A conceptual study of offshore fresh groundwater behaviour in the Perth Basin (Australia): Modern salinity trends in a prehistoric context

Study Region: Fresh groundwater is thought to occur off the coast of Perth, Western Australia, in the confined Leederville and Yarragadee aquifers. Onshore hydraulic heads suggest that offshore groundwater may be augmenting onshore groundwater extraction, which is a critical component of Perth's water supply. Study focus: To assess offshore freshwater conditions, we apply variable-density flow and transport modelling to a simplified cross-sectional representation of the Perth Basin offshore aquifers, developed using available hydrogeological information. New hydrological insights for the region: Simulations suggest Perth's offshore fresh groundwater was emplaced during glacial periods (when sea levels were up to 120 m lower than today), and the interface between seawater and freshwater is likely still moving landward in response to paleo-conditions, albeit slowly (i.e., a maximum rate of 0.74 m/y was predicted). Onshore groundwater extraction is predicted to have increased the rate of inland interface movement by up to 75%, compared to the rate under paleo-conditions alone. Simulations including the offshore Badaminna Fault suggest that this feature truncates the offshore extent of fresh groundwater and reduces the rate of inland interface movement. The results of this investigation demonstrate that paleo-stresses may impose stronger controls than modern, human-induced factors on offshore freshwater extent in the Perth Basin, and that offshore faults may play a critical role in controlling offshore freshwater extent.