The Impact of Unsaturated Zone Thickness on the Severity of Saltwater Intrusion in Coastal Aquifers from Overtopping

Storm-surge inundation and wave overwash, collectively overtopping, causes the land surface to be inundated with seawater, resulting in vertical infiltration of saline waters into underlying coastal aquifers, which can have severe consequences on freshwater resources. Unsaturated zone thickness plays an important role in determining the severity of saltwater contamination resulting from an overtopping event. This study uses numerical modeling to investigate vertical saltwater infiltration and salinization, and the subsequent aquifer recovery to overtopping events for coastal aquifers with different unsaturated zone thicknesses. Groundwater models were developed using the numerical code SUTRA to solve saturated-unsaturated, density-dependent groundwater flow and solute transport. Our results indicate that thicker unsaturated zones allowed a greater salt mass to infiltrate and salinized area to develop within a coastal aquifer following inundation. Models with thicker unsaturated zones also required an extended amount of time for the saline plume to flush through the fresh aquifer. Sensitivity analyses were also performed to investigate the impact of recharge, inundation time, and permeability. Our results found that high recharge rates resulted in less saline contamination and recovered to pre-inundation conditions more quickly. In addition, aquifer contamination and flushing times were relatively insensitive to inundation durations. Aquifers with higher permeability also received greater saline contamination but had shorter recovery times. The model results provide a generalized assessment of coastal aquifer susceptibility to an overtopping event and can inform managers of potential risks associated with overtopping as the frequency and severity of these events increase in response to a changing climate