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CityLink Seattle

Tunneling Our Way to Cleaner Waters

Seattle Public Utilities (SPU) is committed to protecting public health and the environment and enhancing our natural resources. As part of our strategy to protect our local waterways, SPU and King County have begun construction on a 2.7-mile long tunnel, running from Ballard to Wallingford. How will a tunnel protect our local waters? The answer has to do with something called a combined sewer overflow.

Combined Sewer Overflow diagram.
Click to enlarge

What is a Combined Sewer Overflow?
In Seattle’s older neighborhoods, including Ballard, Fremont, Wallingford, and Queen Anne, sewage and stormwater funnel into a single set of pipes. In dry weather, these pipes send combined sewage and stormwater to King County’s West Point Wastewater treatment plant. During heavy rains, however, these pipes can become overloaded. When that happens, they send a mixture of stormwater (90%) and sewage (10%) to outfalls that flow into the nearest body of water. These combined sewer overflows, or CSOs, contain contaminants that can make people sick and harm fish, wildlife, and the environment. SPU has completed several projects around the city to address CSOs and improve the health of our local waterways. The Ship Canal Water Quality Project is by far the largest.


The Ship Canal Water Quality Project
SPU and King County’s Ship Canal Water Quality Project will address CSOs into Seattle’s Salmon Bay, the Ship Canal, and Lake Union. The 2.7-mile long, 18-foot 10-inch diameter tunnel will capture and temporarily hold stormwater and sewage during heavy rains. Once the storm passes, a pump station will send the wastewater to the treatment plant. Once complete, the SCWQP will prevent an average of 75 million gallons of polluted stormwater and sewage from entering our waterways annually – that’s enough to fill 90 Olympic-sized swimming pools! The project is just one of the many ways SPU is reinvesting your dollars back into projects and programs that benefit the health of our community and the environment.

A construction crew works
on building the new 24th Ave NW Pier in Ballard, an early piece of the Ship Canal Water Quality Project.
By significantly reducing polluted stormwater and sewage overflows, the Ship Canal Water Quality Project will create a healthier environment for 100K spawning sockeye salmon who travel through it each year!