PRELIMINARY INVESTIGATION OF A BARRIER ISLAND BEACH AQUIFER AT WALLOPS ISLAND, VIRGINIA USING GROUND PENETRATING RADAR

Ground Penetrating Radar (GPR) is a non-invasive shallow geophysical technique now being used to investigate barrier island sedimentation and patterns of geomorphologic evolution. The technique is also being used to investigate barrier island aquifers. Previous studies, including in Hatteras Island, North Carolina have demonstrated heterogeneities in aquifer dynamics based on the distribution of inter-dune recharge areas, coastal strata geometries, and permeability differences within sedimentary units.

This research is being carried out on a barrier island along the coast of eastern Virginia at NASA’s Wallops Island Flight Facility (WIFF). Here we have collected both shore parallel and shore normal profiles along established transect sites in order to investigate both patterns of sedimentation as well as the extent of the freshwater lens and the position of the salt-water interface. Although analysis and post-processing of more than 60 transect sites is ongoing, investigations have revealed significant heterogeneities in the response of GPR signals detected with Mala’s X3M GPR (250, and 500 Mhz shielded antennas). Ground truthing in the form of excavated pits, shallow monitoring wells outfitted with data recorders, and field surveys at low tide are enabling qualitative and quantitative assessment of initial returns. Although analysis is incomplete, similar to studies elsewhere, it appears that along-shore variations in morphologic features of beach/back-beach areas strongly control the position of the saltwater interface and the discharge points of the freshwater surface aquifer. This study shows that the concept of a freshwater lens needs to be re-evaluated for barrier islands.