6PPD-Quinone Salmon Mortality: Tire Wear Particles and Urban Streams
6PPD-Quinone Salmon Mortality
Researchers traced the mass die-off of coho salmon in Puget Sound urban streams to a single transformation product of a common tire preservative, identifying 6PPD-quinone after screening a chemical mixture thousands of compounds deep.
Every autumn, more than half the coho salmon returning to urban streams around Puget Sound die before they can spawn. In some creeks the entire run is lost. For years nobody could say why.
A team led by scientists at the University of Washington Tacoma, the University of Washington and Washington State University Puyallup published the answer in Science on 3 December 2020.
Ruling out the Usual Suspects
The investigation began with water quality, through a multi-agency effort led by NOAA Fisheries that also drew in the U.S. Fish and Wildlife Service, King County, Seattle Public Utilities and the Wild Fish Conservancy.
Temperature, dissolved oxygen and familiar contaminants such as zinc were all ruled out. What did reproduce the symptoms and the rapid mortality was urban stormwater runoff, which pointed the work toward chemistry rather than hydrology.
Creek samples where salmon were dying shared a chemical signature associated with tire wear particles, and a solution made from those particles proved acutely toxic to salmon in the laboratory.
Finding One Molecule in a Chemical Soup
Tire wear particles carry hundreds of distinct compounds, so the team worked by elimination. They fractionated the solution by chemical property, stripped out classes such as metals, retested each fraction for toxicity and repeated until only a handful of candidates remained.
One dominant compound matched nothing in the reference libraries. The researchers knew its composition contained eighteen carbons, twenty-two hydrogens, two nitrogens and two oxygens, and could get no further for months.
The breakthrough came from asking whether the killer was not an additive at all but something derived from one. Searching known tire rubber chemistry for near matches surfaced 6PPD, an antiozonant used to stop tires degrading prematurely.
A Preservative That Turns Toxic
6PPD works much like a food preservative, protecting rubber from ground-level ozone that would otherwise attack the polymer bonds. It does that by reacting with ozone first, sacrificing itself to spare the tire.
That reaction produces several transformation products, and one of them is 6PPD-quinone, the compound responsible for killing the fish. The additive doing its job is what generates the toxicant.
The problem is not regional. The team detected 6PPD-quinone in roadway runoff from Los Angeles and in urban creeks near San Francisco, which is unsurprising given that 6PPD appears to be used in essentially all tires and tire wear particles accumulate along busy roads everywhere.
What Can Be Done
Two mitigation paths exist. Stormwater can be treated before it reaches a creek, and green infrastructure has proven effective at removing the compound, but building treatment for every road is not realistic.
The other path is reformulating the tire so it does not generate the toxicant. Tires genuinely need antiozonant protection, so the task is finding a replacement chemistry that performs and is properly evaluated for human and aquatic safety before it is deployed at scale.
Species sensitivity also remains open. Coho are devastated while chum salmon are far less affected, and the exposure risk to other organisms inside and outside the Puget Sound basin is still being mapped.
Remove Legacy Tires from the Watershed
Road runoff is only half of the tire chemistry story. The other half is the stockpiled and dumped carcasses leaching into soil and water for decades, and that half is solvable with industrial capacity rather than reformulation.
Klean Industries converts end-of-life tires into recovered carbon black, oil and gas under controlled, oxygen-starved conditions, destroying the waste liability while recovering the materials. Plants are delivered on a KBR turnkey basis: performance-guaranteed, fully bankable and backed by commercial-scale references, including a recovered carbon black facility operating in Poland.
Municipalities and developers can license the technology directly or have KBR build and commission the plant turnkey. Either way the tires stop waiting in a pile beside a watershed.
Contact Klean Industries » GO.
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