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To track submarine ground-water discharge in selected Delmarva coastal bays, USGS scientists and cooperators recently measured the electrical resistivity of the bay sediment. Their measurements revealed that submarine discharge of fresh ground water is occurring at distances from a few hundred meters to 1 km into coastal bay waters, much farther offshore than predicted by standard coastal hydrologic models. The new data will define pathways for freshwater inflow and help resource managers plan remediation efforts to reduce nutrients entering the coastal bays. Accurate information may minimize remediation impacts on local farming, recreation, and other activities in the coastal areas.
A new "streamer resistivity survey" technique was used to measure the electrical resistance of the bay sediment. Normal saltwater sediment is a good conductor of electricity; however, bottom sediment permeated by fresh water is a poor conductor of electricity and acts like a semi-insulator. To conduct the streamer resistivity surveys, scientists towed 360-ft electrical cables called streamers behind small boats. Continuous measurements of electrical potential at variable spacings allowed the scientists to map fresh, salt, and mixed water layers to more than 100 ft below the bay floors.
Streamer resistivity investigations of sub-sea-floor ground water collect data at a rate 30 to 70 times that for comparable measurements on land. In fact, the nearly 300 km of trackline collected in Rehoboth and Indian River bays (Delaware); Assawoman, Isle of Wight, and Sinepuxent bays (Maryland); and Chincoteague Bay (Maryland and Virginia) may exceed the total length of all horizontal resistivity surveys ever collected on land in the United States.
Scientists used a vibradrilling unit from a barge supplied by the Delaware Department of Natural Resources to verify resistivity data collected in October 2001 from Indian River Bay. David Krantz (formerly of USGS, now at University of Toledo, OH) and Dan Phelan (USGS, Baltimore, MD) obtained gamma and electrical-induction logs as much as 90 ft long through steel and plastic casing. These logs confirmed the presence of freshwater layers beneath salty surficial ground water in sediment underlying parts of the bay.
The new vibradrilling logging approach does not require drilling fluid, which can complicate log interpretation. Fluid-conductivity changes delineated by the logs were complemented by interstitial-water and resistivity-probe study of the cores performed by Frank Manheim (USGS, Reston, VA) and John Bratton (USGS, Woods Hole, MA). Advanced chemical and isotopic measurements were performed by J.K. Bohlke (USGS, Reston, VA). Age-dating of water samples by chlorofluorocarbon and sulfur hexafluoride techniques demonstrated ages for submarine ground waters commensurate with ages of ground waters at comparable depths on land.
The current activities represent wide-ranging cooperation among scientists from the USGS, the Delaware and Maryland Departments of Natural Resources, the University of Delaware, and other organizations. Results from these investigations have been presented over the past several months at the Society for Applied Environmental and Engineering Geophysics (SAGEEP) Symposium in Las Vegas, NV, the Geological Society of America meeting in Boston, the USGS Delaware Ground-Water Hydrology Seminar, a Regional Groundwater Planning Conference in Raleigh, NC, and a Regional Groundwater Workshop in Annapolis, MD.
in this issue:
Delmarva Coastal Bays
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