MONITORING AND MODELING THE GROUNDWATER FRESH-SALTWATER BOUNDARY ON KIAWAH ISLAND SC

The integrity of terrestrial ecosystems on barrier islands is dependent on the resilience of the fresh water in the unconfined aquifer. On barrier islands it is essential to understand the thickness of the fresh groundwater lens, groundwater salinity and the salt water wedge. Seawater surrounding the island infiltrates the sub-surface and can displace freshwater from the aquifer into a thin layer within the vadose zone. On Kiawah Island, a 39 km² barrier island on the central coast of South Carolina, freshwater is more abundant toward the middle of the island, the shape of the lens is influenced by the geomorphology and distribution of geologic materials and man-made features. Multiple man-made ponds help to shape and define the freshwater lens. Saltwater inundation during storms, king-tide events, and sea level rise causes saltwater to slowly infiltrate and threaten the quality of groundwater. These conditions could potentially lead to the degradation of both the maritime forests and the built infrastructure through repeated exposure to salt water.

This study use 21 shallow monitoring wells have been installed in the unconfined aquifer, managed ponds distributed across the island along with ocean tidal data to develop a 1 year long data set of high and low tide events on the island. Maps have been produced of the events and an understanding of the fluctuations of the ground water surface have been modeled. The framework developed in this project is capable of classifying the shape and salinity of the groundwater table and helps inform land use decisions on the island of Kiawah with an emphasis on the usage of green space. Monitoring these fluctuations over time will help provide information to guide the selection of green infrastructure installation (e.g., rain gardens and bioretention areas) following low impact development (LID) best practices. The overall goal is to mitigate saltwater intrusion and stormwater flooding to improve the island’s resilience to flooding while recharging the freshwater on the island.