GROUND-WATER QUALITY AND DISCHARGE TO CHINCOTEAGUE AND SINEPUXENT BAYS ADJACENT TO ASSATEAGUE ISLAND NATIONAL SEASHORE, MARYLAND

Part of the mission established for the National Park Service at Assateague Island National Seashore in Worcester County, Maryland, includes preserving and protecting the natural resources of Assateague Island and its adjacent estuaries. In support of this effort, the U.S. Geological Survey conducted a study of ground-water nutrient transport in the surficial aquifer to the estuaries. The study area includes Assateague Island, Chincoteague and Sinepuxent Bays, and the surface-water drainage basins associated with the bays. The purpose of the study was to describe ground-water flow paths that carry freshwater to Chincoteague and Sinepuxent Bays and their tributary streams, and to collect water-quality data associated with these freshwater inputs, particularly nutrient concentrations.

Twenty-eight ground-water monitoring wells were installed in the surficial aquifer within the study area to collect hydraulic head and water-quality data at various depths. Base flow was measured and water-quality samples were collected at 17 nontidal streams to define the concentrations of nutrients being transported to the coastal bays from each site.

Water recharged to the surficial aquifer follows flow paths with lengths ranging from a few hundred feet to several miles. Many of the shorter flow paths end by discharging ground water as base flow to streams, while longer flow paths end by discharging ground water directly to Chincoteague Bay or the Atlantic Ocean. Traveltime for ground water flowing to the end of deep flow paths in the surficial aquifer may be 30 years or longer.

Dissolved nitrate is the dominant nutrient in ground water in the study area. Nitrate concentrations in ground-water samples collected from wells ranged from below 0.05 mg/L as N to as high as 15.5 mg/L as N. Nitrate concentrations in samples of stream base flow ranged from below 0.05 mg/L as N to 5.29 mg/L as N, and showed a significant, positive statistical correlation with the percentage of the stream basin area used to cultivate row crops.