Lower Duwamish Waterway
Source Control Investigation

Slip 4
Early Action Area 3

Slip 4 view looking SE into the slip (Ecology 2005) Slip 4 is located approximately 3 miles upstream from Harbor Island, just north of Boeing Plant 2. The slip encompasses 6.4 acres and is approximately 1,400 feet long with an average width of 200 feet. The northwest side of the slip is mostly covered with docks and a berthing area.

Sediments in Slip 4 have accumulated chemical contaminants from numerous sources. These sources are both historical and potentially ongoing. Chemicals entered the slip in different ways, such as through direct discharges, spills, bank erosion, groundwater discharge, surface water runoff, or other non-point discharges.

In the inner portion of the slip, PCB concentrations have been greater than state standards since at least 1990. Phthalate contamination is also present. Under EPA and Ecology oversight, the City of Seattle and King County are investigating the potential sources of sediment contamination at Slip 4 and preparing for cleanup.

Current Status

Ecology recently completed an Interim Source Control Status Report for the adjacent Slip 4. Enough source control work has been completed to allow for the EPA to move forward with sediment cleanup at the head of Slip 4 this year. EPA will provide information about the Slip 4 cleanup as additional construction details become available. Work to control sources in this area will continue. For more information about EPA's Slip 4 work, visit http://yosemite.epa.gov/r10/cleanup.nsf/ldw/slip+4+early+action+area

Georgetown Steam Plant Flume

The Georgetown Steam Plant (GTSP) is located at 6700 13th Avenue South in Seattle, Washington. It is on the east side of the Lower Duwamish Waterway. The Site is bordered to the west by Ellis Avenue South, the south by North Boeing Field, and the northeast by King County International Airport.

When built, the GTSP was next to the Duwamish River. When the river was straightened in 1916 to form the Duwamish Waterway, the GTSP Flume (Flume) was constructed to carry cooling water to Slip Four.   

The city of Seattle (City) is preparing to clean and replace the Flume.  The City will conduct part of the Flume project as an Independent Remedial Action under the Washington Department of Ecology. The remainder is being performed under Environmental Protection Agency (EPA) oversight as part of an Administrative Settlement Order on Consent (ASAOC).

The City has completed a State Environmental Policy Act (SEPA) environmental checklist for the project and has issued a Determination of Non-significance (DNS).  The checklist and a fact sheet about the project can be viewed below:

Cleanup Sites in Early Action Area 3-Slip 4

Related Information

Additional Resources

This page last updated 01/04/2017