CHARACTERIZING GROUNDWATER FLOW ACROSS THE BARRIER ISLAND - SALTMARSH INTERFACE

The groundwaters of barrier islands are exceptionally diverse in their flow and composition. A multitude of factors including evapotranspiration, geologic constraints, tidal forcing, and meteoric inputs influence subterranean flow. These factors create a groundwater salinity gradient throughout the island-marsh setting which directly influences the spatial distribution of dominant vegetation on the island. Rising sea levels and variation in local climate may alter the hydrologic balance on barrier islands, potentially shifting salinity gradients and ecological baselines on short timescales. The intrusion of saltwater in the porewater-groundwater system could result in shifting of these ecotones and potential destabilization of barrier sediments, as well as degradation of water quality used for municipal services.

An interdisciplinary method was adopted to characterize the groundwaters of Waties Island, a barrier island located in northeastern SC. A 150-meter-long study transect located on the landward side of the island encompassed the ecological shift between maritime forest and salt marsh. Three shallow groundwater wells were located along the transect, with a midpoint well in the high marsh where the greatest subterranean mixing was expected. Water level, temperature, and conductivity were recorded in each well at 20-minute intervals. Modal analysis of the data revealed tidal and meteorological drivers of groundwater level across the transect. Time series electrical resistivity (ER) surveys conducted along the transition from forest to marsh revealed horizontal mixing of groundwater within sediment layers of highly permeable material. Vibracores facilitated analyses of the relative permeability of sediments along the transect and augmented the interpretation of ER data. Together, these data resolved the impact tidal and meteoric drivers have upon groundwater level in addition to characterizing resulting subterranean flow given geologic and biotic constraints.