Cleaning up: Range of cleanup options considered for Lower Duwamish Waterway site

Join us at a public open house on Tuesday, Dec. 5

This past summer, the   Lower Duwamish Waterway cleanup reached a key milestone: pollution sources are controlled enough to begin the first phase of cleaning up the riverbed’s contaminated sediments. There’s only one location in this initial area that isn’t quite ready for sediment cleanup, which is the waterway adjacent to the Boeing Isaacson Thompson site in Tukwila. But we’ve been studying different ways to clean up the site and control pollution, and we’d like your input on the preferred option.

Boeing Isaacson Thompson

Aerial map showing parcel boundaries of the Boeing Isaacson Thompson cleanup site

The Boeing Isaacson Thompson cleanup site is made up of two parcels currently owned by Boeing and one owned by the Port of Seattle. Historically, a number of industrial activities took place here, including wood treatment and disposal of smelter slag and fire brick. Contaminants in the site’s soil and groundwater include metals, PCBs, petroleum hydrocarbons, and volatile and semi-volatile organic compounds.

Cleanup efforts began several decades ago. In the early 1990s, the property owner attempted to stabilize the contaminated soil by adding concrete. However, too much concrete was mixed in with the soil, which raised the pH of the groundwater. The higher pH is making the arsenic in the soil soluble, so arsenic is leaching into the groundwater. The arsenic is then transported by groundwater into the Duwamish Waterway, where it’s accumulating in the river’s sediments.

Aerial map showing contaminated areas in the cleanup site.

Several different approaches to cleanup have been considered. The preferred option is a combination of activities that include:

  • Installing a permeable barrier to treat the contaminated groundwater before it enters the river
  • Excavating a portion of the contaminated soil
  • Containing the soil that remains on the site and preventing it from eroding into the river

South Park Landfill

Another cleanup site in the Lower Duwamish Waterway area, South Park Landfill, is also having a public comment period. The legal agreement between us and the City of Seattle (called an Agreed Order) and the Interim Action work plan are being updated to reflect the City’s current, less extensive plans for redevelopment.

Learn more at our open house

We’re hosting an open house on Tuesday, Dec. 5, 2023, 5:30 – 7:30 p.m. Drop by in-person for food and conversation about the Boeing Isaacson Thompson and South Park Landfill cleanup sites, or join us online. Phone interpretation will be available in Spanish, Chinese, Vietnamese, and Khmer. All ages welcome!

In-person: Duwamish River Community Hub, 8600 14th Ave. South, Seattle, WA 98108

Online: Register on Zoom