A Geographic Information System Analysis Of Submarine Groundwater Discharge On The Eastern Shore Of Virginia

Recent research has indicated that submarine groundwater discharge (SGWD) is a significant component of the hydrologic cycle in coastal plain regions, and that discharged groundwater can be contaminated with nutrients and pesticides. Although discharge has been measured directly on a local level, a convenient method for determining the spatial distribution of SGWD on a regional scale using readily available data is needed. A geographic information systems (GIS) was applied to the problem of regional-scale determination of SGWD and nutrient loads for the Eastern Shore of Virginia. Computerized data layers included land use, hydraulic gradient and soil permeability. The GIS model was used to predict spatial distribution of SGWD as well as the contaminant flux from nearshore agriculture and on-site waste treatment technologies. Comparison with field data indicated excellent agreement for SGWD, but only moderate agreement for contaminant fluxes. The GIS model predicts that 90% of the impact from nearshore land use occurs in the southwestern quadrant of the Eastern Shore. The model identified areas susceptible to contamination through discharge to a resolution of about 33 meters, showing that GIS can function as a local as well as regional management tool INTRODUCTION Nonpoint sources contribute approximately 66 percent of the phosphorus and 77 percent of the nitrogen to the Chesapeake Bay (U.S. EPA, 1992). While significant progress has been made in reducing phosphorus inputs from nonpoint sources to the Chesapeake Bay, reduction of nitrogen loadings has yet to be achieved (U.S. EPA, 1992). If nonpoint sources of nitrogen are to be controlled and reduced effectively, resource assessment, management, and mitigation strategies must be developed. Nonpoint source pollution control programs have traditionally targeted surface phenomenon, such as erosion control and reduction of surface water runoff. Recently, efforts have begun to couple land use activities, groundwater discharge, and surface water quality. Consequently, a growing body of research has implicated submarine groundwater discharge (SGWD) as a significant source of fresh water, nitrogen and pesticides to coastal waters and more specifically to the Chesapeake Bay system (Simmons, 1989; Libelo et al., 1991; Simmons et al., 1991; Reay et al., 1992; Gallagher et al.,1996).