A WATERSHED RUNOFF LOADING METHODOLOGY FOR POLYCHLORINATED BIPHENYLS

For the initial stage of the Delaware Estuary TMDL for Polychlorinated Biphenyls (PCBs), estimates were needed for watershed runoff loads of PCBs delivered to the Delaware River and Bay from tributaries for which no monitoring data were available. For these tidewater tributary basins, an alternative approach was developed to estimate Polychlorinated Biphenyl (PCB) loads using data available from existing international stormwater data bases and from some locally-collected stream discharge water quality data bases. The approach is based on studies conducted both in the region and elsewhere in the United States, and is used to estimate average daily PCB loadings for comparisons with other pollutant sources in the total maximum daily load assessments.Runoff volume from the unmonitored tributaries is estimated using two methods. The first is a simplified rainfall-runoff depth estimate determined from a modified rational formula approach. The second method uses runoff volumes from USGS streamflow gages in watersheds characteristic of the physiographic provinces and sub-regional hydrologic conditions found within the tidewater tributary basins. By performing baseflow separations to determine surface runoff volumes for the selected characteristic gages, unit area surface runoff volumes were calculated and applied across the urban/suburban/commercial portions of the tidewater tributaries.To estimate yields of PCBs from urban areas in the basins, event mean concentrations of PCBs in urban runoff were derived through the retrieval and careful review of over 200 references that yielded 12 investigations with EMC results for PCBs. These investigations were conducted from 1979 to 1997 in approximately 20 cities located in Canada, the United States, France, Germany, and Japan. EMC results were obtained from approximately 70 runoff data collection sites and represented approximately 130 station-storms.The literature search for PCB EMCs yielded no information suitable for estimating loads from rural areas. To provide estimates for PCB contributions from rural areas, a simple USEPA indirect loading methodology was employed. For this application, the atmospheric deposition rates were taken from the published and unpublished works of researchers at Rutgers University, who have conducted atmospheric deposition monitoring in the Delaware Estuary drainage (Van Ry, et al., 2002). Selecting only the rural site information from their data, estimates were performed using wetfall, dryfall, and an allowance for surface adsorptive processes, assumed to be approximately equal to 50% of the total wetfall plus dryfall. Pass through loading rates for PCBs in rural watersheds initially were assigned as 10% for land surfaces, and 90% for water surfaces.Application of this loading methodology in the initial Stage of the PCB TMDL for the Delaware Estuary yielded a total load estimate of about 9 kilograms of Penta-PCB for a 577-day period, or about 19% of all estimated external Penta-PCB loads to the estuary.