Upon the Minderoo pastoral station, Big Bend and Peepingee borefields are located alongside the Ashburton River which meanders over the Ashburton Coastal Plain in the Shire of Ashburton, Western Australia. Bores pump from a productive, heterogeneous, multi-layered aquifer system comprising fluvial and gravel interbedded with clay and silt about 30 m thick. Aquifer recharge is mainly by infiltration of intermittent river flows typically from December to August.
The current scheme comprises primary boreholes and a 3.2 m high weir constructed late 2010 for MAR to augment the groundwater storage and support sustainable development of groundwater resources in Minderoo water supply area for irrigation purposes. Extensive hydrogeological investigations undertaken since 2003 over both borefield areas have included airborne TDEM surveys, groundwater exploration and borefield development programs.
Increasing groundwater demand for both irrigation and water supplies for the town of Onslow, combined with the intermittent nature of river flow has necessitated further plans for expansion of the MAR scheme by constructing additional weirs and testing their efficiency and beneficial influence by means of numerical simulations. A finite - element numerical groundwater model was developed to simulate the groundwater regime and assess the reliability of the Big Bend and Peepingee Borefields with and without additional managed aquifer recharge weirs.
The borefield performance and the river aquifer interaction was simulated through 8½ years of typical river flow/aquifer recharge conditions. To test the uncertainty of the predictions to hydrological uncertainty, all modelling scenarios were followed by a 600 day simulated river drought.
The simulated performance of the Big Bend and Peepingee borefields through a 600 day river drought period suggests that both borefields should be able to each sustain abstraction at 2,000 kL/day, albeit that water levels in the Big Bend borefield would drawdown more than 50% of the available saturated aquifer thickness (filling factor) by the end of the drought period and some of the lower yielding bores (in low-K areas) in both the Big Bend and Peepingee Borefields would fail. The construction of a 3.2 m high, leaky MAR weir enhances groundwater seepage in the Ashburton alluvial aquifer, forming discernible groundwater mounding up to 5 km on either side of Ashburton River. The induced mounding takes several months to dissipate. Furthermore, each additional MAR weir has a cumulative effect on regional groundwater mounding, which is most pronounced immediately adjacent to each weir.