Alluvial aquifer interactions

The alluvial aquifer of the Hunter subregion is considered as a regional discharge zone for the aquifers within the region (EPA, 2013), and when formulating WSPs for the region the alluvial aquifer is considered as its own unit (DPI, 2009; 2013). This treatment of the alluvial aquifer implies an interaction, a transfer of water, from the groundwater to surface water system through the alluvial aquifer (the distribution of the alluvium in the Hunter subregion is shown in Figure 37, Section 1.1.4). For example, the current WSP for the Regulated Hunter River Alluvium (DWE, 2009) gives rules on the drilling and operation of new bores in the alluvium near groundwater‑dependent ecosystems. In general they must be more than 40 m from a first order or second order stream, unless they are drilled into underlying parent material and slotted at 30 m or deeper, and no impact on baseflow is demonstrated. This rule assumes that the aquifers can transfer water between each other, and affect stream baseflow in the regulated sections.

The main recharge mechanisms for the alluvial aquifer are: river leakage to the alluvium, direct rainfall recharge, and upward flow from Permian fractured rocks. River leakage is generally considered to be the largest recharge component, and in various modelling studies it has been fitted as up to four times greater than diffuse rainfall recharge (Worley Parsons, 2009; Heritage Computing, 2012). Further, in these studies and other meta-analyses the connection between the alluvial aquifer and underlying fractured rocks has been considered bi-directional (EPA, 2013).

Mining operations have been seen to affect groundwater gradients in the alluvial aquifer. In the case of a study at North Wambo Underground Mine, changes in the observed behaviour of hydrograph rise and recession was closely correlated to changes in mine operations and expansion of underground mining areas (Heritage Computing, 2012). Alluvial bores sufficiently downstream from operations showed no apparent effects. At the open-cut Mount Arthur Coal Mine, dewatering of Permian fractured rocks from mining operations led to a reversal of the pre-mining gradient between the alluvial aquifer and Permian fractured rocks, so that water levels were now higher in the alluvial aquifer (Australasian Groundwater and Environmental Consultants, 2013, p. 37, Figure 14). This did not result in a significant change in water levels in the alluvial aquifer despite occurring in a wetter period locally, which indicates either compensating leakage from the river, or only a limited connectivity between the two aquifers.

Last updated:
18 January 2019
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