SEATTLE — For decades, toxic tire dust has choked coho salmon before they can spawn in their natal streams. Now, King County scientists say they have made a "potential breakthrough" in how to save them.

Preliminary results from a recently completed study show certain soil mixes can effectively filter a toxic chemical out of stormwater, boosting coho salmon's survival rates significantly. The key may be in special soil mixes containing sand, coconut fiber and biochar, a charcoal-like organic fertilizer, King County scientists said this week.

The results suggest there is a potential solution to a seemingly intractable issue of how coho salmon can survive in urban streams where pollution from traffic is so ubiquitous.

Scientists first identified toxic tire dust, or more specifically the chemical 6PPD-quinone, as a salmon-killer in 2020. For decades, observers had watched in agony as adult coho salmon mysteriously gasped at the surface of urban waters before dying.

In the 2020 study, scientists, including some with the Center for Urban Waters in Tacoma, the University of Washington and Washington State University, were able to isolate 6PPD-quinone as the chemical at fault among some 2,000 chemicals in roadway runoff.

The researchers found the chemical is so lethal it kills 40% to 90% of returning coho to some urban streams before they spawn. The chemical was the culprit behind the deaths of coho in about 40% of the Puget Sound area.

"We're talking concentrations that are about a few drops in an Olympic-sized swimming pool could kill half of a population of coho that you might put in that pool," said Chelsea Mitchell, a senior ecotoxicologist with King County.

6PPD-quinone is a byproduct of a preservative in tires that is created when the tires come in contact with ozone, either from the atmosphere or a vehicle's exhaust, Mitchell said.

The research around how toxic tire dust affects other fish is growing. The chemical also kills other species like rainbow trout and whitespotted char, she said.

In the recent King County study, researchers — including Mitchell and county science section manager Josh Latterell — sought to evaluate how effective certain soil mixes approved for stormwater treatment are at filtering out toxic tire dust.

In the study, researchers gathered polluted stormwater runoff from under the busy Ship Canal Bridge on Interstate 5. At a lab at Western Washington University, researchers filtered the untreated water through four different soil mixes — including a compost and sand mix and three "high performance" mixes consisting of the coconut fiber, sand and biochar.

All four soil mixes filtered out 6PPD-quinone, with the high-performance mixes performing the best. Then, at a King County lab, researchers exposed 20 juvenile coho salmon to untreated and treated water. What they found made their jaws drop, Latterell said.

Of the baby coho that were exposed to untreated stormwater runoff, only one or two survived. However, of the coho that were exposed to treated water, all 20 survived.

"To me, this is just like a mic-drop moment," he said. "If you had a time machine and took this back 25 years to when people first started looking into prespawn mortality, they would be dancing in the streets to see this."

King County researchers are still finalizing a report on their results and intend to share it with the Washington Department of Ecology, which provides guidelines on stormwater treatment in the state.

The compost and sand soil mix, which is used widely around the state, is prone to releasing nutrients and minerals into waterways, Mitchell said. Since 2013, Ecology has recommended against using it near sensitive waters.

Now, researchers are testing a high-performance soil mix outside of the lab at a site near Bellingham. While there is still more work to make sure the mix is safe, Latterell said he hopes to see more stormwater treatment projects using it by 2027 or 2028.

Latterell said he remembers being a graduate student in 1999 when his colleagues were just starting to notice that coho seemed to be mysteriously dying in creeks.

"It's just been really gratifying to see how determined and systematic scientific inquiry has led to ... understanding what's going on here," he said.

King County is working to identify roadways with higher concentrations of toxic tire dust and create a list of areas that need stormwater treatment.

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Information from the Seattle Times archive was used in this report.

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