Duwamish/Diagonal Sediment Remediation Project

Location: Duwamish River, Seattle, Washington

Funding Source: City of Seattle/Metro Settlement

Project Goals:

• Determine preferred remedial alternative.
• Remove contaminated sediment.
• Cap with clean material to original grade.

Site Summary

The project site is located at the King County Duwamish Combined Sewer Overflow (CSO) and the City of Seattle Diagonal Way CSO/Storm Drain (SD) outfall on the Duwamish River, near Kellogg Island. The Duwamish/Diagonal sediment remediation project was selected by the Elliott Bay/Duwamish River Restoration Program (EB/DRPP) based on criteria identified in the Panel's Concept Document (see summary). The goal of the EB/DRRP include remediation of contaminated sediments associated with CSOs and SDs, and control of potential sources of contamination from outfalls.

In 1994, a cleanup study workplan was developed. It identified nine contaminants of concern, including PCBs, mercury, phthalates, and zinc. Field work was conducted in 1994 to 1996 to determine the extent of contamination around the outfalls. In 2001 a cleanup study report was released. A public meeting was held in August 2003 to discuss the proposed cleanup. After public comment, the size of the proposed cleanup was increased from five to seven acres. A total of 68,250 cubic yards of sediments was dredged from a depth of three to five feet and disposed of off-site at an approved location. The dredged area was then capped with clean material to isolate contamination located beneath the dredge depth.

Post-dredging sampling in the vicinity of the remediated area indicated that PCB concentrations were increased at a few locations. After discussions with the DOE and EPA, King County placed a thin-layer cap (six to nine inches) of clean material over a four-acre area. Capping was completed in February 2005. Monitoring is underway for both the seven-acre dredged and four-acre thin cap areas.

Restoration Activities

• 1994 - Cleanup Study Workplan issued.
• 1994 to 1996 - Field work conducted to determine extent of contamination.
• 2001 - Cleanup Study Report issued.
• 2003 - Public meeting held in August.
• 2003 to 2004 - Dredging of the seven-acre area began in November 2003 and capping was completed in January.
• 2005 - Placement of a thin-layer cap on the additional four acres was completed in February.