IMPROVED UNDERSTANDING OF NITROGEN FATE AND TRANSPORT IN SUPPORT OF EXPLAINING WATER-QUALITY TRENDS IN THE CHESAPEAKE BAY WATERSHED

Understanding spatial variability in contaminant fate and transport is critical to relating changing water quality over time to changes in land-use, sources, management practices, or other potential causes. An approach was developed to capitalize on previously calibrated spatially-referenced regression (SPARROW) models to improve the understanding of nitrogen fate and transport in the 166,000-square-kilometer (km²) Chesapeake Bay watershed. A continuous function of four hydrogeologic, soil, and other landscape properties significant (alpha=0.10) to nitrogen transport from uplands to stream corridors was evaluated and compared among each of the more than 80,000 individual catchments (mean area, 2.1 km²) within the watershed. Budgets (including inputs, losses or net change in storage in uplands and stream corridors, and delivery to tidal waters) also were estimated for nitrogen applied to these catchments through upland agricultural or atmospheric sources. Most such nitrogen inputs are removed, retained, or otherwise processed in watershed uplands rather than transported to surface waters. Nitrogen transport to streams is most efficient (conservative) in parts of the Delmarva Peninsula and in the north-central and western areas of the Chesapeake Bay watershed (particularly in the lower Susquehanna and upper Potomac River watersheds), where groundwater recharge is relatively high and soil conditions conducive to denitrification are relatively uncommon. Combining SPARROW results with previous budget estimates suggests that 55 percent of upland losses occur through denitrification in soils or groundwater, while a further 23 percent is attributable to crop or timber harvest and 6 percent to volatilization. Remaining upland inputs represent a net annual increase in landscape storage in soils or biomass exceeding 10 kilograms per hectare in some areas.