Norfolk CSO Remediation

Status: Remediation Completed


The Norfolk combined sewer overflow and storm drain is located near the south end of King County Airport on the Duwamish River. The Norfolk combined sewer overflow outfall, which is part of the King County wastewater system, discharges runoff and untreated sewage during heavy rains when the sewer system is filled with water. The storm drain, which discharges through the same outfall, discharges runoff from the surrounding subbasin. River-bottom sediment around the outfall contained high concentrations of certain contaminants, primarily mercury, bis (2-ethylhexyl) phthalate, and polychlorinated biphenyls, or PCBs.

Although the sources of these contaminants are unknown, King County led the cleanup of the Norfolk site under the supervision of the Elliott Bay/Duwamish Restoration Program, or EBDRP. The EBDRP committee is made up of representatives from U.S. Fish and Wildlife, Washington State Department of Ecology, Suquamish Tribe, Muckleshoot Tribe, City of Seattle, National Oceanographic and Atmospheric Administration, and King County. One goal of EBDRP is to remediate contaminated sediment from King County's and City of Seattleā€™s combined sewer overflows and stormwater--and the cleanup of Norfolk is part of that goal, as was the Duwamish/Diagonal combined sewer overflow/storm drain project.

The area just offshore of the Norfolk combined sewer overflow/storm drain (CSO/SD), outfall was originally identified as a priority cleanup area by EBDRP in 1991 because of sediment contamination associated with the Norfolk CSO/SD discharge. In 1994, King County investigated sediment contamination near the Norfolk CSO/SD outfall and determined that contaminated sediment should be dredged and the area capped with clean sediment. Dredging began in February 1999.

Contaminated sediment was removed from the site by mechanical dredge and dewatered onshore in a containment area. Sediment with a PCB concentration greater than 45 parts per million (ppm) was transported to a Subtitle C landfill for disposal. Sediment with a PCB concentration less than 45 ppm was transported to a Subtitle D landfill for disposal. A total of 5,190 cubic yards of sediment was removed during the remediation, about 1,900 cubic yards of which were transported to a Subtitle C landfill as hazardous waste.

Most sediment was removed to a depth of 3 feet; however, remediation in the PCB hot spot areas required removal of sediment up to 9 feet deep. The area was backfilled with clean sand. The project was monitored for five years to evaluate possible recontamination of the backfill sediment because of continuing combined sewer overflow or stormwater discharges. During monitoring, elevated PCBs were found at the northern portion of the site near a private storm drain. Actions were taken to control sources from the drain, and the sediment directly in front of the drain was removed. Backfilling within the dredged area was completed by March 1999.

In addition, just north of the Norfolk cleanup area The Boeing Company used a specialized vacuum excavator in 2003 to remove approximately 60 cubic yards of PCB-contaminated sediment near Boeing outfall DC2. This outfall is sometimes referred to as the south storm drain outfall.

LDWG lead: King County

King County's Norfolk webpage:

Lead Agency: National Oceanic and Atmospheric Administration


  • Clean Up Study Report issued October 1996
  • Dredging began February 1999
  • Dredging completed March 1999
  • Project completed April 1999
  • Monitoring continuing until April 2004

King County's Duwamish homepage: