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Sediment Management Program

Remediating and rehabilitating the region's waterways

For more information about the Sediment Management Program, please contact Caryn Sengupta.

Wastewater Treatment Division 
King Street Center
Suite 0505
201 South Jackson Street
Seattle, WA 98104

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Norfolk CSO sediment remediation project

norfolk sediment remediation


The project is part of the Elliott Bay/Duwamish Restoration Program, or EBDRP. The program is the result of a 1991 legal settlement reached by the City of Seattle and King County (then the Municipality of Metropolitan Seattle, or Metro) with the National Oceanic and Atmospheric Administration, or NOAA.

The settlement projects include sediment remediation, habitat development and improvements, and pollution source-control measures. The U.S. Fish and Wildlife Service, Washington State Department of Ecology (Ecology), Suquamish Tribe, Muckleshoot Tribe, NOAA, City of Seattle and King County administer the program and make up the EBDRP Panel.

The U.S. Environmental Protection Agency, Ecology, King County and City of Seattle worked together on source control, project planning and design for the Norfolk CSO Sediment Remediation Project.

Project description norfolk sediment remediation

The Norfolk Combined Sewer Overflow (CSO) cleanup project removed contaminated sediments from the Duwamish River to enhance and restore the river habitat.

The Norfolk CSO, or combined sewer overflow, is located near the south end of Boeing Field on the Duwamish River. The CSO is part of the county wastewater system, and discharges run-off and untreated sewage during periods of heavy rainfall when the sewer system is inundated with water. The submerged sediments around the CSO were found to contain high levels of certain contaminants, mainly mercury, bis (2-ethylhexyl) phthalate, and PCBs.

Site remediation began in February 1999 and was completed in March 1999. Activities consisted of dredging contaminated sediment and backfilling the dredged area to original grade with clean sediment. Contaminated sediments were removed from the site by mechanical dredge and dewatered onshore in a containment area.

Sediments with a PCB concentration greater than 45 parts per million (ppm) were transported to a Subtitle C landfill for disposal. Sediments with a PCB concentration less than 45 ppm were transported to a Subtitle D landfill for disposal. A total of 5,190 cubic yards of sediment was removed during the dredging, approximately 1,900 cubic yards were transported to a Subtitle C landfill as hazardous waste. Sediment was generally removed to a depth of three feet.

The project was monitored for a period of five years to evaluate possible recontamination of the backfill sediment as a result of continuing CSO discharges. After the monitoring period is completed, the site will be evaluated as part of the early action sites in the Superfund area.

In 2003, The Boeing Company conducted a cleanup of river sediments near the Norfolk CSO site using a specialized vacuum excavator, approximately 100 cubic yards of sediment was removed.