Field observations and numerical simulations of groundwater dynamics within Cabretta Island, a barrier island off the coast of Georgia

The demand for knowledge of groundwater resources and nutrient exchange in coastal settings has increased in response to rapid population growth. I present field and numerical results that (1) provide a better understanding of groundwater dynamics during calm weather periods, and (2) investigate the impacts of high energy storms on groundwater properties within Cabretta Island, a barrier island in Georgia. Field and numerical results showed that tidal exchange is the predominant driving force for groundwater flow in the marsh and beach, reaching velocities greater than 10 cm/d and 30 cm/d respectively. Topography along with atmospheric exchange generated flow velocities of 5 cm/d in the upland. Results showed that groundwater flow is downward in shallow marsh mud sediments, horizontal in the confined aquifer below the marsh, downward in the upland, and predominantly horizontal beneath the beach.;In August 2008, Tropical Storm Fay came within 200 kilometers of the study site, bringing a one meter storm surge and 8.5 centimeters of precipitation. Groundwater monitoring data showed that the storm significantly altered hydraulic gradients and increased hydraulic head by 50 centimeters in the marsh and 1 meter in the dunes. Both field and numerical results suggest high energy storms have the potential to drastically change flow directions and salinities from pre-storm conditions, which also directly impacts rates of nutrient transport to coastal ecosystems. Results from this study have implications that suggest the distribution of marsh macrophytes are hydraulically dependent, stratigraphy determines freshwater lens geometry within transgressive barrier islands, and topography of the beach-upland transition has an effect on groundwater salinities within the intertidal zone of a barrier island beach.