2.3.2.1 Scope and overview

The scope of this section is to collate and summarise the connections in the hydrological cycle of the Gloucester subregion (see companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)), and to describe the flows and pathways that may be affected by coal mining and/or CSG development. The range of both existing and potential flows and pathways for water in the subregion will be summarised, without lengthy discussion of any individual component. Section 2.3.5 discusses specific pathways in the context of coal mining and CSG development, whereas companion product 2.5 for the Gloucester subregion (Herron et al., 2018) details complete water balances of all components.

The spatial extent of the flows and pathways is limited to the preliminary assessment extent (PAE) of the Gloucester subregion as defined in companion product 1.3 for the Gloucester subregion (McVicar et al., 2015). This covers a surface area of about 350 km² underlain by the geological Gloucester Basin, a north–south elongated sedimentary basin, along with a 1 km buffer either side of the Gloucester River flowing north to its confluence with the Manning River, and a similar buffer for the Karuah River flowing south to Port Stephens (McVicar et al., 2015, p. 20, Figure 6).

Open-cut coal mine operations in the Gloucester subregion have occurred at the Stratford Mining Complex since 1995, with four pits mined but only two currently (as of June 2015) operational, and at Duralie Coal Mine since 2003 (see companion product 1.2 for the Gloucester subregion (Hodgkinson et al., 2014)). The existing coal mine operations, along with any pits that have ceased production, are considered under the baseline coal resource development (baseline). All descriptions of the current and future coal mine operations are in Section 2.3.4.

From a groundwater perspective there are essentially three hydrogeological units in the Gloucester Basin:

The key to the pathways in the Gloucester subregion is that it is a geologically closed basin, so that any water that enters must be expressed as discharge within the basin. The important layer controlling the surface water – groundwater interactions is the shallow weathered and fractured rock layer (SRL). This layer underlies the alluvium entirely, and outcrops extensively across the rest of the surface of the Gloucester subregion. Groundwater recharge to the SRL is primarily via rainfall and, to a lesser extent, from surface runoff at the margins of the subregion (i.e. the overland surface water flowing towards the basin draining the surrounding mountain ranges). The rivers and streams that are hosted within the alluvial aquifer are typically gaining and connected with local groundwater (McVicar et al., 2014, p. 79, Section 1.1.6). This aquifer is recharged via river leakage during high flow and flood events, by diffuse rainfall infiltration, and upward discharge from the SRL.

For the near-surface system the water pathways centre on hydraulic pressure and open-cut mining. CSG exploration and production reduce the groundwater level locally due to pumping out excess water to allow gas to flow and be collected. Local pressure drops can be transferred to the SRL and then to the alluvium where any diffuse pathway exists, and via fractures and structural features. If the underlying groundwater level of the SRL decreases enough, then it may induce flow downward and reduce stream discharge potentially turning a gaining stream into a losing stream. There may also be the possibility that a leaky well would allow the transfer of water to any layer above it, including to the surface.

For open-cut mine operations, local mine site dewatering is required so that coal can be mined and the mine is not flooded from incoming flows. This will induce water to flow toward the pit locally, and again will have an effect on the SRL, and potentially the alluvium if the proximity and hydraulic properties allow it. In such cases, water that would normally be discharged to the river and support baseflow in the surface water network could be drawn away.