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Tire preservative found responsible for coho salmon deaths in urban streams

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In the Pacific Northwest, coho salmon must undergo a dangerous journey from the ocean to toxin-infested urban and suburban streams in order to breed. More than half of them die before they can spawn — in some streams, salmon populations have been completely eliminated. 

Researchers at the UW Tacoma and Seattle campuses, as well as Washington State University, have recently published a study revealing why adult coho salmon in Puget Sound’s urban streams are dying at such an alarming rate

Rainfall in the region carries bits of old tires into neighboring streams, which by itself, seems toxic enough. The culprit, however, is located specifically in the mix of chemicals that leach from tire wear particles: a preservative called 6PPD.

In the past, researchers noted that acute mortality events in salmon populations were linked to stormwater runoff, but the exact chemical causing these deaths had not been identified. A multi-agency effort, headed by the National Oceanic and Atmospheric Administration (NOAA) Northwest Fisheries Science Center (NWFSC) Ecotoxicology Program, hoping to identify the killer had begun long before the UW study was published. 

In order to narrow down what could be afflicting salmon populations, the researchers compared water samples from creeks where coho salmon were dying to search for common trends. They concluded that tire wear particles contained the lethal chemical behind salmon deaths. However, tire wear particles contain a melting pot of different chemicals, and the researchers needed to single out the killer. 

“We manipulated the water composition by passing the water through chemical sorbents and media,” Edward Kolodziej, senior co-author and associate professor in the UW department of civil & environmental engineering, said. “By tracking where the toxicity went, we could learn what chemical formula it was.”

The researchers identified a substance called 6PPD, which is used to keep tires from breaking down too quickly. While helpful on the road, 6PPD quickly transforms into a toxic chemical called 6PPD-quinone once it reacts with the very pollutants it protects tires from.

“6PPD works by reacting with ground-level ozone before the ozone can degrade and weaken the tires, which helps make them safer,” Kolodziej said. “However, once 6PPD reacts with ozone, the transformation product it makes is toxic, and then washes off the tires and roads when it rains.”

Unsurprisingly, 6PPD-quinone is not limited to the Puget Sound region. This substance was found to be present in runoff and urban streams along the Pacific coast, in cities like Los Angeles and San Francisco. As 6PPD is widely used in global tire production, traces of the chemical are likely to be present in streams near highways around the world.

Although research is yet to be done on the effects of 6PPD-quinone on other fish and wildlife, Kolodziej hopes to conduct some of those studies to better understand what species might be more or less sensitive to this chemical.

How can we protect vulnerable salmon populations and other creatures living in urban streams? Treating roadway runoff is a good preventative measure, and tests have shown that there are robust and environmentally-friendly stormwater technologies for removing 6PPD-quinone. However, it would be costly and difficult to collect and systematize all of the stormwater in the Pacific Northwest.

“The long-term solution is to use green chemistry principles to find non-toxic alternatives to 6PPD and use them in tires,” Kolodziej said. “It’s better if consumer products like tires, especially those which enter the environment in so many ways, are made from chemical ingredients whose environmental safety and fate has been carefully evaluated.”

Reach contributing writer Rachel Sun at Twitter: @rachickenn

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