THE INFLUENCE OF GEOLOGIC AND HYDROLOGIC CONNECTIVITY IN COASTAL AQUIFERS ON OFFSHORE FRESHENED GROUNDWATER (Invited Presentation)

Recent advances have expanded our understanding of offshore freshened groundwater systems and highlighted their potential as a resource along coastlines worldwide. Recent studies suggest two mechanisms for the offshore occurrence of freshwater: 1- during past sea-level lowstands, freshwater was emplaced within continental shelves, and its persistence reflects a land-sea disconnect which implies disequilibrium with present-day sea level; 2- fresh groundwater is discharging offshore due to an active land-sea connection, which implies equilibrium with present-day sea-level; or 3- a combination of the two. This functioning – the nature of the land-sea hydrologic connection – is a critical factor in understanding the potential future use of the resource. We conducted numerical simulations of variable-density groundwater flow and salt transport through aquifers extending from onshore to offshore. We assessed the influence of geologic connectivity on the salinity of offshore groundwater for systems in equilibrium with sea level. Results show that preferential flow through connected high-permeability subsurface conduits results in freshened groundwater tens of kilometers offshore in some cases. These systems host active discharge of both fresh and saline groundwater, the latter of which is enhanced in the presence of heterogeneity. Incorporation of onshore and offshore pumping shows that geologic connectedness is a critical factor in assessing and managing the potential use of the resource, which may be vulnerable due to unpredictable hydrologic connectivity. We suggest that understanding the nature of the onshore-offshore hydrogeologic connectivity is a critical frontier in exploration of offshore freshened groundwater systems.