Reassessing the management of groundwater use from sandy aquifers: acidification and base cation depletion exacerbated by drought and groundwater withdrawal on the Gnangara Mound, Western Australia

The combined effects of low rainfall, groundwater withdrawal in excess of 300 GL/year and reduced recharge in areas covered by pine plantations has caused the water table in a sandy unconfined aquifer on the Gnangara Mound in Western Australia to drop by up to 5 m and aquifer storage to decline by about 500 GL over the last 20 years. Groundwater has become acidic in areas of high drawdown, with pH values typically being less than 5.0 at the water table, and elevated concentrations of SO₄ ²⁻, Al, Fe, Zn, Cu, Ni and Pb. Trends of increasing acidity and base cation concentrations in deep water supply wells in the Mirrabooka wellfield indicate that about 0.7 keq/ha/year of base cations are being leached from soil within cones of depression of pumping wells. These results indicate that the assessment of the sustainable yields of aquifers under conditions of low rainfall needs to consider geochemical interactions between groundwater, aquifer sediments, soils and vegetation, and not be just based on aquifer hydraulics and water-balance changes.