A recently completed King County study may mark a turning point in efforts to protect coho salmon from a persistent urban pollutant: toxic tire dust. For decades, coho returning to urban streams like those in the Puget Sound region have faced up to 90% mortality due to a chemical called 6PPD-quinone—a compound formed when tire preservatives react with ozone. The chemical’s toxicity is extreme; even trace concentrations can be fatal.

Researchers evaluated four soil mixes designed for stormwater treatment, including one standard compost-sand mix and three high-performance variants incorporating sand, coconut fiber, and biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More. All filtered out 6PPD-quinone to some degree, but the high-performance mixes—especially those containing biochar—were the most effective.

In a controlled lab experiment, only 1 or 2 out of 20 juvenile coho salmon survived exposure to untreated stormwater collected from beneath Seattle’s I-5 corridor. In contrast, all 20 salmon survived exposure to water filtered through the biochar-based mixes. These results offer compelling early evidence that engineered soils could mitigate one of the most lethal stressors in urban salmon habitats.

While more testing is underway at a field site near Bellingham, and broader implementation is still a few years off, these findings support the growing role of biochar as a critical filtration material. King County scientists are now working to map high-risk roadways and prioritize areas for stormwater treatment retrofits.

The research may inform state-level updates to stormwater guidelines by 2027 or 2028.

LEARN MORE: King County scientists identify a potential breakthrough for treating salmon-killing tire chemical