Duwamish/Diagonal Combined Sewer Overflow/Storm Drain Remediation

Status: Remediation Completed


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 area to include another area immediately upstream to minimize the potential for recontamination from any later remediation. After further evaluation, the Elliott Bay/Duwamish Restoration Program panel expanded the project.

The project site is on the east side of the Lower Duwamish Waterway, upstream of Harbor Island and immediately downstream of Kellogg Island. The 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 two areas total approximately 7 acres.

The project isolated contaminated sediment near the following four outfall pipes:

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

The area dredged is known to contain polychlorinated biphenyls, or PCBs. All dredged sediment (66,000 cubic yards) was below the Toxic Substances Control Act PCB limit so it could be shipped by train to the Roosevelt Landfill in Klickitat County. The dredged area was backfilled with sand (with some armoring near an active slip) and topped with a special near-shore habitat mix. Some dredging residuals were identified outside of the dredge site during post-construction sampling. During the next opportunity, a thin layer of sand was placed over approximately 4 acres south and west of the dredge site to address those dredging residuals and to compare enhanced natural recovery to monitored natural recover in that portion of the Lower Duwamish Waterway.

LDWG Lead: King County

King County's Duwamish/Diagonal Web Page:


Lead Agency: Washington State Department of Ecology


  • January 2001: Cleanup Study Report issued
  • July 2002: Cleanup action decisions issued
  • November 2003: Dredging began
  • January 2004: Dredging completed
  • March 2004: Project completed
  • February 2005: Thin-layer placement completed
  • 2004 - 2009: Continued monitoring

King County's Duwamish Home Page: