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U.S. Geological Survey Open-File Report 2009-1151

Continuous Resistivity Profiling and Seismic-Reflection Data Collected in 2006 from the Potomac River Estuary, Virginia and Maryland


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The data collected in this study indicate that plumes of reduced-salinity groundwater are commonly present along the shorelines of Chesapeake Bay and the Potomac River Estuary in the areas that were surveyed. High-resistivity (low-salinity) anomalies were strongest and extended the greatest distance offshore in upstream areas of the Potomac River Estuary, especially along the Maryland (north) shore. Anomalies commonly showed plumes extending approximately perpendicular from the shore to distances of 500-1000 meters or more offshore. Anomalies were not common in areas under the deepest channels of the estuary, and terminated more abruptly on the flanks of these channels than in upstream areas with more gradual seafloor slopes.  Buried paleochannels, often extending from sub-tributaries into the main channel of the Potomac River Estuary, were usually underlain by low-resistivity sediments, interpreted as containing brackish to saline groundwater. This observation indicates that incision by rivers during sea-level lowstands may have breached confining units, and that subsequent filling of paleovalleys with sediments did not produce conditions favorable for isolating submarine fresh groundwater from saline surface water.  These observations suggest that submarine groundwater recharged on land is more likely to be present offshore in upstream areas of the Potomac River Estuary and its tributaries. In addition to discharge along the shore, such groundwater may discharge diffusely or in a more localized manner along the flanks of modern channels and buried paleochannels, carrying nutrients and other pollutants from land with it.

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