STREAMING RESISTIVITY SURVEYS AND CORE DRILLING DEFINE GROUNDWATER DISCHARGE INTO COASTAL BAYS OF THE DELMARVA PENINSULA

This paper describes new techniques for the delineation of submarine ground-water discharge – a source of excess nutrient supply to coastal bays of the Delmarva Peninsula. Streamer resistivity surveys using a dipole-dipole “DC Resistivity” system developed by the Zonge Co. were towed behind small boats. The system incorporated a multichannel cable and 2-D, “smooth model” inversion postprocessing. Effective measurement depth varied from 10 to 50 m in the shallow bays. Nearly 300 km of surveys were conducted in Rehoboth, Indian River, Chincoteague, Sinepuxent, Isle of Wight, and Assawoman Bays. Extensive freshwater distributions appeared in inversion model profiles as high resistivity layers (>10 ohm-m) beneath low-intermediate resistivity layers (.6-3 ohm-m). Subsurface freshwater tongues were associated with either elevated topography (e.g. 2 m or more) or creek mouths. Many freshwater anomalies were limited to a few hundred meters from shore, but in some places distinct anomalies were observed 1 km offshore or more. Subsurface brackish waters characterized most of subseafloor of the bays. In one area of Indian River Bay hypersaline brine (2x salinity of normal sea water) was identified at depth. Hoverprobe coring confirmed fresh interstitial waters in Herring Creek (Rehoboth Bay, DE) and near Public Landing (Chincoteague Bay, MD). In the latter area subseafloor fresh water with high nitrate indicated seaward movement of oxygenated fresh water. Shallower sediments showed undetectable nitrate but large ammonia concentrations. Fresh sub-bay ground water beyond limits suggested by a diffuse Ghyben-Herzberg hydrologic model seems to be associated with the presence of finer bottom sediments along the western margin of the investigated bays. Organizations supporting the above studies (beyond those associated with the above investigators) include the University of Delaware, Delaware Geological Survey, Delaware Sea Grant, Maryland Department of Natural Resources, U.S. National Park Service (Assateague Island National Seashore), Johns Hopkins University Applied Physics Laboratory, and the Virginia Institute of Marine Science.