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Sediment Management Program

Remediating and rehabilitating the region's waterways

For more information about the Sediment Management Program, please contact John Phillips.

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

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Duwamish/Diagonal sediment remediation project

Introduction Duwamish Dredge

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 are working together on source control, project planning and design for the Duwamish/Diagonal Sediment Remediation Project.

Project description

The initial study for this project, issued in January 2002, proposed a cleanup area of about 5 acres. In February 2002 during public review of the study, comments were received that recommended expanding the site to include an adjacent 2 acre area upstream. After further evaluation, the EBDRP Panel expanded the project.

The project site is on the east side of the Duwamish River, upstream of Harbor Island and immediately downstream of Kellogg Island in the lower portion of the Duwamish River. The project site is within the south industrial section of Seattle.

The original area extends about 750 feet along the shoreline upstream and downstream of the Duwamish/Diagonal Way outfalls. It is 260 feet wide. The added site extends about 500 feet along the shoreline upstream and downstream of the former Diagonal Avenue Treatment Plant outfall and the Diagonal Avenue South storm drain outfall. It is 160 feet wide and does not touch the shoreline.

The project isolated contaminated sediment near four outfall pipes on the Duwamish River in Seattle:

  • Diagonal Way storm drain
  • Duwamish combined sewer overflow
  • the former Diagonal Avenue Treatment Plant outfall
  • Diagonal Avenue South storm drain.

These areas were dredged to an average depth of 5 feet and 66,000 cubic yards of contaminated material was removed. The dredged material contained polychlorinated biphenyls, or PCBs, but all dredged sediment was below the Toxic Substances Control Act (TSCA) PCB limit. The contaminated sediment handled by Rabanco waste disposal and shipped by train to the Roosevelt Landfill in Klickitat County.

The project isolated any remaining contaminated sediment from the environment by building an isolating sediment cap. the cap surface was also designed to provide beneficial salmon foraging habitat.

A follow up action was conducted in February 2005. A thin layer of sand was placed around a portion of the dredged area to reduce the level of contaminants from the previous dredging activity.