MAPPING SOIL SALINITY USING BROADBAND ELECTROMAGNETIC INDUCTION IN A SALTWATER MARSH, OYSTER POINT, NC

The importance of describing the spatial variability of groundwater recharge is especially important in saline environments. Techniques are needed to obtain freshwater recharge estimates and to map subsurface stratigraphy in coastal environments to aid in creation of accurate aquifer flow models and in the search for potable groundwater resources. Traditional subsurface investigations give varying degrees of spatial resolution, and often are associated with high cost and labor requirements. Electromagnetic induction (EMI) is used in a wide-range of geological and hydrological applications to map soil electrical conductivity. Little is known, however, about EMI capabilities in highly conductive environments. This study tests the ability of a handheld, broadband EMI sensor to detect freshwater recharge in a salt marsh in Oyster Point, NC. Using a Geonic GEM-2 instrument, transects were conducted laterally across the marsh ridge-swale topography, recording transmission frequencies of 6030 Hz, 15030 Hz, 37410 Hz, and 93090 Hz. Lithology was mapped from boreholes along transects and indicated a 0.5-5 meter thick clay subsurface above the fine sand aquifer. Sediment samples were analyzed for bulk density and conductivity. EMI conductivity profiles reflected the gradient from the ridge to swale as a function of both increased clay content and pore water salinity. This study allows for increased understanding of EMI sensor depth of exploration, as well as its applicability in future investigations of freshwater recharge in coastal environments.