Permeable pavements could reduce tire pollutants that kill salmon

The porous structure of permeable sidewalks may help protect coho salmon by preventing tire wear particles and associated contaminants from entering runoff, according to a Washington State University study.

Researchers demonstrated that four types of permeable coatings can act as giant filters, retaining more than 96% of the mass of applied tire particles. They also captured several tire-associated chemicals, resulting in an average 68% reduction in 6PPD-quinone, a contaminant that kills coho salmon in urban waterways. The results of the study were published in the journal Total Environmental Science.

“Pressure on existing stormwater management technologies is becoming problematic, especially with climate change and increased development,” said lead author Chelsea Mitchell, who recently earned a Ph.D. in environmental and natural resource sciences from WSU. “Permeable sidewalks are a very promising type of green infrastructure for stormwater because they could address this type of pollution where it is generated, rather than downstream.”

In 2020, a team led by scientists from WSU and the University of Washington at the Washington Stormwater Center discovered that 6PPD, a chemical found in tires, converts to 6PPD-quinone when exposed to ozone or sunlight. Even in small concentrations, 6PPD-quinone is fatal to salmon.

For the latest study, WSU scientists led by Ani Jayakaran, a professor at WSU’s Puyallup Research and Extension Center, worked in an active parking lot at Tacoma’s School of Industrial Design, Engineering and Art, Washington, by performing a series of tests on four permeable asphalt or concrete sidewalks. The sidewalks were created in collaboration with Boeing, as well as Tacoma Public Schools and the City of Tacoma.

First, the researchers imitated a rain event by splashing water across sidewalks, thereby measuring the background level of existing pollutants. The next day, they placed crushed used tire treads on a dosing area. They simulated a rainstorm again, measuring the amount of retained deposited particles and associated chemicals.

A third water-rinsing experiment helped researchers measure the likelihood that 6PPD-quinone and other chemicals would continue to leach from retained tire particles during future rainstorms.

“Permeable pavements make a difference in managing 6PPD-quinone and its source: tire wear particles,” said Jayakaran, co-author of the study. “6PPD-quinone is hydrophobic and we believe the chemical is absorbed by the internal surfaces of the pavement system.”

Due to their void spaces, permeable pavements are inherently more fragile than their traditional counterparts, making it difficult to withstand heavy traffic flows. Researchers at the WSU Voiland College of Engineering and Architecture had previously demonstrated that using waste carbon fiber composites from Boeing aircraft wings could make pavements stronger.

The new research could have a huge impact, according to the authors. Salmon are important to the culture of Indigenous nations and to respecting their treaty rights to harvest salmon from waterways. This technology could also have implications for human health.

“Tire particles are so fine that they can be airborne and possibly enter the human body, even in a slight breeze. And even in small doses, 6PPD-quinone is toxic to salmon,” Jayakaran said. “Both could impact human health, particularly in communities that live near busy roads.”

Although the study results are encouraging, Mitchell and Jayakaran emphasize that more testing is needed and that widespread use of the new infrastructure presents some challenges. Permeable sidewalks are sometimes installed in a new housing development or road repair, but the cost of replacing existing roads and parking lots is high.

“We are not suggesting that permeable sidewalks are an appropriate replacement for all roads,” Jayakaran said. “There is still much work to be done to increase their strength and utility, and it is certainly not a silver bullet. However, our research is very promising and we are opening a very promising direction for the future management of 6PPD- quinone and tire wear particles.