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Executive Constantine applauds unanimous vote to adopt long-term pollution control plan


A long-term plan to control combined sewer overflows (CSOs) that includes the scientific study of local waterways to help prioritize future project investments recently won the unanimous support of the King County Council.


King County Executive Dow Constantine saluted the King County Council for unanimously voting to approve a long-term plan to control combined sewer overflows that includes the scientific study of local waterways to help prioritize and refine these water quality project investments.

“With this green-light on an updated CSO plan, we accelerate work on the Lower Duwamish, ensure that clean-up investments are informed by the latest science, and deliver the best outcomes and value for our customers as we meet Clean Water Act requirements,” said Executive Constantine.

Executive Constantine transmitted his long-term plan to control CSOs to the County Council in June. Combined sewer overflows, or CSOs, occur in older parts of Seattle, discharging a combination of wastewater and stormwater into rivers, lakes, and Puget Sound during heavy rains when sewers are full.

The plan calls for King County to design and build nine separate projects at an estimated cost of $711 million to reduce the number of combined sewer overflow discharges at 14 County-owned locations to meet Washington state requirements by 2030.

“We maintained our critical commitment to high water quality standards for Puget Sound and surrounding waterways through adoption of our updated CSO plan,” said Councilmember Larry Phillips, Chair of the Regional Water Quality Committee. “The county’s work to prevent CSO overflows has reduced the volume of untreated wastewater in our waterways from over 20-30 billion gallons per year in the 1970s to the current volume of 800 million gallons per year. This plan will continue to bring the volume down to meet federal and state water quality standards and protect our waterways, beaches, and basements.”

Public input reflected in the plan includes:

  • Making CSO control a higher priority in the Lower Duwamish River area to coincide with federally-coordinated sediment cleanup.
  • Implementing opportunities for Green Stormwater Infrastructure techniques where they can complement and potentially reduce the cost of traditional storage and treatment options.
  • Collaborating with the City of Seattle on projects to control stormwater that enters the regional sewer system and other joint projects where it makes economic sense to do so.
  • Ensuring King County is committed to meeting federal and state water quality requirements by 2030.

Along with the plan, the County Council approved the Executive’s call for a science-based water quality assessment and monitoring study to be carried out in parallel with the implementation of the CSO projects.

Subject to County Council approval, the study would potentially be utilized within the new, integrated planning framework developed by the U.S. Environmental Protection Agency (EPA). Federal, state, tribal, and environmental leaders will be invited to make recommendations to optimize water quality improvements within the CSO discharge areas. The goal of the proposed study is to inform decisions about the integration and sequencing of projects to meet federal and state water quality standards and improve water quality in the CSO discharge areas. 

Since the 1970s, King County has successfully reduced volumes of uncontrolled CSOs in our waterways by more than 90percent. 

The Executive’s plan will comply with state and federal water quality requirements to control the remaining CSOs by 2030.

Learn more about King County’s CSO Control Program at


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