Ship Canal Water Quality

What’s happening now?

Seattle Public Utilities and King County Wastewater Treatment Division are building an underground storage tunnel to significantly reduce the amount of polluted stormwater (from rain) and sewage that flows into the Lake Washington Ship Canal, Salmon Bay and Lake Union from our sewer system.

Learn more about the Ship Canal Water Quality project and construction taking place in your neighborhood.

Background

In some parts of Seattle, sewage and stormwater (rain) share a set of pipes; this is called a combined sewer. During heavy rains (What? Rain? Here?) the water often exceeds the pipes' capacity (known as an overflow to us sewer nerds), sending untreated sewage (yep, that means poop) and stormwater into the Ship Canal. These overflows can harm fish, wildlife, the environment and can contain pollution.

In 2018, 84% of the city's overflows came from the combined sewers in Crown Hill, Ballard, Fremont, Wallingford, Queen Anne, Downtown and Capitol Hill. During a heavy storm, the new tunnel will capture and temporarily store more than 29 million gallons of untreated stormwater and sewage until the treatment plant is ready for it. The tunnel will improve water quality regionally by keeping more than 75 million gallons of polluted stormwater (from rain) and sewage from flowing into the Lake Washington Ship Canal, Salmon Bay and Lake Union on average each year.