The objective of this study was to improve the conceptualisation of fresh groundwater occurring in the offshore extension of confined aquifers underlying Perth, Western Australia. These aquifers (the Leederville and Yarragadee) are a critical water resource for the city of Perth.
Onshore geological and hydrogeological data were used in conjunction with offshore geological information from petroleum exploration wells to develop a preliminary representation of the offshore stratigraphy and structure. The stratigraphy was used in the development of a two-dimensional variable-density flow and solute transport numerical model. This model extends from 10 km inland to the continental shelf, 90 km offshore.
Simulations provide evidence for long-term sea-level variations (associated with glacial cycles) being a key driver of offshore fresh groundwater emplacement in the Perth basin confined aquifers. Simulations also indicate that the position of the interface between seawater and freshwater may not have reached equilibrium with present day sea levels. As such, the interface may still be moving landward in response to the rise in sea level that has occurred since the last glacial maximum (around 20,000 years ago).
This study offers insights into the key drivers of offshore fresh groundwater distribution in the Perth basin aquifers and highlights the importance of transience in this system.