SEEPAGE OF GROUNDWATER NITRATE FROM A RIPARIAN AGROECOSYSTEM INTO THEWYE RIVER ESTUARY

Intensive research in Chesapeake Bay has indicated that reductions in nitrogen inputs to the bay will be necessary to restore water quality to levels needed for resurgence of bay living resources. Fall-line wat er quality monitoring efforts have characterized diffuse-source nitrog en inputs from a large percentage of the bay drainage basin, but relat ively little information exists regarding rates of nitrogen delivery t o tidal waters from coastal plain regions. Extensive nitrate contamina tion of shallow groundwater due to agricultural activities, coupled wi th the dominant role of subsurface flow in discharge from Coastal Plai n regions of the drainage basin, creates the potential for high rates of nitrogen delivery to tidal waters via groundwater seepage. This stu dy utilized intensive hydrologic and water chemistry monitoring from A pril 1992 through September 1994 to determine the spatial characterist ics of the groundwater-estuarine interface, as well as the rates of su bsurface nitrogen transport from an agricultural field into nearshore waters of the Wye River, a subestuary of Chesapeake Bay. The hydrogeol ogic characteristics of the study site resulted in groundwater dischar ge to the Wye River occurring almost exclusively within 15 m of the sh oreline. Calculated groundwater discharge rates were found to vary wid ely in the short term due to tidal fluctuations but in the long term w ere driven by seasonal changes in groundwater recharge rates. The zone of groundwater discharge contracted shoreward during summer months of low discharge, and expanded to a maximum width of approximately 15 m during high discharge periods in late winter. Average discharge rates were more than five times higher in winter versus summer months. Groun dwater nitrate concentrations entering the discharge zone were relativ ely stable throughout the study period, with little evidence of denitr ification or nitrate uptake by riparian vegetation. Consequently, nitr ogen discharge patterns reflected the strong seasonality in groundwate r discharge. Annual nitrate-N discharge was approximately 1.2 kg m(-1) of shoreline, indicating drainage basin rates of nitrogen delivery to tidal waters of approximately 60 kg ha(-1).