THE USE OF NITROGEN STABLE ISOTOPES TO ASSESS THE SOURCES AND DISTRIBUTION OF INORGANIC NITROGEN IN THE GROUND WATER OF AN AGRICULTURAL AREA LOCATED ON THE EASTERN SHORE OF VIRGINIA.


Robert J. Tapper.


On the Eastern Shore of Virginia, upland agricultural areas are linked to adjacent tidal creeks and marshes by the flux of ground water nutrients. As such, there is great potential for ground water discharge to alter nutrient cycling, trophic structure, and secondary production in this environment. Nitrogen isotope studies in ground water systems have been employed to identify sources of nitrate contamination and to identify redox reactions involving nitrogen species. The present study will utilize nitrogen stable isotopic analysis to determine the source and cycling of inorganic nitrogen inputs to a tidal creek and marsh from an agricultural field.

The proposed investigation will be conducted at the Brownsville Research Site, which is owned by the Nature Conservancy and located approximately two to three miles east of Nassawadox, Virginia, on the Delmarva Peninsula. The research site is used extensively for agricultural purposes, and is surrounded on three sides by tidal salt marshes and Phillips Creek. A series of nested wells will be installed at the center and perimeter of the agricultural field, creekbank, and tidal marsh, thereby creating a field-marsh and field-creek transect.

The specific objectives of the study are:

    1. To characterize concentrations of ammonium and nitrate, del-15Nammonium and del-15Nnitrate as ground water is discharged from the upland agricultural field to the adjacent tidal creek and marsh.
    2. To estimate the rate of ground water discharge and nutrient influx at the field-marsh and field-creek transects.
    3. To evaluate short term (tidal cycle) and monthly variation in concentrations of ammonium and nitrate, del-15Nammonium , del-15Nnitrate, and nutrient influx at the field-marsh and field-creek transects.