Numerical investigation of offshore freshened groundwater dynamics in a changing marine environment

Offshore freshened groundwater (OFG) is a potential alternative water resource for coastal populations facing water insecurity caused by high population density, pollution, or converging stressors from atmospheric and oceanic climate change. However, OFG systems are globally less explored than terrestrial groundwater systems, and no prior studies have been conducted on the Canadian continental shelf. Recent observations from two boreholes in the Gulf of St. Lawrence, Canada indicate a salinity decrease from 30 g/L to 20 g/L and 26 g/L, respectively, within the uppermost 5 m of marine sediment. To investigate the mechanisms driving OFG formation in the region, we conducted three-dimensional numerical simulations of coupled variable-density groundwater flow and salt transport, incorporating detailed analyses of the region's topography, stratigraphy, and geological structures. Our findings indicate that sub-ice-sheet recharge and meteoric recharge are the primary processes responsible for OFG emplacement since the Last Glacial Maximum. The modeled OFG volume is 57,950 km3, while the total submarine groundwater discharge across the entire Gulf is estimated at 1.15 km3/ year. Anisotropy in geological formations facilitates hydraulic connections between onshore and offshore aquifers, promoting active recharge of OFG. This onshore-offshore connectivity is reinforced by lower hydraulic conductivity, thicker unconsolidated marine sediments, greater anisotropy, and interlayered aquitards. Sensitivity analysis demonstrates that the system's behavior varies across the range of plausible geologic connectivity, transitioning from active flow dynamics in highly connected settings to a more passive regime in less connected configurations. This study highlights how geological and hydrological factors influence the distribution and availability of OFG resources in the region. Understanding the existence and function of the OFG system could inform the potential development of an alternative freshwater resource for the province of Prince Edward Island that is entirely groundwater reliant and faces competing water demands from domestic water users and the intensive agricultural industry.