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Now available: New study results to help tackle tough water quality problems

By November 28, 2017August 26th, 2020No Comments

Decades of infrastructure investments are paying off for regional water quality. Even with big population gains, our waterways are healthier than they were 40 years ago.

While this is welcome news, a recent study on water quality that was led in part by King County scientists shows there are still a number of actions to take to keep our waters healthy for the next generation of Puget Sound residents.

The Water Quality Assessment Study – How Data Informs Decisions

Protecting water quality is WTD’s top mission and controlling combined sewer overflows, or CSOs, is part of that.

CSOs are stormwater mixed with small amounts of sewage that still happen in older parts of Seattle during heavy rains.CSO

Every five years, King County updates its plans for reducing the frequency of these overflows. These plans explain the solutions and schedules for future projects to control the overflows. King County just completed a Water Quality Assessment and Monitoring Study to inform the 2018 CSO Long-Term Control Plan Update.

The study focuses on the water bodies where King County has CSOs that overflow more than an average of once a year.1Graphic

Here’s What We Did

The Water Quality Assessment gathered decades of monitoring data to understand long term trends. King County scientists collected and analyzed hundreds of new water samples in order to understand current conditions. They researched how planned programs will affect water quality in the future.

Here’s What We Learned – Improving Water Quality Trends

The water quality investments the region made over the last 40 years have paid off. Even though more people live here, the water has:

  • Less bacteria that can make people sick
  • Fewer nutrients that can cause toxic algae blooms
  • More dissolved oxygen for fish to breathe



But there is more to do. Despite improvements, water does not always meet state water quality standards for bacteria, dissolved oxygen, and temperature or human health standards for banned industrial chemicals called PCBs. The study shows King County’s and Seattle’s plans to finish CSO projects and meet state standards will reduce bacteria entering the water bodies by 80 percent.

Sharing the data with the region

CSOs are only one pathway for pollution. Improving CSOs will have the biggest effect on bacteria. Other pathways and pollutants need attention and action. At King County, we are committed to doing our part. And we are committed to sharing what we learn and partnering with others to achieve our region’s goals. The data that King County scientists gather, analyze, and report can be used to address other pollution pathways.


Working together for clean water

Our region has a good track record tackling tough water quality problems. We have done it before. The long-term improvements in bacteria, nutrients, and dissolved oxygen show the work is paying off.

Let’s keep doing it.

Learn more about the Water Quality Assessment study online at