Rainstorms are washing a tire-rubber byproduct called 6PPD-quinone into Santa Cruz County creeks at sharply rising concentrations, with testing showing jumps of roughly 240% at Felton and Zayante Creek and about 230% at Branciforte Creek following heavier rain. The chemical is highly toxic to coho salmon, a species that has been listed as endangered in the county since 2005.
What Sampling Found in Local Creeks
The numbers come from water testing described by Good Times, which reported that city sampling covered four locations around the county focused on coho recovery streams, including Zayante Creek, Branciforte Creek and the San Lorenzo River. Crews collected the first round of samples on November 8, 2022, shortly after rains began that season, then returned on February 3, 2023, following more intense flooding. At three of the four locations, concentrations of the chemical more than doubled between the two sampling events.
Branciforte Creek recorded the highest levels of the three waterways where concentrations doubled, according to the same report. The San Lorenzo River near downtown was the lone exception, with levels actually dropping from 6.2 nanograms per liter to less than 2 nanograms per liter between the two tests, the outlet noted.
How a Tire Additive Becomes a Fish Killer
The chemical begins its life as 6PPD, an additive used to keep tire rubber from breaking down when exposed to ozone. As tires wear on pavement, 6PPD reacts and oxidizes into 6PPD-quinone, a research team writing in Science explained, describing it as an oxidation product of an additive meant to protect tire rubber. Rain then carries worn tire particles containing the compound downstream, per the same Good Times account of how the chemical travels from roadway to creek.
The toxicity is well documented in laboratory settings. The Science study found that tire-tread-particle leachate caused 98.5% mortality among juvenile coho salmon within just 24 hours of exposure, and the same team calculated a median lethal concentration of 0.8 ± 0.16 micrograms per liter. Separately, Good Times reported that 50% of adult coho can die in the presence of 95 nanograms per liter, with juveniles dying at 41 nanograms per liter — figures that differ from the Science study's threshold, a discrepancy the dossier does not resolve beyond noting both sets of numbers come from differing tests.
Rain Events Drive the Chemical Downstream
Research published in Environmental Science & Technology Letters found that transport of the chemical in a coho-bearing stream increased rapidly following rainfall and the resulting rise in stream flow, reinforcing why storm timing matters so much for monitoring. That pattern lines up with what Santa Cruz County's own sampling showed: levels generally rose at sites tested after the storms, with the exception of the downtown San Lorenzo River stretch.
The San Francisco Chronicle has also been tracking the broader threat posed by 6PPD-quinone to California's salmon-bearing watersheds, reporting that the compound comes from tire chemicals washed into waterways and builds up on roads before spilling into rivers and creeks during storms.
Regulators Still Studying the Problem
California's Department of Toxic Substances Control began the process of listing motor-vehicle tires containing 6PPD as a Priority Product in May 2022, according to Good Times, a step that requires manufacturers to start considering alternatives to the chemical. Regulatory filings reviewed by California Safer Consumer Products show the state is weighing an alternative that would remove 6PPD from tires without replacing it with another chemical, alongside possible changes to tire design or manufacturing.
At the federal level, the U.S. Environmental Protection Agency's action plan states that available information indicates 6PPD-quinone is toxic enough to quickly kill some fish, and the agency has developed a testing method intended for widespread use while its Office of Water works on draft screening values meant to protect sensitive salmon and other aquatic life.
Potential Fixes on the Horizon
Chemists are racing to develop replacements for 6PPD itself. Chemical & Engineering News reports that alternatives to the antiozone agent are emerging, though the publication cautions that bringing a replacement to market broadly could take years. In the meantime, EPA research posted to its risk assessment portal suggests that mapping where and when the chemical hotspots form in a watershed could help communities prioritize green infrastructure, such as targeted stormwater treatment, to intercept the compound before it reaches sensitive streams.
Santa Cruz County's coho runs remain a small fraction of their historic size, and the creeks sampled in this reporting — Zayante, Felton, Branciforte and the San Lorenzo — are among the waterways where the fish still attempt to spawn each year despite the rising chemical loads documented after storms.
