Click on figure for larger image.

Figure 1.Location map showing the continuous resistivity profiling (CRP) survey tracklines. The different colors indicate different days of surveying. The background image is from ArcGIS Online, ESRI_Imagery_World_2D, accessed September 2010. Copyright 2009, ESRI, i-cubed, GeoEye.

Like many coastal water bodies, the Corsica River Estuary, a tributary of the Chester River and Chesapeake Bay (fig. 1), is experiencing nutrient over-enrichment and associated water quality problems. The Corsica River watershed was targeted by the State of Maryland for implementation of intensive restoration efforts to improve water quality beginning in 2005 (Maryland Department of Natural Resources [MD-DNR], 2003). Several studies have documented elevated nutrient concentrations in groundwater throughout the Delmarva Peninsula between Chesapeake Bay and Delaware Bay (Shedlock and others, 1999; Denver and others, 2004). These studies, and others conducted previously in Chesapeake Bay and its tributaries (Reay and others, 1992; Reay and Simmons, 1992; Staver and Brinsfield, 1996; Speiran, 1996; Robinson and others, 1998; Hussain and others, 1999; Charette and Buesseler, 2004; Sanford and others, 2009; Cross and others, 2010), indicate that submarine discharge of groundwater recharged in agricultural and urban areas of the watershed is likely to be a source of excess nitrogen loads to the Corsica River Estuary. Excess nitrogen in estuary waters can contribute to blooms of harmful algae, such as the toxic dinoflagellate, Karlodinium veneficum, which may have been responsible for several fish kills in the Corsica River Estuary in recent years (Glibert and others, 2008). Septic systems and effluent from the Centreville, Md., Wastewater Treatment Plant also introduce nutrients to groundwater in the watershed. Site-specific studies, such as the one described here, are necessary for broader characterization of the distribution of, and controls on, submarine groundwater discharge (Taniguchi and others, 2002), and for determination of the relative significance of this part of the regional and global nitrogen cycles (Slomp and Van Cappellan, 2004; Hulth and others, 2005).

The Corsica River Estuary is approximately 8 kilometers (km) long and 0.5 to 1 km wide. Its salinity is generally between 5 and 15 parts per thousand (ppt), and it has an astronomical tidal range of 0.6 meters (m); wind-induced tides can be significantly greater. The estuary is oriented east-west and flows into the Chester River and ultimately into the Chesapeake Bay (fig. 1). The average depth of the estuary is <1 m in its upper reach, with a maximum depth of approximately 5 m near the mouth. The surface area of the Corsica River watershed is 102 square kilometers (km²), including 5.6 km² of water. Agricultural land makes up 64 percent of the watershed, 28 percent of the land in the watershed is forested, and 7 percent is developed, primarily in and near the town of Centreville, Md. (MD-DNR, 2003).

A description of the stratigraphic units and associated surficial and shallow confined aquifers in the region is provided by Cushing and others (1973), Owens and Denny (1979), and Hobbs (2004). The area is underlain by several confined aquifers and an unconfined surficial or Quaternary aquifer (Drummond, 2001). Relatively fine-grained sediments likely deposited during the last interglacial sea-level highstand, or a subsequent relative highstand (Kent Island Formation; Owens and Denny, 1979; Hobbs, 2004; Pavich, 2006; Scott and others, 2010), are exposed at the surface along the low-relief marine terrace that lies adjacent to the western half of the Corsica River Estuary. This area is part of the St. Michaels Lowland District of the Atlantic Coastal Plain Physiographic Province of Reger and Cleaves (2008). The relatively thin deposits of the Kent Island Formation overlie the Upper Paleocene Aquia Formation, based on outcrops along the Chester River shoreline (Cleaves and others, 1968). The dissected uplands along the eastern half of the estuary comprise coarse, fluvial deposits of the Upper Miocene/Lower Pliocene Pensauken Formation. They are underlain by marine silt of the Lower to Middle Miocene Calvert Formation, which is exposed in the bases of stream valleys incised into the uplands of the Corsica watershed, but which appears to be thin or absent beneath the low terrace in the western part of the watershed (Cleaves and others, 1968). The uplands lie on the western edge of the Denton Plain District of the Atlantic Coastal Plain Physiographic Province (Reger and Cleaves, 2008). In some areas of the uplands, a silt mantle of apparent late Pleistocene glacial origin (loess) is also present (Wah, 2003). Shallow sediments in the estuary itself consist of silt and clay in the offshore areas, with sand and gravel common along the shorelines (NOAA, 2008).