Loading estimates of PCBs from upstream sources transported by the Green-Duwamish River, WA to the Lower Duwamish Waterway
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- Terrestrial and Aquatic Ecology
The sediments in the Lower Duwamish Waterway (LDW) in Seattle, Washington are contaminated with polychlorinated biphenyls (PCBs) from decades of intense industrial, commercial, and residential activities. Previous studies predicted that greater than 99 percent of new sediment entering the LDW will originate from upstream sources. The U.S. Geological Survey (USGS) is conducting field monitoring to estimate sediment loads and sediment-associated toxic chemical loads from upstream sources in the watershed that are transported by the Green-Duwamish River to the LDW. Two new real-time stream gaging stations in tidally-influenced reaches of the Green-Duwamish River upstream of the LDW were installed (at River Mile, RM, 5 and 10). Each station utilizes an Acoustic Doppler Velocity Meter and turbidity sensor to provide continuous, publicly-available discharge and turbidity data. During a range of hydrological conditions representing seasonal, storm-, and dam-related variations in flow and/or turbidity, the USGS is collecting representative samples of water, suspended sediment and bed sediment from the upstream station (RM 10) for analysis of a broad suite of contaminants including PCB Aroclors and 209 congeners. Concurrent with the chemistry sampling, the USGS is measuring discharge, suspended sediment concentration and particle size distribution, and general water quality parameters. The resulting data is being used to develop regression relations between discrete samples and continuous turbidity and discharge to improve estimates of event, seasonal, and annual sediment loading and sediment-associated chemical loading from the Green-Duwamish River to the LDW. During a spring 2013 storm event with peak flows of 5000 cubic feet per second, a suspended sediment-associated loading of approximately 200 mg/hr of PCBs was estimated, which was more than 50 times higher than low-flow loading estimates. This presentation will provide preliminary results from the first phase of the study and describe current monitoring activities.