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 groundwatermaybeaugmentingonshoregroundwaterextraction,whichisacriticalcomponent of Perth’s watersupply. 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 seal evels were up to120m 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 0.74m/y was predicted). Onshore groundwaterextractionispredictedtohaveincreasedtherateofinlandinterfacemovementby upto75%,comparedtotherateunderpaleo-conditionsalone.Simulationsincludingtheoffshore BadaminnaFaultsuggestthatthisfeaturetruncatestheoffshoreextentoffreshgroundwaterand reducestherateofinlandinterfacemovement.Theresultsofthisinvestigationdemonstratethat 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.