Slip 4 Early Action Area | Region 10 | US EPA

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Slip 4 Early Action Area

Lower Duwamish Waterway Sitewide Information

Early Action Cleanup Areas

Slip 4 Contacts

Karen Keeley (keely.karen@epa.gov)
Project Manager
206-553-2141

Renee Dagseth (dagseth.renee@epa.gov), Community Involvement Coordinator
206-553-1889

En Espaņol: Si desea hablar con alguien que habla Espaņol, llame a: Michael Ortiz (ortiz.michael@epa.gov)
206-553-6234

A river otter checks out progress of the habitat restoration project at Slip 4, which was completed in February 2012.
What is an Early Action?

Early Action cleanup areas are parts of a Superfund site that EPA has determined may become a threat to people or the environment before the long-term cleanup for the site is completed.

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Site Summary: The Slip 4 Early Action Area is part of the Lower Duwamish Waterway Superfund site. It is located on the east side of the Duwamish Waterway, about 3 miles upstream from the point where the waterway divides to flow around Harbor Island and into Elliott Bay near downtown Seattle. The slip is located just north of Boeing Plant 2 and covers about 6.4 acres. Sediments in about 3.6 acres of Slip 4 were found to be contaminated with PCBs.


Construction Complete: The City of Seattle has cleaned up the contaminated sediments and banks of Slip 4. With EPA oversight, the City’s contractor dredged and removed contaminated sediments, including the eroding banks, and placed engineered caps of clean sand and gravel over the remaining sediments. An aging pier was demolished, two beaches were created, and more shallow-water areas were created to improve habitat in Slip 4. Construction started on October 3, 2011, and was completed on February 7, 2012. Total project costs were about $8 million. The cleanup was funded by The Boeing Company and the City of Seattle, and used Model Toxics Control Act matching grant funds from the Washington Department of Ecology.

The cleanup included this work:

  • Dredged and removed approximately 9,800 cubic yards of bottom sediment and bank soils.
  • Capped approximately 3.6 acres to isolate sediments not removed.
  • Removed and disposed of an estimated 130 tons of creosote-treated timbers and piles and other debris.
  • Removed and recycled an estimated 3,278 tons of concrete and 78 tons of steel from a pier.
  • Excavated the banks in a manner so as to create stable slopes and expand habitat.
Overall, the cleanup will include a net gain of over an acre of shallow and riparian habitat for threatened Puget Sound Chinook and Coastal/Puget Sound bull trout.

Water quality monitoring was performed throughout the duration of the project, and only one brief exceedance of turbidity criteria was observed (during placement of clean cap material). The City of Seattle will establish monitoring and controls to protect the sediment cap over the long-term. Upland plantings will occur in Fall 2012.

Background: Beginning in 2003, the City of Seattle and King County initiated investigations of sediment contamination in Slip 4. In May 2006, EPA selected a cleanup plan for Slip 4 sediments. The plan required that sediments with the highest contamination be removed, and that the remaining sediments be "capped" in place. The banks were also required to be cleaned up. In January 2007, Ecology determined that sources of PCBs to the storm drain system that discharge to Slip 4 were not adequately controlled to protect the cleanup in Slip 4 if it were to be implemented at that time. In September 2010, EPA and The Boeing Company implemented treatment for contaminated stormwater from North Boeing Field, which drains to Slip 4. In March 2011, Ecology determined that sources of PCBs were adequately controlled and the cleanup could proceed.
North Boeing Field

The North Boeing Field storm drain system carries stormwater to Slip 4 through more than seven miles of catch basins, drains, inlets, and oil-water separators. Studies by Ecology, City of Seattle, and Boeing showed the North Boeing Field storm drain system is the biggest source of PCBs to river sediments in Slip 4.

Construction Complete: In September 2010, under an agreement with EPA, Boeing installed a new interim stormwater treatment system at North Boeing Field. With the installation of this stormwater treatment system (using chitosan-enhanced sand filtration), approval was given for the cleanup of sediments in Slip 4 to proceed in 2011. This system, which treated approximately 35 million gallons of stormwater between September 2010 and October 2011, reduced the amount of toxic PCBs and other chemicals in stormwater discharged to Slip 4.

In October 2011, Boeing upgraded the stormwater treatment system from 500 gallons per minute to 1,500 gallons per minute. In its first month of operation, the system treated 12 million gallons of water. In addition, a small passive stormwater treatment system, using a biofilter media bed, is currently being operated as a pilot study. The media mix is a blend of rhyolite sand, zeolite, and granular activated carbon; in a second mix, the carbon has been replaced by walnut shells.

Boeing continues to perform source control actions to reduce PCB concentrations in storm drain solids.

Georgetown Steam Plant Flume

Construction complete: In summer 2009, the City of Seattle cleaned up and replaced the Georgetown Flume, which discharges stormwater to Slip 4. Since the early 1900’s, the Flume operated as a 2,450-foot long system of wood-fortified and concrete-lined open ditches and buried piped segments that connected the Georgetown Steam Plant to the Duwamish Waterway at Slip 4. The City of Seattle demolished the existing structure, removed contaminated sediment from within the flume, removed contaminated soil from areas adjacent to the flume, eliminated all unauthorized drains into the flume, and constructed a new drainage system with a new outfall and tide valve at Slip 4. All cleanup levels were met, and the work was completed ahead of schedule.

Review the Removal Action Completion Report (PDF) (685pp, 89MB) - February 2010.

Background: The demolition and removal of the 2,450-foot long system (built in the early 1900’s) of wood-fortified and concrete-lined open ditches and buried piped segments that connected the Georgetown Steam Plant to the Lower Duwamish Waterway at Slip 4 was completed in September 2009. The project goal was to remove contaminated sediments from within the flume and to construct controls so it no longer served as a potential conveyance for contamination to reach Slip 4. Part of the work was conducted as an Independent Remedial Action and part was conducted under EPA oversight for Slip 4 Early Action cleanup Order.

Technical Documents
aerial view of the Lower Duwamish Waterway Slip 4 Action Area site

Aerial photo of the Slip 4 Action Area site (photo courtesty of Washington Department of Health

Sediment Cleanup

Final Water Quality Certification (PDF) (11 pp, 202K) - May 2011
Interim Source Control Status Report (PDF) (30 pp, 2.2MB) - March 2011
Lower Duwamish Waterway Slip 4 Interim Source Control Status Report (PDF) (3 pp, 180K) - March 2011
Lower Duwamish Waterway Slip 4 Early Action Area Biological Assessment (PDF) (94 pp. 13.1MB) - February 9, 2007; Revised by the U.S. EPA October 7, 2010

100% Design Analysis (appendices are available from the Region 10 Superfund Records Center)

Sampling and Analysis Plan for Boundary Definition; Addendum: Pre-Design Investigations Sampling (PDF) (43pp. 3.9 MB) - June 2006 Action Memo for Non-Time-Critical Removal Action - May 5, 2006 Engineering Evaluation/Cost Analysis - February 10, 2006 Revised Draft Technical Memorandum Proposed Boundary of the Early Action (PDF) (38pp, 2.1MB) - January 2005

Source Control

North Boeing Field stormwater treatment
Georgetown Steam Plant Flume

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