By Eric Wagner, Freelance Writer

The Ship Canal Bridge is a double-deck steel truss that looms nearly two hundred feet above Portage Bay, between Lake Washington and Lake Union in Seattle. More than 200,000 cars pass over this bridge every day, as they have for fifty years, whipping north or south along the twelve lanes that make up Interstate 5. Stand under the bridge at North Passage Point Park, on the shore of Portage Bay, and you can hear the surging hum of all those tires as they whoosh far above you. And if you happen to be in the park during one of the region’s many rainstorms—even one that seems brief and light—you can watch veils of water cascade from the bridge into the bay below overhead. Below your feet, meanwhile, you hear rushing water as it gathers from thousands of acres of Green Lake, Aurora, and north Seattle streets, consolidated into a single pipe that dumps it, untreated, into the canal.

That stormwater, scientists now know, is full of toxic chemicals. But one in particular has caught their attention: N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine; or, as it’s called in shorthand, 6PPD-quinone. An industrial chemical, 6PPD-quinone is used as an antioxidant in rubber tires. Every time a car or truck drives, friction causes tiny bits of tire to flake off onto the road. Those bits and all the toxins they contain—including 6PPD-quinone—lie there until stormwater picks them up and carries them away as runoff to the nearest stream, creek, lake, or river.

Pollution from cars including 6PPD is shed on roadways and then swept into local waterways as stormwater. CreditL Hannah Letinich/TNC

The range of 6PPD-quinone’s effects once in water is still being sketched out, but what is known is sobering. Ecotoxicologists have identified 6PPD-quinone as the main cause of pre-spawn mortality for coho salmon when they return from the ocean to freshwater. One study found that in the most extreme cases, coho can die within a day of exposure to high levels of the toxin.

“6PPD-quinone is one of the five or six most toxic compounds to aquatic organisms ever identified,” says Ed Kolodziej, the engineering professor at the University of Washington who led the team to first describe its effects. “So its impacts go far beyond just salmon.”

With the lethal nature of stormwater in general and 6PPD-quinone in particular becoming better understood, mitigating its impacts is urgent and critical. The approach is two-pronged, involving both source control and treatment. Source control means, in essence, removing 6PPD-quinone from tires, so it cannot even get to the water in the first place. But the compound is so deeply embedded in the production cycle of a ubiquitous product that extracting it could take years, even decades. In the meantime, stormwater will continue to wash off the roads, and salmon and other organisms will continue to die.

That leaves treatment as the most realistic near-term solution. “I like to describe it as a Yes And approach,” says Catherine Gockel, of the Environmental Protection Agency. “We need to get 6PPD out of tires and we need to find a safer alternative. And in the meantime, there’s green stormwater infrastructure that we know alleviates the worst effects of the pollution.”

As stormwater passes through a series of raingardens below the Aurora Bridge, toxins are naturally filtered out, reducing pollution that enters the Puget Sound. Credit: Courtney Baxter/TNC.

“Everyone understands 6PPD and the importance of getting it out of tires,” says Jessie Israel, the Puget Sound Conservation Director, The Nature Conservancy in Washington. “But sometimes it seems we’ve forgotten that we only found 6PPD in the first place because we were learning that green infrastructure works for filtering roadway runoff.” Run even the gnarliest stormwater through a column of compost, soil, and sand, or let it spend some time in a bioswale, and it loses its acute toxicity to salmon. “We now know 6PPD is a smoking gun for coho, but I expect there’s additional noxious firepower in roadway runoff, creating health impacts for both people and nature. The good news is bioretention works. So the question becomes: With limited resources, do we focus only on 6PPD? ” Israel says. “Or do we prioritize green infrastructure in areas of highest overall loading, and highest need for targeted outcomes toward getting to lot more impact, sooner?”

Government managers from the state to the federal level agree that more emphasis on green infrastructure could go a long way to addressing the region’s pressing stormwater issues. “When you use green infrastructure, you also get flow control benefits, you get other water quality benefits,” says Derek Day of the Washington Department of Ecology. “With 6PPD, one of the only effective management strategies is bioretention, so what you want to do is identify toxic hotspots and then see which of them are having the biggest effect on salmon.”

Indeed, green infrastructure projects shrewdly placed can act as force multipliers when it comes to stormwater mitigation. TNC scientists recently published a study in Proceedings of the Royal Society B that showed 70% of stormwater pollution along the whole west coast of the United States could be addressed by treating just 1.35% of the land area. And with The Nature Conservancy’s newly released Stormwater Heatmap, an interactive mapping tool that can help visualize hotspots of pollution generation and runoff throughout the Puget Sound watershed, finding those areas that would allow managers to get the most bang for the buck just became a whole lot easier.

The Stormwater Heatmap provides a clear picture of stormwater pollution across the Puget Sound.

“Getting more green infrastructure in the landscape is an important first step, and second to that is optimizing the performance of treatment systems so we treat more area in a smaller space,” says Jen McIntyre, a professor at Washington State University who studies stormwater chemistry and mitigation. Through innovative engineering solutions that maximize the efficiency of green infrastructure on a small site footprint, stormwater parks and facilities are popping up across the region, treating acreage and water volumes that would have seemed impossible just a few years ago.

This is where the Ship Canal Bridge and North Passage Point Park come in. The Nature Conservancy has identified the site as an ideal place for a new kind of public-private partnership, with a hybrid stormwater treatment facility and public space. “The drainage basin as in area is such that more than 2,000 acres drain to this single point under the bridge,” says Paul Fendt, a civil engineer with Parametrix, an environmental consultancy firm working with The Nature Conservancy to design the park. “And we saw an opportunity to make a public space that was innovative from a stormwater treatment standpoint, and also educational and just pleasant to be in. It would be a way of to make stormwater treatment something that the public not only sees and appreciates, but also recreates in.”

The hope is that, supported with funding from the recent federal infrastructure bill, a stormwater park could treat almost a billion gallons of stormwater every year. In this, a little investment could go a long way. And as is painfully clear every time the rains come and stormwater flows from the bridge into the bay, there is precious little time to waste.

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