While stormwater runoff pollutants in general aren't great for aquatic animals, chemicals from tire particles are particularly harmful to salmon. A recent study now shows that permeable pavements could keep most of those toxins from ever reaching the fish.
Ordinarily, when rainwater runs along city streets, it carries pollutants from those streets down the storm drains and out into the local waterways. One proposed method of addressing that problem involves replacing existing road surfaces – or building new ones – with what are known as permeable pavements (aka porous pavements).
These surfaces aren't as dense as their traditional counterparts. Instead, they're full of pores that allow the rainwater itself to flow through into the underlying soil, while trapping many of the accompanying pollutants.
One of the major sources of roadway pollution is the tiny particles of rubber which are constantly shed by cars' tires. A chemical in those particles, called 6PPD, becomes a toxin known as 6PPD-quinone (6PPDQ) when exposed to sunlight or ozone on the road.
A previous Washington State University study found that 6PPDQ is deadly to coho salmon, which are native to the state. With that fact in mind, the university's Prof. Ani Jayakaran and colleagues set out to see if permeable pavements might help keep the chemical out of the local coho habitats.
For the study, sections of four different types of permeable pavements – made of asphalt or concrete, with or without added carbon fiber waste – were installed in a Tacoma school parking lot. Drains under each section collected the water that flowed through.
Initially, water simulating a downpour was flushed through the bare pavements, to set a baseline. The next day, however, ground-up tire particles were sprinkled across the surfaces, after which they got another flushing. A third flushing followed, in order to see if the trapped particles might continue to leach out chemicals.
When the water from the drains was analyzed, it was found that the pavements retained over 96% of the tire particle mass, plus they trapped an average of 68% of the 6PPDQ that was produced by the particles. Because the chemical is hydrophobic (repelled by water) the scientists believe it may have actually been absorbed into the pavements.
All of that being said, permeable pavements (so far) aren't are strong as regular concrete or asphalt. Additionally, changing existing roads over to them would be a huge undertaking. Nonetheless, the scientists are pleased with their findings.
"We’re not suggesting that permeable pavements are an appropriate replacement for all roadways," said Jayakaran. "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."
The research is described in a paper that was recently published in the journal Science of the Total Environment.
Source: Washington State University
