DETERMINING NITROGEN FLUXES IN LOW-RELIEF COASTAL STREAMS: ASSESSMENT OF PROJECTING LOCALIZED MEASUREMENTS TO REGIONAL-SCALE MODELING

Environmental problems caused by nitrogen (N) pollution of natural waters, including eutrophication, are currently receiving public and legislative attention. Determining N fluxes from specific watersheds for use in N management and reduction plans is a current challenge. The goal of this study was to assess methods of predicting regional-scale N fluxes from incomplete observations of numerous watersheds based on a thorough study of a subset of those watersheds. Low-relief coastal streams draining into seaside lagoons on the Delmarva Peninsula of Virginia comprised the area of study; 54 such streams have been mapped. Rating curves were developed for 4 streams encompassing the latitudinal range of the Peninsula to quantify discharge (Q) from remote pressure measurements, and stream N concentrations (C) were measured over varying multi-year periods under diverse stream-flow levels. A C/Q relationship was developed and applied to annual stream Q records to determine N flux from the studied streams. Weaknesses in the C/Q data were the dearth of storm samples and unreliable stage-Q relationships at high flow. Additional rain-event sampling of the 3 streams with the shortest periods of record improved the C/Q relationship. The dominant factor in determining N flux to coastal waters was found to be Q. High variability in Q magnitude for these small streams overwhelmed the relatively lesser chemical variability in determining annual flux. Watershed contributing area, as determined using ArcGIS, was used as proxy for stream Q from unmonitored streams scaled to the 4 study streams. Feasibility of projecting individual stream N flux measurements to make regional-scale flux estimations was investigated through sporadic efforts to quantify N flux for 12 other streams in the study area. Determination of the validity of using representative streams in modeling much larger-scale fluxes is crucial in refining current methods for quantifying N load delivered to receiving water bodies.