A geographic information system analysis of submarine groundwater discharge on the eastern shore of Virginia
Recent research has indicated that submarine groundwater discharge (SGWD) occurs in the Chesapeake Bay, and that discharged groundwater can be contaminated with nutrients and toxic substances. Although discharge has been measured directly on a local level, a convenient method for determining the spatial distribution of SGWD on a large scale has not yet been devised. Although current watershed mass balance approaches have been used to estimate gross SGWD, this method cannot identify areas at high risk for contaminant input. A family of innovative computer mapping programs called Geographic Information Systems (GIS) have emerged from the software industry that serve not only as mapping tools but also as databases designed to manage spatial data) The GIS approach allows straightforward manipulation and presentation of data that is spatially related. In the research presented here, a GIS was applied to the problem of large-scale determination of SGWD and nutrient loads for the Eastern Shore of Virginia. Computerized data layers including land use, hydraulic gradient and soil permeability were used to create a GIS model of SGWD on Virginia's Eastern Shore. The model was used to predict spatial distribution of SGWD as well as the nutrient loading from nearshore agriculture and on-site waste treatment technologies. While sufficient field data for reliable calibration and verification of the model did not exist, the available data and the output of the GIS model were consistent. The methodology developed predicts that 90% of the impact from nearshore land use occurs in the southwestern quadrant of the Eastern Shore. In addition, the model determined locations where urban land use (septic tanks) may present a nutrient and coliform contamination danger. 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. The GIS framework is also expandable. The model can be modified to accommodate new data as research continues into the phenomenon of submarine groundwater discharge. This synthesis represents the first large-scale groundwater discharge analysis of the Eastern Shore and exists as part of an ongoing investigation to characterize groundwater-surface water interactions of the Eastern Shore.