Groundwater acidification near the water table of the Superficial aquifer, Gnangara Mound, Swan Coastal Plain, Western Australia

•Improves the understanding of the groundwater acidification issue in a poorly buffered aquifer.•Groundwater pH < 5 identified on a regional scale, mainly due to pyrite oxidation.•Groundwater chemical analysis recorded negligible alkalinity (<10 mg/L as CaCO3).•Groundwater chemical analysis recorded high sulfate to chloride ratios (>0.5 mg/L units).•Groundwater chemical analysis recorded elevated concentrations of aluminium and iron.

This study improves the spatial understanding of the magnitude of the groundwater acidification issue in a poorly buffered sandy aquifer in Western Australia. The study provides more information to assess the risk the acidification poses to groundwater dependent ecosystems of the Gnangara Mound. Groundwater acidification (pH < 5) was identified on a regional scale, mainly due to pyrite oxidation, with the lowest pH of 3.13. Groundwater was fresh (TDS < 500 mg/L) with low concentrations of base cations, negligible alkalinity (<10 mg/L as CaCO3), high SO42-/Cl- ratios (>0.5 mg/L units), low Ca to Al molar ratios and elevated concentrations of Al and Fe.Acidic groundwater in the capture zones of wetlands may pose a long term risk to ecological values. Acidic groundwater in poorly buffered systems also poses a risk for users of untreated groundwater. Land and water use management should consider poorly buffered aquifers to be highly sensitive, particularly where rainfall and recharge trends have declined over a period of time. A comprehensive and consistent groundwater sampling program should be developed to fully delineate the groundwater acidification issue near the water table of the Gnangara Mound to assist the formulation of groundwater allocation plans.