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King County did the assessment to make sure that combined sewer overflow projects (CSO) are well-planned and timed to optimize water quality improvements in the basins where King County’s CSOs discharge. The assessment will:

King County has been working to reduce CSOs for decades. Learn more about projects that are currently under way.

Every 5 years, King County updates its Long Term Control Plan for CSOs. The Water Quality Assessment and Monitoring Study will support decision-making about the remaining CSO control projects in King County’s 2018 CSO Long-term Control Plan Update.

Yes. King County has a “consent decree” agreement with the federal government that outlines the time frame and schedule for completing the CSO program.

The study focuses on the water bodies where King County has CSOs that overflow more than an average of once per year: Elliott Bay, Lake Union, the Lake Washington Ship Canal, and The Duwamish Estuary.

  • Water does not always meet state water quality standards for
    • Bacteria –Often exceeds in the Duwamish Estuary and parts of Elliott Bay and occasionally in parts of the Lake Union Ship Canal.
    • Temperature -- Occasionally exceeds standards.
    • Dissolved Oxygen – Occasionally exceeds standards in the parts of the Duwamish River where dissolved oxygen standards apply.
  • Water does not always meet human health standards for:
    • Banned industrial chemicals called PCBs.
  • However, long term trends show that bacteria, nutrients, and dissolved oxygen have been improving.
  • A “controlled” CSO overflows no more than one time each year, based on long-term average. The assessment shows this will significantly reduce bacteria loading to the waterways on rainy days.
  • Completing the CSO projects will reduce the amount of fecal coliform bacterial entering the water bodies by 80 percent. CSO control will not substantially affect the other impairments, including temperature and dissolved oxygen.
  • Controlling CSOs will reduce other pollutants (in addition to bacteria) entering the water bodies. But other pathways will continue to be larger contributors of those pollutants.

CSOs are only one pathway for pollution to get to water bodies. The assessment examined 15 pathways (including CSOs) and 14 pollutants. The largest pathways for each pollutant include:

Pollutant Largest pathway for this pollutant
Bacteria CSOs
Total PAHs (chemicals found in creosote and fossil fuels)
Creosote treated pilings
Boat paint containing copper to prevent organisms from attaching to the boat

The remaining pollutants:

  • nutrients
  • solids
  • arsenic
  • phthalates (used in plastics)
  • lead
  • mercury
  • PBDEs (flame retardants)
  • PCBs (industrial chemicals banned in 1979)
Stormwater and upstream watersheds

On February 9, 2017, an equipment malfunction during heavy rains led to flooding at the West Point Treatment Plant, which resulted in limited treatment capacity for 3 months. Untreated bypasses occurred on three days: Feb 9, 15, 16. Effluent discharged outside of those dates received full disinfection and primary treatment until the plant was performing full secondary treatment by the end of April. King County increased monitoring at West Point and other locations for a time shortly after the incident. Results are available at West Point marine water quality monitoring page.

These events at West Point do not affect the overall findings of the Water Quality Assessment:

  • The assessment looked at decades of data and focused on ongoing and long-term trends. The West Point incident was short-term in comparison and would not change the long-term trends.
  • West Point is outside of the area examined by the assessment, which specifically focused on waterbodies where King County has uncontrolled CSOs.

However, the study provides a reference for understanding the incident at West Point in the context of regional water quality.

Janice Johnson
CSO Control Program