Norfolk Combined Sewer Overflow (CSO) Sediment Remediation Project
Location: Duwamish River, Tukwila, WA
Funding Source: City of Seattle/Metro Settlement
Reduce overflow effluent volume of approximately 70 million gallons to 7.30
Determine preferred remedial alternative.
Remove contaminated sediment.
Restore intertidal habitat.
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 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: 
(4184KB)  (4204KB) 
(3558). Appendices not included.
August 1999released 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:  
October 1999 - End of first dry season, released Six-Month Post-Construction
April 2000 - End of first wet season, released Year One Annual Monitoring
April 2001 - End of Year Two, In November 2001,
released Year Two Monitoring Report. Also in two smaller files: 
April 2002 - Year Three.
April 2003 - Year Four.
April 2004 - Year Five.