A TIERED WATERSHED PCB RUNOFF LOADING APPROACH FOR UNMONITORED TRIBUTARIES IN A TMDL PROCESS

Runoff volume from unmonitored tributaries is estimated using three methods for the initial stage of the Delaware Estuary TMDL for Polychlorinated Biphenyls. 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. The third approach will employ an application of the USEPA SWMM5 model.Event mean concentrations of PCBs in urban runoff were derived to estimate yields of PCBs from urban areas. To provide estimates for PCB contributions from rural areas, a simple USEPA indirect loading methodology was employed.