A team led by researchers at the University of Washington Tacoma, UW and Washington State University Puyallup have found that a highly toxic oxidation product of tire rubber particles turns streams toxic and may be responsible for the annual die-offs observed among migrating adult salmon across the US Pacific Northwest. A paper on the work is published in Science.

For decades, researchers have observed regular acute mortality events affecting adult coho salmon (Oncorhycchus kisutch) that migrate into urban waterways contaminated with stormwater runoff. In the most urbanized watersheds, urban runoff mortality syndrome (URMS) is estimated to kill 40-90% of returning salmon before they have a chance to spawn.

While URMS has been tied to stormwater runoff and potentially linked to tire tread wear particles (TWPs)—one of the most significant sources of microplastics in freshwater—the one or more toxicants responsible for killing salmon have remained elusive.

Using liquid chromatography mass spectrometry and nuclear magnetic resonance, lead author Zhenyu Tian, a research scientist at the Center for Urban Waters at UW Tacoma, and colleagues explored the compounds associated with roadway runoff and TWP to search for toxic compounds.

They found that 6PPD, the primary antioxidant chemical used in tire rubber, reacts with ozone to form the previously unidentified compound 6PPD-quinone.

Ozone breaks the bonds holding the tire together. 6PPD reacts with ozone before it can react with the tire rubber, sparing the tires. But when 6PPD reacts with ozone, the researchers found that it is transformed into multiple chemicals, including 6PPD-quinone.

According to the authors, 6PPD-quinone is highly toxic and is deadly to juvenile salmon at concentrations of roughly 1 microgram per liter. Retrospective analysis suggests that this deadly compound is widespread in stormwater-impacted waterways across the US West Coast.

It is unlikely that coho salmon are uniquely sensitive, and the toxicology of 6PPD transformation products in other aquatic species should be assessed.

… If management of 6PPD-quinone discharges is needed to protect coho salmon or other aquatic organisms, adaptive regulatory and treatment strategies along with source control and “green chemistry” substitutions (i.e., identifying demonstrably non-toxic and environmentally benign replacement antioxidants) can be considered.

More broadly, we recommend more careful toxicological assessment for transformation products of all high production volume commercial chemicals subject to pervasive environmental discharge.

—Tian et al.

Resources

• Zhenyu Tian, Haoqi Zhao, Katherine T. Peter, Melissa Gonzalez, Jill Wetzel, Christopher Wu, Ximin Hu, Jasmine Prat, Emma Mudrock, Rachel Hettinger, Allan E. Cortina, Rajshree Ghosh Biswas, Flávio Vinicius Crizóstomo Kock, Ronald Soong, Amy Jenne, Bowen Du, Fan Hou, Huan He, Rachel Lundeen, Alicia Gilbreath, Rebecca Sutton, Nathaniel L. Scholz, Jay W. Davis, Michael C. Dodd, Andre Simpson, Jenifer K. Mcintyre, Edward P. Kolodziej (2020) “A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon” Science doi: 10.1126/science.abd6951