Storm Surges Cause Simultaneous Salinization and Freshening of Coastal Aquifers, Exacerbated by Climate Change

Abstract

AbstractOcean surge events are known to threaten coastal aquifers through vertical infiltration, with the degree of salinization depending on hydrogeologic factors. Another salinization process in coastal aquifers is lateral saltwater intrusion, which may also be affected during surges as the inundation alters the aquifer hydraulic heads. While these processes have been considered individually, here we consider the interplay between them and the longer‐term impact of climate change, which is projected to increase the frequency of surges in the future. Using numerical modeling, the location of the lateral freshwater‐saltwater interface and the total salt storage are calculated for single and repetitive surge events with different recurrence times to predict the long‐term effect of surges. Results point to two novel mechanisms: (1) Following a single overwash event, salt storage in the aquifer peaks due to vertical salinization, and a second, lower peak occurs on a longer time scale. This second peak is due to the surge‐induced rebound motion (seaward and then landward) of the interface. (2) The projected increase in surge frequency due to climate change can potentially induce long‐term migration of the interface seaward, independent of a change in sea level, depending on the aquifer permeability. Together with this freshening effect, the total salt load in the aquifer increases due to repetitive vertical salinization. Thus, we show for the first time the combined effect of storm surges and climate change on both vertical and horizontal movement of salt in coastal aquifers, with important implications for water management along global coastlines.