Quantifying Long-Term NPS Pollutant Flux in an Urbanizing Watershed

Long-term nonpoint source (NPS) pollutant flux is described within the rapidly developing Occoquan watershed west of Washington, D.C. Data consist of up to 24 years of observed rainfall, integrated pollutant discharge, and land use/land cover from four headwater basins of the Occoquan River. Three of the four study basins, ranging in size from 67 to 400 km2, are predominantly forest and mixed agriculture. The fourth basin, the 127 km2 Cub Run watershed, has urbanized rapidly over the past 20 years. Higher annual NPS sediment and nutrient fluxes in Cub Run after 1983 are linked to increased soil disturbance from urban construction and increased storm volumes resulting from increased mean impervious percent. Over the long-term, storm fluxes of NPS particulate P, soluble P, particulate N, and soluble N make up 92, 67, 89, and 50%, respectively, of the total fluxes of those constituents, with between 88 and 98% of mean annual total suspended solids fluxes delivered by storm flow. Higher NPS pollutant fluxes in Cub Run basin after 1983, and specifically during the growing season, indicate a seasonal impact of replacing vegetated cover with impervious surface.