Norfolk Combined Sewer Overflow (CSO) Sediment Remediation Project

Location: Duwamish River, Tukwila, WA

Funding Source: City of Seattle/Metro Settlement

Project Goals:

• Reduce overflow effluent volume of approximately 70 million gallons to 7.30 million gallons.
• Determine preferred remedial alternative.
• Remove contaminated sediment.
• Restore intertidal habitat.

Site Summary

The Norfolk Combined Sewer Overflow (CSO) sediment remediation project was selected by the Elliott Bay/Duwamish Restoration Program (EB/DRP) based on criteria identified in the Panel's Concept Document (see summary). The goals of the EB/DRP include remediation of contaminated sediments associated with combined sewer overflows (CSOs) and storm drains, and control of potential sources of contamination from outfalls.

The Norfolk CSO site is located in the Duwamish River, above Turning Basin No. 3 at approximately river kilometer 10 in the City of Tukwila (see Location map). The site is located upstream of the river reach maintained for commercial navigation and has maintained its natural channel and some riparian habitat. The shoreline is characterized by a steeply sloping, erosional bank maintained with large concrete riprap and piling walls. The bank joins a gently sloping intertidal mud shoreline that is exposed during extreme low tides.

A site characterization and cleanup study was performed in 1994 and 1995, and the cleanup study report was issued in 1996 (see Cleanup area map). Chemicals of concern at the site included mercury, 1,4- dichlorobenzene, bis(2-ethylhexyl)phthalate and polychlorinated biphenyls (PCBs), all present at concentrations exceeding State of Washington Sediment Management Standards (SMS) sediment chemical criteria values that define the cleanup screening levels. PCB "hot spot" concentrations at the site also exceeded Toxic Substances Control Act (TSCA) limits for hazardous waste disposal. This sediment remediation project was designed to remove those sediments.

Site remediation began in early February 1999 and was completed in late March 1999 (see Closure Report). Remedial activities consisted of dredging contaminated sediment and backfilling the dredged area to original grade with clean sediment from the Duwamish River Turning Basin. 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 remediation, approximately 1,900 cubic yards of which were transported to a Subtitle C landfill as hazardous waste. Sediment was generally removed to a depth of three feet however, remediation in the PCB hot spot areas required removal of sediment up to nine feet in depth. The project was monitored for a period of five years to evaluate possible recontamination of the backfill sediment as a result of continuing CSO or storm water discharges. The schedule follows:

• October 1996-Issued Norfolk CSO Sediment Cleanup Study. In three files: [1] (4184KB) [2] (4204KB) [3] (3558). Appendices not included.
• August 1999—released Norfolk CSO Sediment Remediation Project Closure Report (1517KB)
• April 1999 - establish baseline conditions, Panel Publication 22. In August 1999, released Norfolk CSO Project, Monitoring Report. Also in four smaller files: [1] [2] [3] [4].
• October 1999 - End of first dry season, released Six-Month Post-Construction Monitoring Report.
• April 2000 - End of first wet season, released Year One Annual Monitoring Report.
• April 2001 - End of Year Two, In November 2001, released Year Two Monitoring Report. Also in two smaller files: [1] [2].
• April 2002 - Year Three.
• April 2003 - Year Four.
• April 2004 - Year Five.