2.6.2.3.2 Hydrogeological conceptual model

The conceptual understanding of the Namoi subregion is defined in companion product 2.1-2.2 (Aryal et al., 2018a) and companion product 2.3 (Herr et al., 2018) and summarised in Section 2.6.2.1. This section pertains to the conceptualisation of groundwater flow in the alluvial and deeper sedimentary layers in the parts of the Surat and Gunnedah basins that are included in the numerical groundwater modelling for the Namoi subregion. The main geological domains in the Namoi subregion are, from oldest to youngest, the Permian Gunnedah Basin, the Jurassic to Cretaceous Surat Basin and the Cenozoic alluvium. Hydrogeology in this region can be conceptualised as consisting of three distinct but connected groundwater flow systems comprising shallow alluvial groundwater sources, deep groundwater sources primarily in the Pilliga Sandstone and other confined aquifers, and the surface water sources within the Namoi River and connected streams and creeks.

Quaternary-age alluvial deposits occur along the Namoi River and creeks feeding into the river. They are important sources of fresh groundwater for the subregion and have higher hydraulic conductivities than the underlying sedimentary rocks. The aquifers in the alluvium are major groundwater sources supporting agriculture in the Namoi subregion. The major regional groundwater source in the Surat Basin in the Namoi subregion is the Pilliga Sandstone. Hydrogeologically, sandstone formations typically act like aquifers (i.e. units capable of transmitting and storing useful quantities of groundwater), whereas shale and siltstone layers have hydraulic properties typical of aquitards. Non-alluvial near-surface rock units are typically more weathered and have higher hydraulic conductivities than deeper rock units and are commonly only partially saturated.

The subregion boundary to the eastern side is defined by the Hunter-Mooki Thrust Fault, which separates the geological Sydney Basin from the Gunnedah and Werrie basins (see Section 2.3.2.2.1 of companion product 2.3 for the Namoi subregion (Herr et al., 2018)). Since this is the edge of the basin, it is assumed to be a zero-flow boundary, as discussed further in Section 2.6.2.4. However, contouring of the base of the alluvial sediments indicates they form continuous units across the Hunter-Mooki Thrust Fault, extending beyond the eastern boundary of the Namoi subregion, similar to the surface water catchment. Regional-scale groundwater flow generally follows the direction of the topography from an east to north-westerly to westerly direction.

Losses from the Namoi River (including flood recharge) and irrigation recharge are the major inputs to the groundwater system in the Lower Namoi Alluvium. In Section 2.1.5 of Aryal et al. (2018a), river connectivity to the alluvial aquifers is described in detail. High levels of historical groundwater use have impacted on the surface water – groundwater interaction in Lower Namoi, converting the river to be a predominantly losing stream. CSIRO (2007) estimated that the total average impact on tributary streamflow by 2100 would be a loss to groundwater of 19 GL/year more than that included in the river planning models examined. Discharges of groundwater to gaining streams (i.e. baseflow) sustain flow in the Upper Namoi reaches where the watertable is shallow. Along the eastern extent of the Great Artesian Basin outcrop, it is considered that Pilliga Sandstone is providing baseflow to the river (Herczeg, 2008) but estimates of their contribution to total flow are highly variable. Because of this uncertainty, model parameters that control baseflow are varied in the uncertainty analysis (see Section 2.6.2.7).

Coal mining is undertaken using open-cut and longwall mining methods in the six major baseline mines and eight modelled additional coal resource developments of the Namoi subregion. These methods of coal extraction involve mine dewatering, resulting in aquifer depressurisation. The methods of extraction modify subsurface physical flow paths, particularly above longwall mines where hydraulic enhancement is an inevitable consequence of collapsing the longwall panels. The effects of these changes are drawdown of the watertable (and confined aquifers) and changes in the magnitude and timing of exchanges with streams that are connected to groundwater.

Details of the datasets and data analyses that have informed the conceptualisation and development of the groundwater model are provided in Section 2.1.3 and Section 2.1.5 of companion product 2.1-2.2 for the Namoi subregion (Aryal et al., 2018a). They include the mapped extent of the Namoi alluvium (Section 2.1.3.1.3), the generation of a spatially varying rainfall-recharge surface for the subregion (Section 2.1.3.1.4), results from the analysis of hydraulic conductivity measurements by lithology (Section 2.1.3.1.2), and an assessment of the surface water – groundwater interactions (Section 2.1.5). Details of the mine footprints and flow rates (i.e. the assumed pumping rates to dewater mines) used to represent the hydrological changes due to mining are provided in Section 2.1.6.