Slip 4 Early Action Area | Region 10 | US EPA

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Slip 4 Early Action Area

Slip 4 Contacts

Elly Hale (
Project Manager

Julie Congdon (, Community Involvement Coordinator

Site Summary

The Slip 4 Early Action Area is part of the Lower Duwamish Waterway Superfund site in Seattle, Washington. The site is located on the east side of the waterway - just north of the Boeing Plant 2 site - and covers about 6.4 acres. Sediments in about 3.6 acres of Slip 4 were contaminated with PCBs.

In 2006, EPA selected a cleanup plan that required sediments with the highest contamination to be removed, and remaining sediments would be "capped" in place. In 2007, Ecology determined that sources of PCBs to the storm drain system that discharge to Slip 4 were not adequately controlled to protect the proposed cleanup. After EPA and The Boeing Company implemented treatment for contaminated stormwater from North Boeing Field - which drains to Slip 4 - Ecology determined that sources of PCBs were adequately controlled and the cleanup could proceed.

What is an early action area? Early actions are parts of a larger Superfund cleanup that EPA has determined may become a threat to people or the environment before the long-term cleanup for the site is completed.

Construction is 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 work included:

  • 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 occurred late in 2012.

North Boeing Field

The North Boeing Field (NBF) 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 was a continuing source of PCBs to river sediments in Slip 4.

In September 2010, under an agreement with EPA, Boeing installed a new interim stormwater treatment system at NBF. 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. The system preferentially treats storm flows from the onsite NBF North Lateral, while also treating storm drain base flow and a portion of all the storm flow that drains to the King County lift station and to Slip 4, including offsite flow from the King County International Airport. In 2012 through 2015, approximately 176 to 195 million gallons of water were treated and discharged each year. The 4-year cumulative capture and treatment of 66 percent of all stormwater that flows through the lift station exceeds the original design basis of 59 percent, indicating the capacity of the treatment system remains appropriate.

Throughout the first four years of operation (see February 2016 report), all water samples of the system effluent have been non-detect for PCBs (except one), and performance standards and goals for the project have been achieved. The one water sample that was above the PCB limit of detection (0.005 ug/L total PCBs), had a measured PCB concentration of 0.006 μg/L, which is still below the limit of quantitation (0.010 μg/L total PCBs).

On March 2, 2016, EPA determined that all work under the settlement agreement with Boeing had been completed, and that the Completion Report and four Annual Performance Evaluation Reports documented compliance with the objectives specified in the settlement agreement. Boeing will continue to operate the stormwater treatment system as a Treatment Best Management Practice under the Washington Department of Ecology’s Industrial Stormwater General Permit (ISGP). Boeing also continues to perform source control actions to reduce PCB concentrations in storm drain solids.

Georgetown Steam Plant Flume

Construction complete at Georgetown Steam Flume: 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.

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 the Slip 4 Early Action cleanup Order.


Some technical reports and other documents for the Slip 4 Early Action Area are available below:

Sediment Cleanup Documents

Source Control Documents

North Boeing Field stormwater treatment:
Georgetown Steam Plant Flume:

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