oa Hydrogeology of a Tertiary-Quaternary alluvial sequence in western Victoria, and the potential for upward leakage to induce dryland salinity

Many low-lying areas of the Mount William Creek valley are affected by dryland salinity due to shallow watertables across the floodplain. The valley contains a sequence of alluvium consisting of a basal gravel and sand unit of Tertiary age, overlain by Quaternary silts and clays. This sequence lies unconformably on a variety of Palaeozoic basement rocks, which outcrop as rolling hills and mountains towards the catchment boundaries. Potentiometric surface configurations and bore hydrograph data show that the basal Tertiary sequence and the overlying Quaternary sequence do not act as a single unconfined aquifer. Rather, they behave as two aquifers, with the Quaternary silts and clays having some confining capacity over the Tertiary sequence. Hydraulic head differences demonstrate that there is potential for downward leakage between these two units at the floodplain margins and upper catchment throughout the year, and that potential for upward leakage exists in the lower catchment following the winter peak in recharge. The area where upward leakage from the Tertiary sequence can be invoked as a salinity-causing mechanism is spatially and temporally restricted, but it does correspond to some large areas of land salinisation. The high groundwater pressures causing potential upward leakage can easily be attributed to head gained in the adjacent Grampians ranges. However, geological restrictions to lateral groundwater flow through the system may also contribute to the high groundwater pressures in the Tertiary sequence.