On the Eastern Shore of Virginia, upland agricultural areas are linked to adjacent wetland areas by the flux of groundwater nutrients. As such, there is great potential for groundwater discharge to alter nutrient cycling, trophie structure, and secondary production in this environment. in this study, groundwater was monitored at a series of nested wells installed in the center and perimeter of an agricultural field which borders a tidal creek and is adjacent to a salt marsh. Nitrate concentrations are higher on the creek side of the field than on the marsh side (mean NO3- = 373 uM/L and <2 uM/L, respectively). Ammonium concentrations are greater on the marsh side of the field than on the creek side (mean NH4+ = 23 uM/L and 2 uM/L, respectively). Concentrations of nitrate generally decrease and ammonium concentrations increase in the direction of ground water flow. The observed nitrate concentrations can be as high as 700 uM/L and suggest possible contamination from human activity.
Del15 nitrate analyses suggest no simple relationship between isotopic composition and concentrations of ammonium and nitrate. The d15N ammonium values of selected samples suggest that ammonium is formed by mineralization of organic nitrogen. The d15N values of selected samples suggest that nitrate is derived from soil or fertilizer sources, and is modified by denitrification. The marsh/upland ecotone is a sharp ecosystem N-economy transition which, at the Virginia Coast Reserve, is not yet overwhelmed by heavy agricultural input.
• Steve Macko and R. J. Tappe Graduate Student