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A water quality assessment and monitoring study is under way to ensure that future projects to control pollution are well-planned and timed to improve water quality.

The assessment will:

  • Inform King County’s Protecting Our Waters Program.
  • Investigate scientific information about water quality at relief points in the sewer system called “combined sewer overflows,” or CSOs, where sewage mixed with stormwater can flow to area waterways on very rainy days. 
  • Examine local water quality concerns near King County CSOs in Elliott Bay, Lake Union, the Lake Washington Ship Canal, and the Duwamish River.

Assessment components

  • Review of existing data and literature
  • Additional monitoring studies to fill data gaps 
  • Synthesis report to pull the information all together

The study is being conducted by King County, is reviewed by an independent science and technical review team to ensure quality work, and is guided by a transparent public participation process

How will the assessment be used?

The assessment will:

  • Inform King County’s 2018 CSO Plan Update.
  • Establish baseline conditions for mandatory post-construction monitoring of CSO control projects.
  • Make technical information available to many organizations working to improve water quality.
scientist on the Duwamish
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Study schedule

Water quality assessment and monitoring study schedule

Collaboration with the City of Seattle

All King County Protecting our Waters projects are located within the City of Seattle. The city has a similar program for Protecting Seattle’s Waterways happening simultaneously. The two agencies continue to work together to get the most out of these water quality investments and coordinating with each other on studies like this one.

Erika Peterson
Water Quality Project Manager

206-477-5525