QUANTIFYING NPS POLLUTANT DELIVERY IN AN URBANIZING HEADWATER BASIN Mark Dougherty Randel L. Dymond Carl E. Zipper PhD candidate, Civil and Environmental Engineering, Virginia Tech Assoc. Professor, Civil and Environmental Engineering, Virginia Tech Assoc. Professor, Crop and Soil Environmental Sciences, Virginia Tech ( mdougher@vt.edu ) KEYWORDS: NPS flux, urbanization, storm, precipitation INTRODUCTION In spite of several national nonpoint source (NPS) studies (U.S.EPA, 1983; Driver and Tasker, 1988), research in diffuse pollution that includes both urban and rural sources has been on the fringe of environmental engineering research (Novotny, 1999). A number of studies have measured pollutant fluxes from large mixed land use watersheds, demonstrating the importance of land use management in controlling the magnitude of TSS-related fluxes, and that most TSS fluxes occur during large or intense storm events. However, few studies in literature have the combined long-term precipitation and integrated pollutant discharge data necessary to evaluate NPS pollutant flux as a function of precipitation (Correll et al., 1999b). This research investigates fundamental watershed relationships using a unique assembly of long-term spatial and water quality data. The study area consists of four headwater catchments in the Piedmont physiographic province of the Chesapeake Bay drainage. The basins are part of the 1530 km 2 Occoquan River watershed in northern Virginia (figure 1). The three western basins, ranging in size from 67 to 400 km 2 , are predominantly forest and/or mixed agriculture. The fourth basin, the 127 km 2 Cub Run watershed, is rapidly urbanizing, with 17% impervious surface and 50% of current land use classed as urban. Figure 1. Location Map: Occoquan River Watershed Study Area, Northern Virginia, USA.