Permeable pavements could protect fish by trapping car tire chemicals

Credit: Jason Berg

Permeable pavements show promise for adsorbing car tire particles and their associated chemicals, and could be more widely used to keep these contaminants from harming coho salmon.

That’s the conclusion of a recent study by Washington State University (WSU) researchers who tested four types of permeable pavements and showed they captured on average 96 per cent of applied tire particle mass. The pavements also retained several associated chemicals, including 68 per cent of 6PPD-quinone.

The chemical is a toxic break down product of N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), a widely-used antioxidant added to car tires to prevent the rubber from degrading and cracking. As 6PPD leaches out, it breaks down into 6PPD-quinone.

Concerns over 6PPD-quinone rose in late 2020 after an earlier WSU study showed it was responsible for killing salmon in Seattle’s creeks after rainstorms washed it from roads into waterways. Up till them, scientists had been stymied to understand what was killing the fish.

6PPD-quinone has since been discovered in urban waterways in Canada, including in Toronto’s Don River and in Saskatoon stormwater and snowmelt.

To better understand whether permeable pavements might mitigate the problem, the WSU team did three experiments in the parking lot of a Tacoma high school to test four types of permeable pavements. Two of the pavements were concrete and two asphalt. One each of the concrete and asphalt permeable pavements were reinforced with cured carbon fibres – ground up leftover material from airplane wings, donated by Boeing.

The researchers added the carbon fibres because permeable pavements are not as strong as traditional pavements. Fine particles are screened out to create the interconnected empty spaces that make it permeable. As a result, permeable pavements are not strong enough for high-traffic roads and are used mostly in parking lots, sidewalks and driveways to help control urban flooding.

The researchers reasoned that if their study showed permeable pavements could help capture tire contaminants, they needn’t be relegated to low-traffic areas, says ecological engineer Ani Jayakaran. He co-led the study with Chelsea Mitchell, who recently completed her PhD in environmental and natural resource sciences at WSU.

In the first experiment, Jayakaran and Mitchell simulated an hour-long rainstorm on all four types of permeable pavement to reveal background levels of tire particles and chemicals. Then they tested the runoff water collected in a catch basin under the pavement. The levels of tire particles and associated chemicals were low, giving them a yardstick against which to measure levels in the subsequent two experiments.

In the second experiment, the pair spread tire particles on the pavements before simulating a rainstorm. The particles reflected the average number found on high-density roads in the Puget Sound area after two weeks with no rain.

In this case, the water that leached out the bottom showed that all four pavements retained on average 96 per cent of applied tire particle mass. The pavements also retained several associated chemicals, including 68 per cent of 6PPD-quinone.

A pair of silver salmon, also called coho salmon, leaps from the icy waters of the Resurrection Bay in Seward, Alaska.

In the third experiment, the team simulated another rainstorm. They wanted to know if a second rainstorm shortly after the first might leach out more of the trapped tire wear particles and associated chemicals. There were low levels of additional leaching, but it tapered off fairly quickly.

“6PPD-quinone is a hydrophobic chemical so it will try to jump out of water as quickly as possible,” says Jayakaran. “We don’t know definitely, but we assume it’s being adsorbed to the pavement walls, the pores of the pavements.”

The team didn’t examine at what point the pavements might become saturated and start leaching out larger quantities of tire wear particles and chemicals. But Jayakaran stressed the pavements would need frequent vacuuming or pressure washing to remove contaminants, which would then be brought to a landfill.

“All green stormwater practices require maintenance,” he says. “In the Puget Sound area, street sweeping is a requirement to mitigate stormwater pollution.”

University of Saskatchewan toxicologist Markus Brinkmann says the study shows permeable pavements can be “a good intermediate solution.”

Tire manufacturers and governments in both the US and Canada are investigating how to replace 6PPD with something less toxic, says Brinkmann, whose research showed 6PPD-quinone in Saskatoon snowmelt puddles. But that could take a decade or more and we need to do something now, he says.

Brinkmann agrees that using carbon fibres to strengthen permeable pavements is also a good idea. “Carbon fibres are now well-studied for toxicity,” he says. “Under some conditions they could be a risk, but if it’s bound in permeable pavement, I don’t see much of a risk.”