U.S. Geological Survey Open-File Report 2010-1094
Continuous Resistivity Profiling Data from the Corsica River Estuary, Maryland
Based on the grid of 67.6 km of CRP data (fig. 1), low-salinity (high-resistivity) groundwater extended approximately 50-400 m offshore from estuary shorelines (fig. 4) at depths of 5 to >12 m below the sediment surface, likely beneath a confining unit (Bratton, 2007). A band of low-resistivity values along the axis of the estuary (fig. 5) indicated the presence of a sediment-filled paleochannel containing brackish groundwater. The meandering paleochannel likely incised through the confining unit during periods of lower sea level, allowing the low-salinity groundwater plumes originating from land to mix with brackish subestuarine groundwater along the channel margins and to discharge. Additional subsurface sampling, penetrating to greater depths and extending farther offshore, would be required to further characterize these offshore parts of the submarine groundwater system. Such investigations have been performed with barge- and hovercraft-mounted drilling rigs in other locations (Bratton and others, 2004; Manheim and others, 2004; Cross and others, 2008; Bratton and others, 2009). Age dating and nutrient analysis of groundwater collected beneath the Corsica River Estuary (John Bratton, NOAA, unpub. data, 2011) indicate that older groundwater recharged on land that may be discharging along the margins of the axial paleochannel beneath the estuary is unlikely to contain high concentrations of anthropogenic nutrients that would contribute to eutrophication. Recirculation of estuary surface water through sediments, however, likely contributes significant ammonium to surface water. Continued input of excess nitrogen through fertilizer application, wastewater disposal through septic systems, and atmospheric deposition of nitrogen to the watershed will eventually lead to increased anthropogenic nitrogen concentrations even in subestuarine groundwater discharging significant distances (>50m) from shore.