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Bridges In Stormwater Spotlight: Pilot Project Points Up Need For More Treatment

Stormwater runoff is the largest source of toxic chemicals entering Puget Sound. It happens when rain hits pavement and carries pollutants such as oil or brake dust into local waterways, and it turns out bridges are an especially big source of that runoff.

A local developer is piloting a solution that will be featured at a Green Infrastructure Summit taking place at UW Bothell Friday.

If you stand beneath the Aurora Bridge in Fremont, just down the hill from the famous Troll statue, you can look up and see gutter pipes that extend up the pylons, 150 feet high. They’re catching runoff from Highway 99. And until just a few months ago, they were all dumping hundreds of thousands of gallons of polluted stormwater directly into Lake Union.

“And it’s really bad water, it’s really contaminated,” said Mark Grey, a property manager with Steven Grey and Associates, which just built a new office building right next to the bridge.

“If we were looking at a sample of it right now, the infamous cup that our scientists took, it looks like a cup of coffee.”    

Credit Courtesy Mark Grey.
Runoff collected from a downspout of the Highway 99 bridge on Troll Avenue in Fremont in April 2017, before the installation of the new bioswales.

That dark brew is full of toxic pollution. Grey became convinced this was a problem after seeing video footage that shows how baby salmon die when immersed in the toxic runoff.

His company took it upon themselves to install landscaping to filter out pollutants under the bridge when they put in their new building next to it.  They re-plumbed beneath the sidewalk.

And now one of the long downspouts coming off Highway 99 is sending about 200,000 gallons of storm water per year into curbside rain gardens called ‘bioswales’ that mimic nature and catch and hold back the worst pollutants.

“This was all asphalt. It was an asphalt jungle and that’s all that was here and the water just ran down the asphalt into the catch basin at the bottom of the hill and went into the lake,” Grey said. “Now we have all this plant and vegetation that’s treating the water and sending it into our lake clean.”  

Credit Bellamy Pailthorp
Bellamy Pailthorp
Grey's company installed these bioswales to treat the runoff, in City of Seattle right of way. They say the intent is also to beautify the neighborhood and educate passersby with informative signage.

One of the biggest challenges in this process was not the cost, but the permitting between state and local jurisdictions.

The company has plans for two additional projects along this bridge that will bring the total stormwater treated to two million gallons a year. In turn, these projects are serving as a pilot for others here and statewide.

Legally, the state isn’t required to treat stormwater that comes off of its bridges, says Ellen Southard, with the non-profit Salmon Safe Puget Sound. That’s why her group and several others including The Nature Conservancy have honed in on this issue.

They’re working first to divert runoff from the six bridges that span Seattle’s Lake Union and the ship canal.   

“It’s a major conundrum in Puget Sound and across the state,” she said. “These six bridges are because this is a key salmon migration corridor and it is our mission to protect the salmon and to protect the way of life of Puget Sound.”

She says salmon are an indicator species for all in the ecosystem, including humans, so when salmon are struggling, we should take note that our health is likely affected, too.

One of the biggest challenges in this process was not the cost, but the permitting between state and local jurisdictions. The runoff comes mostly from a state highway – but it lands in city right of way, where the treatment takes place thanks to new infrastructure that was built by a private company. It took a lot of persistence and repeat meetings to get all of the paperwork right, says Grey.

The parties involved in this pilot project say they hope they’ve created new pathways to more easily divert toxic pollution from bridges statewide in the future.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to