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Toxics Cleanup Program

Lower Duwamish Waterway
Source Control Investigation

Slip 2 - Slip 3
River Mile 1.7-2.0 East

Site Background Click on photo for larger view. 

The RM 1.7-2.0 East source control area is also referred to as Slip 2 to Slip 3.  This source control area is located along the eastern side of the LDW Superfund Site between river mile 1.7 and 2.0 as measured from the southern end of Harbor Island. Several facilities in this source control area are located directly adjacent to the Lower Duwamish Waterway.  From north to south, these facilities are:

  • Glacier Northwest, Inc.(Glacier Northwest),
  • Seattle Biodiesel,
  • Samson Tug and Barge, and
  • Duwamish Marine Center.

Slip 2 is situated between Glacier Northwest and the Duwamish Marine Center.  Located to the east of these properties are East Marginal Way South and other industrial facilities. To the north of Glacier Northwest is James Hardie Gypsum and to the south and southeast of Duwamish Marine Center are the LDW, a vacant lot owned by the Seattle Department of Transportation (DOT) and Slip 3.

There are six outfalls discharging storm water to the RM 1.7-2.0 East source control area.  These outfalls include four private and two public outfalls. One of the public outfalls is the Michigan Street combined sewer outfall (CSOs). Some areas of the Lower Duwamish Waterway are served by combined sewer systems, which carry both stormwater and municipal/industrial wastewater in a single pipe. These systems were generally constructed before about 1970 because it was less expensive to install a single pipe rather than separate storm and sanitary systems. Under normal rainfall conditions, wastewater and stormwater are conveyed through this combined sewer pipe to a wastewater treatment facility. During large storm events, however, the total volume of wastewater and stormwater can sometimes exceed the conveyance and treatment capacity of the combined sewer system. When this occurs, the combined sewer system is designed to overflow through relief points, called combined sewer overflows (CSOs). The CSOs prevent the combined sewer system from backing up and creating flooding problems.

The locations of the outfalls located within the Slip 2-Slip 3 source control area are shown on the map.


Sediments in the portion of the LDW near the RM 1.7-2.0 East source control area have been contaminated by various sources and chemicals.  The contaminants first affected upland media including surface water, groundwater, soil, and air before reaching the LDW sediments.

The following chemicals are considered to be contaminants of concern with regard to potential sediment

Mercury PCBs
Lead  Phthalates (butyl benzyl phthalate)
Dioxins  Semivolatile Organic Compounds (SVOCs)
Furans Poly Aromatic Hydrocarbons (PAHs)
Tetrachloroethene (PCE)  

Source Control

Ecology is the lead for source control for the LDW Superfund site.  Source control is the process of finding and then stopping or reducing releases of pollution to the river.  Ecology looks at the RM 1.7-2.0 East (Slip 2-Slip 3) source control area to determine what sources need to be controlled (see map).  The routes for contaminants to the sediments near this source control area include: direct discharges via piped outfalls, bank erosion from adjacent properties, surface runoff from adjacent properties, groundwater discharge, air deposition, and spills directly to the inlet.

Source Control Action Plan

Ecology developed a Source Control Action Plan for RM 1.7-2.0 East (Slip 2-Slip 3) which describes what source control actions are necessary.  As part of the Source Control Action Plan, Ecology hired a contractor to develop a Summary of Existing Information and Data Gaps report.