without additional investment in model development. This was not possible within the time frame of the Galilee subregion BA.

As described in the following sections there are other approaches to groundwater modelling that can determine cumulative drawdown with estimates of uncertainty within reasonable time frames. Results from these types of models can be used to inform and improve complex regional numerical groundwater models such as the GBH model.

As indicated in Section 2.6.2.1, the analytic element model method is chosen for this study. Table 10 gives a high level overview of the differences and similarities between the GBH model and the analytic element model approach. The section numbers indicated where the model is discussed in this product. The assumptions and limitations of this alternative modelling approach are discussed in the context of the predictions in Section 2.6.2.8.2 .

Table 10 High level overview of differences and similarities between Galilee Basin hydrogeological (GBH) model and analytic element model approach

CDRP = coal resource development pathway