THE ROLE OF UNSATURATED AQUIFER VOLUME ON SALINIZATION DURING OVERTOPPING

Coastal aquifers are important sources of nutrients for coastal ecosystems and serve as freshwater resources across the globe, but rising sea levels, changing climate, land use changes, and continuing coastal population growth are increasingly stressing fresh coastal groundwater. Storm surge and resulting overtopping of seawater onto the land surface can further stress fresh coastal groundwater by vertically introducing saltwater into the subsurface. The degree of aquifer salinization by overtopping is likely to depend on the antecedent moisture content in the unsaturated zone that controls the available unsaturated pore space for seawater infiltration. We characterized the effect of unsaturated aquifer volume on salinization during overtopping using variable-density, variable-saturated numerical groundwater flow and salt transport simulations built with the USGS SUTRA modeling code. We tested the sensitivity of aquifer salinization to changes in unsaturated zone thickness, recharge rates, inundation time, and permeability. Overtopping-induced salinization increased with thicker unsaturated zones because larger unsaturated pore space accommodated a larger volume of infiltrating seawater during inundation. The saltwater flowed downward through the saturated zone, resulting in a larger salinized area and longer aquifer flushing times compared to thinner unsaturated zones. Flushing times were shorter in high-recharge and high-K aquifers despite more saltwater infiltrating into high-K systems. These findings can inform water resource managers to potential risks associated with overtopping as the frequency, severity, and impact of these events increases in response to our changing climate.