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Streamer Resistivity Surveys in Delmarva Coastal Bays

Publisher –
Zonge, 2001.

Authors –
Frank T. Manheim, Coastal and Marine Geology Center, Woods Hole, Massachusetts;
David E. Krantz, Dept. of Earth Science, University of Toledo, Toledo, Ohio;
Donald S. Snyder, Zonge Engineering and Research Company, Tucson, Arizona;
Brian Sturgis, U.S. National Park Service, Assateague Island, Maryland.

Paper – [pdf] IP_MarineStreamer

This paper reports on streaming resistivity (“DC resistivity”) surveys conducted in Maryland and Virginia Atlantic coastal bays in the spring of 2001. Surveys in Assawoman, Isle of Wight, and Chincoteague Bays, MD and VA, were used to study profiles of electrical resistivity of submarine strata to delineate submarine freshwater discharge and submarine saltwater interfaces and salinity distributions in submarine groundwater. The studies follow similar resistivity surveys in Rehoboth and Indian River Bays in spring of 2000 (Krantz and others, 2000; Madsen and others, 2001; and Manheim and others, 2001).

The Delmarva Peninsula coastal studies are part of larger cooperative programs between the U.S. Geological Survey, regional federal and state organizations, and academic institutions. They address the problem of excess nutrient discharge into Delmarva coastal bays. Like the Delaware coastal bays, Maryland and Virginia coastal bays receive excess nutrients due to human activities. The excess nutrients enhance growth of phytoplankton and fouling macroalgae, which impedes boat operation, coats beaches, and lays down organic–rich mats. This organic matter fosters anoxic conditions in the bottom sediments. Growing stagnation alters the habitat for benthic organisms and reduces biological diversity. Recent studies suggest that excessive organic growth inhibits natural mechanisms (like denitrification) that help transform and remove nutrients from the bay systems. Submarine discharge of nitrate-enriched ground waters was inferred from preliminary estimates of land-based hydrologic

flow-nets (Andres, 1987, 1992) in the Delaware coastal bays (Rehoboth and Indian River), and modeled by Cerco and others, 1994. Subsequently, as a part of a large consortium study (CISNET) led by the University of Delaware, T. McKenna of the Delaware Geological Survey (2000) and coworkers performed overflights of Rehoboth and Indian River bays in the winter of 1999 (McKenna and others, 2001). Remote sensing (infrared temperature measurements) of surficial coastal waters in the winter detected a number of areas where warmer water anomalies signified submarine discharge in the near-coastal environment.

A recent summary (Dillow and Greene, 1999) based on land data estimates that roughly 24% (272,000 pounds) of the total nitrate loads from groundwater enters the Maryland bays through submarine groundwater discharge (SGD). This nitrate flux is associated with about 13% of the estimated 100 million gallons per day total water influx estimated to enter the Maryland coastal bays through SGD. The Maryland SGD fraction is a smaller proportion than estimated for the Delaware inland bays (up to 80%). Dillow and Greene (1999) point out that there is uncertainty about the pathways of submarine groundwater discharge. Postulated pathways range from immediate sub-bay coastal margin discharge (corresponding to the Ghyben-Herzberg model), to long-distance transport in aquifers extending under the barrier bar (Assateague Island) and discharging into the Atlantic Ocean.