Seattle's Salmon Death Problem May Be Step Nearer A Solution

Huge numbers of salmon in the streams of Washington state's Puget Sound die every year due to tire pollution, but researchers may have found a way to prevent the massacre.

By making pavements permeable, scientists hope to protect the coho salmon from tire wear particle pollution entering the streams via stormwater runoff.

Four different types of permeable pavement have been shown to act as filters to tire pollution, preventing up to 96 percent of the tire particles from escaping, according to a new paper in the journal Science of the Total Environment.

The use of these permeable pavements resulted in a 68 percent drop in the presence of 6PPD-quinone, a tire-associated chemical known to kill coho salmon in urban streams.

Coho salmon, also known as silver salmon, are native to the northwest of North America, and are anadromous, meaning that they hatch and grow up in freshwater streams before heading out to sea for much of the rest of their lives. Around 0.1 percent of the fish then return to the streams when it's time to breed, laying their eggs and dying.

A paper published in the journal Science in 2020 showed many salmon that returned to the streams, especially after a period of heavy rainfall, died before they could spawn, and this may be due to the presence of tire pollutants in the water. In many years, over half all the salmon died before spawning.

Large rains wash tiny pieces of aging tires from roads into streams, including the chemical 6PPD, which transforms into 6PPD-quinone when exposed to ozone or sunlight. 6PPD-quinone is very toxic to salmon.

"6PPD is a chemical used in tire rubber formulations to protect the tire's structure when the tires are exposed to ozone, which is formed when chemicals in vehicle exhaust interact with sunlight. 6PPD preferentially reacts with ozone, protecting tire rubber from breaking down. When 6PPD reacts with ozone, one (of many) of the byproducts formed is 6PPD-quinone," paper co-author Anand Jayakaran, a professor and green stormwater infrastructure specialist at Washington State University, told Newsweek.

"6PPD-quinone kills coho within hours at extremely low concentrations, less than 0.1 parts per billion. We are still learning why coho are so sensitive, but researchers at WSU (including lead author Stephanie Blair) found that stormwater exposure disrupts the blood brain barrier in coho, causing blood to permeate the brain. Other research has recently shown that a range of genes in coho regulating the blood brain barrier proteins are significantly impacted by 6PPD-q exposure. Our understanding of the toxicity is still incomplete, but researchers worldwide are working to disentangle the mechanisms causing this toxicity."

Now, researchers have developed four permeable pavements made of either asphalt or concrete that may help to prevent 6PPD-quinone from entering the streams.

"We studied permeable pavements for their ability to treat tire particles and leachable tire chemicals, like the tire chemical 6PPD-quinone. Permeable pavements retained most of the tire particle mass that we deposited onto the pavements and reduced the concentrations of associated leachable tire chemicals, including 6PPDq. Our results support previous research that showed that permeable pavements could be a major environmental sink for tire particles," Jayakaran said.

These permeable pavements are more porous, allowing chemicals shed from tires to be captured by the roads rather than washed into waterways.

The researchers tested the permeable pavements by applying ground-up tire tread across them, then simulating a heavy rainstorm, and measuring where the chemicals ended up. They found that 96 percent of the tire particles were retained by the roads, and there was a subsequent 68 percent drop in the presence of 6PPD-quinone in the runoff.

"Permeable pavements are like conventional pavement but are made by omitting the smaller particles from the pavement aggregate. This makes them porous with interconnected pores that allow water to pass through. Previous research has found that permeable pavement is effective at filtering out sediments and particulate pollutants from stormwater, and this current research suggests that tire particles are filtered out similarly," Jayakaran said.

"The mechanisms for how these pavements remove chemical pollutants like 6PPD-quinone are still poorly understood, but these chemicals likely adsorb to the internal surfaces and particles within the pavement's pores. Researchers in the U.K. previously found that specific organic contaminants can be biodegraded by microorganisms within the pore spaces of permeable pavements, so this may be a longer-term mechanism for treatment. However, further study is needed to determine whether 6PPD-q could be biodegraded in permeable pavements."

While this will help prevent massive die-off of the coho salmon, the permeable pavements could also benefit human health.

"Tire particles are so fine they can become airborne and possibly enter the human body through even a gentle breeze. And even in small doses, 6PPD-quinone is toxic to salmon," Jayakaran said in a statement. "Both could impact human health, especially in communities who live near busy roads."

The researchers hope to test the permeable pavements much more extensively, and also note that using this new material in practice would be challenging, due to the cost and effort of replacing roads. The permeable pavement material, due to the level of porosity, is also more prone to crumbling than traditional road surfaces.

"We're not suggesting that permeable pavements are an appropriate replacement for all roadways," Jayakaran said. "There's still a lot of work to be done to increase their strength and utility, and this is certainly not a silver bullet. However, our research holds great promise, and we're pointing in a very hopeful direction for future management of 6PPD-quinone and tire wear particles."

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Update 02/05/24, 04:45 a.m. ET: This article was updated with comment from Anand Jayakaran.

Correction 02/05/24, 12:50 p.m. ET: This article's headline was updated to reflect the fact that the permeable pavements are one prospective solution to the problem.