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Project update

May 2022

Soil sampling at tank site near 1st Avenue bridge starting week of May 16

In early May, crews will mark the locations of existing utilities and sample locations with paint to prepare for groundwater and soil testing. As early as May 16, crews will use a truck- or track-mounted drill rig, like the one pictured below, to collect soil samples and construct groundwater monitoring wells. Once drilling is complete, crews will restore the surrounding surface. 

Read more HERE .

a drill rig mounted on a truckTruck-mounted drill rig

King County WTD is building a storage tank near the 1st Ave S Bridge that will reduce water pollution from the South Park and Highland Park drainage basins into the Duwamish River.

Read about the project’s history, what’s happening now, and how you can share your thoughts with the project team .

For questions about this project or for language assistance needs, please reach out to Kristine Cramer at or Amelia Bahr at

Project need

Like many cities around the country, older parts of King County’s sewer system use the same set of pipes to carry both sewage and stormwater to a treatment plant. During storms, the pipes can fill with stormwater that runs off roofs, driveways and streets. When the system is overwhelmed, it is designed to overflow. These overflows are called Combined Sewer Overflows, or CSOs.

To keep the sewer system working and to prevent sewer backups, the excess water and sewage is released into our local water bodies through CSOs. However, CSOs pose a risk to public health and the environment. Over the past several years, King County has been planning a project to reduce CSOs from the South Park drainage basin into the Duwamish River. Our requirement is to reduce CSOs to no more than one overflow per year on average.

Project overview

The West Duwamish CSO Control Project is moving forward with a storage tank to hold stormwater and wastewater during large storms. The tank will prevent raw sewage from spilling into the Duwamish River during these storms. After the storm, the water will flow to West Point Treatment Plant for cleaning.

Read about the project’s history, what’s happening now, and how you can share your thoughts with the project team .

Map diplaying location of new storage facility near the 1st avenue bridge (State Route 509), on the west bank of the Duwamish Waterway. Two additional underground structures, next to West Marginal Way SW, are part of the project..


Contact us

Contact Kristine Cramer, community services lead at:


Amelia Bahr at:

If you’d like to receive email or text updates on the West Duwamish CSO Control project:

Project location

West Duwamish CSO control map

Back to the capital projects overview map .

Combined sewer systemWhy does sewage overflow on the rainiest days?

Person talking to a King County staff person at information boothKing County RainWise team at the 2021 Duwamish River Festival