Lower Duwamish Waterway
Source Control Investigation
Early Action Area 1
Combined Sewer Overflow/storm drain (CSO/SD)
The Duwamish/Diagonal source control area is located on the east side of the
river approximately one-half mile south of Harbor Island. The area is
adjacent to storm drains and combined sewer overflows owned and operated by
Seattle Public Utilities (SPU), King County, and the Port of Seattle (Port). The
Port also owns upland property adjacent to the area.
The Duwamish/Diagonal Way drainage basin for the combined sewer service area
includes about 4,900 acres. Sewage and storm water collected in the
combined sewer basin normally flows to a treatment plant; however, when there is
a large storm event, the combined sewer overflow drains into the Duwamish
Waterway. The storm drain basin includes about 2,600 acres. Storm water
in this basin always flows into the Duwamish.
This area was identified as a cleanup priority by the Elliott Bay/Duwamish
Restoration Program (EBDRP) in the mid-1990s. This designation was based
on concerns about contaminated sediments (mud at the bottom of the river) near
the cleanup site (see Upland Contaminated Sites, Port of Seattle Terminal 106,
and Port of Seattle Terminal 108).
In 1993, the Duwamish/Diagonal sediment cleanup project began. Based on the
results of studies conducted in 1994 and 1996, PCBs, mercury, bis(2-ethylhexyl)
phthalate (BEHP), and butyl benzyl phthalate were identified as the major
chemicals of concern in the sediments. A draft site assessment and a
cleanup study report were prepared by EBDRP (King County 2000, 2001).
The draft Duwamish/Diagonal Cleanup Study Report (King County 2001) proposed a
cleanup of 4.8 acres of sediments. During public review in February 2002, some
comments recommended the site be expanded to remove an area upstream which had
high concentrations of contaminants. This area was called a “hot spot”.
The concern was that the five acre cleanup area would be recontaminated with
PCBs when the “hot spot” was dredged (removal of sediment) in the future.
The project was expanded to seven acres to remove the upstream “hot spot”
(Ecology 2002c). Partial cleanup at the Duwamish/Diagonal Way site began
in November 2003 and was completed in March 2004. Cleanup consisted of
dredging contaminated sediments from the site and placing an engineered sediment
cap to isolate remaining sediment contamination.
Follow up Action
In February 2005 a follow up action was conducted due to unacceptable levels of PCB contamination surrounding portions of the dredged area.
A thin layer of sand was placed over the most contaminated area to reduce the exposure of marine organisms to contaminants and accelerate natural recovery of the area.
The King County Closure Reports (July 2005) are
More information related to Duwamish Diagonal can be found on King County’s Webpage:
King County's Duwamish Diagonal Sediment Remediation Project (King County).
Source Control Action Plan Completed
Ecology identified potential sources in the Duwamish/Diagonal Drainage basin
that could recontaminate sediments. Ecology reviewed the files for properties
near the site. These reviews provided information used to create the Source
Control Action Plan. You can view the property reviews
here. Ecology developed a
Source Control Action Plan
for this site in December 2004. Appendix A of this document gives a summary of
the source control actions needed to protect the bank and sediments once they
have been cleaned up.
See Duwamish/Diagonal plan action tables.
Summary of Existing Information and Data Gaps Report Completed.
The Source control Action Plan identified 338 facilities in the
Duwamish/Diagonal Drainage basin that needed to be evaluated for their potential
to recontaminate sediments. Ecology hired a contractor to review readily
available information and develop a
Summary of Existing Information and Data
Terminal 108 is located on the Duwamish Waterway immediately north of Diagonal
In January 2007 the Port conducted an investigation of the soil and
groundwater in the Oregon Street right of way. The study area is located
on the northern side of Terminal 108 and includes the area overlying the
Diagonal/Duwamish combined sewer overflow and storm drain. That report can
be viewed below.
Port of Seattle T-108 Soil and Groundwater Data Report Oregon Street
Right-of-Way (January 2007)
The Port of Seattle conducted an independent groundwater investigation
at Terminal 108 from 2006-2007. This investigation was performed to determine
if groundwater is a source of contamination to the sediments. The results of
this investigation are available below.
Port of Seattle T-108 Groundwater Investigation Final Report (October 2007)
The Port of Seattle prepared a Source Control Strategy Work Plan for Terminal
108. This plan was prepared to achieve the goals in Ecology’s source
control strategy for the Lower Duwamish Waterway. These goals include
preventing recontamination of the sediments to levels that exceed the Washington
State Sediment Management Standards (SMS). This work plan is available
Port of Seattle T-108 Source Control Strategy Work Plan (February 2008)
As part of the Source Control Strategy for Terminal 108, the Port of Seattle
developed an Environmental Conditions Report to document historical information
and data for the property. The Terminal 108 property was used for many different
purposes in the past including sewage treatment plant operations, dry cement
storage and transport, and gas station equipment storage. The Environmental
Conditions Report is available below.
Environmental Conditions Report (January 2009)
The Port of Seattle is also developing Source Control Strategy Plans for
Terminal 108. A separate plan is being developed for each of the two parcels of
the property- the Eastern Parcel and the Western Parcel. The Eastern Parcel plan
is still under development. The Western Parcel Source Control Strategy Plan is
Terminal 108—Western Parcel Source Control Strategy Plan (October 2009)
This page last updated January 1, 2017