Connectivity mapping

The majority of surface water – groundwater interactions in the Gloucester subregion are considered to occur within the alluvial deposits adjacent to major creeks and rivers (Parsons Brinckerhoff, 2013a). The alluvium typically comprises a layer of clay 3 to 4 m thick, overlying 8 to 12 m of coarse sands and poorly mixed gravels, and is not expected to exceed 15 m total depth. The system as a whole is an unconfined to semi-confined aquifer. It is heterogeneous with highly variable conductivity, but is overall quite permeable with rapid recharge, throughflow and discharge. The average hydraulic conductivity is 10 m/d, with a range of 0.3 to 100 m/d (Parsons Brinckerhoff, 2013b).

Hydrographs from bores drilled and screened in the alluvium show a response to high rainfall and streamflow events, and the ponding associated with these events. Some observations of the vertical head gradients indicate that recharge to the alluvium is highest at the margins of the geological basin (Parsons Brinckerhoff, 2013a). This is due to surface runoff from the elevated and rocky areas there. Due to the heterogeneous nature of the alluvium, bore hydrograph responses can be divided into two broad groups. In the relatively more permeable areas there is a rapid rise in response to recharge and a steep recession to previous levels occurs within one or two months. In the less permeable areas that have greater clay content and semi-confined conditions, hydrographs respond rapidly when recharge passes a threshold, then recede slowly over several months, sometimes not fully recovering before the next major event (Parsons Brinckerhoff, 2013a).

Stream gauging data, when compared with nearby groundwater level data, indicate that the Avon River and Dog Trap Creek are gaining streams under most conditions. Adjacent bore water levels are consistently one to two metres above the stream level indicating the streams are discharge features for shallow groundwater in the local alluvium (Parsons Brinckerhoff, 2013b). Both upward and downward gradients were observed in nested bores in the Stratford area, indicating the possibility of both upward and downward leakage between the alluvium and the shallow weathered rock aquifer. Due to the low permeability of the shallow rock aquifer, any contribution to surface water was considered to be only a minor component (Parsons Brinckerhoff, 2013a). Using a five-layer MODFLOW model, Parsons Brinckerhoff (2013a) estimated a net upward flux from the shallow rock aquifer to the alluvium in the Gloucester subregion of 1.7 GL/yr.

Last updated:
5 January 2018