NITRATE LOADING AS A FUNCTION OF STREAM DISCHARGE ALONG THE EASTERN SHORE OF VIRGINIA

A predictive model was developed to determine annual nitrate loading to sea-side lagoons from streams along Virginia's Eastern Shore under base-flow conditions. Nitrogen is added through a variety of nonpoint sources, eventually reaching the Chesapeake Bay to the west and lagoons to the east of the Delmarva Peninsula via base flow from low-relief streams. The purpose of this study is to quantify the contribution of these streams to the enrichment of nitrate concentrations in the sea-side lagoons and to determine the hydrologic and watershed land-use factors that most affect nitrate loading.

Thirteen watersheds were selected for nutrient sampling and discharge measurements, ranging in size from 213 ha to 1971 ha. Selections were made so that a range of watershed areas and land-use categories within each class of watershed areas would be obtained. Nine synoptic surveys were performed between July 2001 and May 2006, measuring N03^- concentration and discharge at stream locations downgradient in the watershed but uninfluenced by tidal effects.

We determined relationships between stream discharge and watershed area in each of our sampled watersheds and used a regression of these relationships to estimate discharge for the entire eastern portion of the Eastern Shore under base-flow conditions. Discharge values varied spatially among streams by as much as a factor of 23 and temporally for a single stream up to a factor of 14. Nitrate concentrations ranged from 0.14 mg/L to 9.4 mg/L. In all but one sampling survey, the two streams with the lowest nitrate concentrations drained watersheds with the greatest fraction of forested land as compared with cropland, developed land, and miscellaneous other land uses. Estimated annual nitrate loadings to the lagoons based on individual surveys ranged from 5.2 x 10⁴ kg to 2.2 x 10⁵ kg, varying primarily due to seasonal streamflow fluctuations. Comparisons between nitrogen loading predictions made using measured stream nitrate concentration values and spatially-averaged concentration values in individual watersheds differed by less than 9% in all instances, indicating that stream discharge is a better predictor for overall nitrate loading than specific nitrate concentration.