Consistently low annual rainfall, increasing groundwater abstraction, and the conversion of native Banksia woodland to pine plantations over the last 30 years has led to widespread acidification of groundwater at the water table on the Gnangara Mound, an important source of drinking water for the city of Perth in Western Australia. The acidification of shallow groundwater is being caused by a falling water table exposing sulphide-rich wetland sediments to oxidising conditions, and to the release of stored acidity from podsolized aeolian sands within the unsaturated zone beneath pine plantations. The sands are of Pleistocene age, are highly siliceous and contain no residual carbonate minerals. Ten to fifteen cm thick spodosol horizons within sand profiles above the water table contain significant total sulphur levels (2-5%), have high concentrations of leachable Al3+ (5-7 mg/L), and have a low pH (3.1 – 4.3). Progressively increasing concentrations of calcium and magnesium in groundwater pumped from production bores indicates that the ANC of the soil is declining at a rate of about 0.07 keq/ha/year due to the leaching of base cations. The progressive oxidation of organic matter and sulphides and increasing acidity of soil profiles is also decreasing Ca/Al mole ratios in soil solution. These factors together with the low Cation Exchange Capacity (CEC) of sandy soils (0.01-1.6 cmol+/kg) and increased dissolved organic matter (DOM) strongly affect the fate/mobility of metals and metalloids in groundwater, posing threat to groundwater resources and the health of both terrestrial and aquatic ecosystems on the Gnangara Mound.