Ground-Water Quality and Discharge to Chincoteague and Sinepuxent Bays Adjacent to Assateague Island National Seashore, Maryland
By Jonathan J.A. Dillow, William S.L. Banks and Michael J. Smigaj
Abstract
The U.S. Geological Survey, in cooperation with the National Park Service's Assateague Island National Seashore, conducted a study of the transport of nutrients in ground water in the surficial aquifer to estuaries adjacent to Assateague Island National Seashore. The study area includes Assateague Island, Chincoteague and Sinepuxent Bays, and the surface-water drainage basins of 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, particularly nutrient concentrations, associated with these freshwater inputs.
Twenty-eight ground-water monitoring wells were drilled and constructed in the surficial aquifer and underlying confining bed within the study area to collect water-level and water-quality data at various depths. Base flow was measured and water-quality samples were collected at 17 non-tidal streams within the study area during the winters of 1999 and 2000 to determine the nutrient concentrations being transported to the coastal bays from each stream.
Ground-water flow paths in the surficial aquifer are highly variable in length, ranging from less than 500 feet to lnger than 5 miles. Many of the shorter flow paths end by discharging ground water as base flow to both the nontidal and tidal sections of the streams, whereas longer flow paths end by discharging ground water directly to the Chincoteague Bay or the Atlantic Ocean. Travel-times inferred by hexaflouride gas analyses for ground water followingshorter flow paths before discharging to a stream range from less than 1 year to as long as years. Ground water flowing along longer flow paths may take 30 years or more to reach the discharge point at the end of the flow path.
Dissolved ammonia and dissolved nitrate are the dominant nutrients in ground water in the study aera. Ammonia concentrations as high as 23.4 milligrams per liter as nitrogen were detected in anixic ground-water samples. Nitrate concentratons in ground-water samples collected from wells ranged from below 0.05 milligram per liter as nitrogen to 15.5 milligrams per liter as nitrogen, and were highest in oxic ground water. Ammonia is less mobile in ground water than nitrate, so if it remains in the reduced state, transport to the discharge zones would be slower.
Nitrate concentrations in samples of stream base flow ranged from below 0.05 milligrams per liter as nitrogen to 5.28 milligrams per liter as nitrogen, and showed a significant, positive correlation with the percentage of the stream basin area used to cultivate row crops. The majority of base-flow nitrate concentrations were above 0.4 milligrams per liter as nitrogen, the upper limit of the level found in natural waters in the study area, which indicates that the water quality of stream base flow at most sampled stream sites in the area is affected by anthropogenic activities. Previous U.S. Geologic Survey estimates of ground-water nitrate loads to Chincoteague and Sinepuxent Bays assumed an average stream base-flow nitrate concentration of 0.72 milligrams per liter as nitrogen. The average stream base-flow nitrate concentration in samples collected during this study was 1.75 milligrams per liter as nitrogen. In addition, the presence of ammonia and organic nitrogen in stream base-flow samples may have implications for the eutrophication of the coastal bays, as these nitrogen species are available as nutrients to microorganisms and plants.
