EFFECTS OF COASTAL STORMS ON FLOW AND SALINITY DISTRIBUTIONS IN A BARRIER ISLAND GROUNDWATER SYSTEM

Waves and tides along sandy shorelines can generate a mixing zone of saltwater and freshwater in beach aquifers. The intertidal mixing zone is a biogeochemically active region in the beach subsurface that moderates chemical and carbon fluxes entering the ocean along fresh groundwater flow paths. Fresh groundwater discharging to the ocean plays a significant role in the coastal water budget, and thus characterizing salinity dynamics in beach aquifers is important for quantifying mass loads to the ocean. Stronger coastal storms with intensified wave action and heavy precipitation from climate change will likely affect pore water salinities in the intertidal zone and may enhance or suppress beach biogeochemical processing. Here, a numerical variable-density groundwater flow and salt transport model was used to simulate groundwater flow and salinity dynamics in the barrier island located at the U.S. Army Corps of Engineers Field Research Facility in Duck, NC. The model simulated a 2-D cross-shore transect of the barrier island, and included tides, time-varying wave setup, and daily groundwater recharge for 2021-2022. In the simulations the intertidal mixing zone expanded horizontally and vertically during periods of intensified wave conditions. The growth of the mixing zone coincided with density-driven downward transport of vertically stretched saltwater plumes into the deeper aquifer. Waves had a greater influence than tides and large recharge events on the intertidal salinity distribution. In particular, large recharge events associated with coastal storms had a negligible impact on beach pore water salinity. The results may help to improve estimates of fluxes of land-sourced chemicals such as nutrients, metals, and organic contaminants to coastal ecosystems in a changing climate. This research is supported by NSF and USCRP.