.Toxic Chemicals Found in Local Waterways

City of Santa Cruz found high levels of a chemical that is lethal to Coho salmon species in a recent study

Coho salmon, which once thrived in the San Lorenzo River but are now on the endangered species list, may be threatened by this year’s heavy rains.

After particularly heavy rainfalls, in February Santa Cruz city officials with the water department identified concerningly high levels of a tire chemical in waterways around the county. 

Chemicals from tires were washed into the river during the months of storms and could kill the fish, which are also called silver salmon. They live most of their lives in saltwater, but are born, spawn and die in freshwater. 

The chemical, known as 6PPD-quinone, is so toxic for salmon species it has been paired with an especially lethal term: Urban Runoff Mortality Syndrome. It’s a phenomenon whereby fish are observed mysteriously dying shortly after entering select waterways.

Scientists aren’t concerned about the longevity of Coho salmon in the rest of the state. But here in the county, the salmon have been listed as endangered since 2005, when they were recategorized from threatened.  

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In efforts to forestall Coho extinction in the Santa Cruz area, several agencies, such as NOAA Fisheries and the Santa Cruz County Environmental Health Department, have collaborated over the years to manage their population. The departments regularly monitor population levels, habitats and potential threats to their spawning cycle. 

This toxin could jeopardize the recovery of this species. 

Due to the nascency of this investigation, as well as the heavy stream flows impeding monitoring efforts this year, little is known about current Coho population counts. Little is also known about the potential long term trends of this chemical in Santa Cruz County, but officials speculate the damage could be severe.  

The Tire Toxin 

For decades, researchers in the Pacific Northwest were baffled as every year they watched large portions of Coho salmon die while migrating upstream to spawn. Once the salmonids entered certain waterways, they tended to gape their mouths, gasp at the surface and swim in circles as if disoriented, before dying hours later. 

Scientists narrowed down the culprit to contaminants in storm runoff and then later, to car tires. In 2020, scientists finally isolated the molecule, now known as 6PPD-quinone. 

6PPD-quinone is a preservative used by tire manufacturers for decades to ensure longevity in tires. Tire particles containing this chemical then wear off and are transported downstream via storm runoff. In California alone, tires produce between 98,750 and 185,650 metric tons of tire particles per year. 

The tire rubber is a plastic, so the tire particles are actually considered microplastics (plastic particles smaller than 5mm). SFEI estimates that tire particles may account for the largest source of microplastic pollution in the world.

As it happens, car tires contain a number of chemicals. Potentially thousands of chemicals, says ecotoxicologist with the San Francisco Estuary Institute (SFEI) Dr. Ezra Miller. 

Storm water runoff could contain even more.

“You name it, it’s probably in stormwater,” Miller says, who adds that storm runoff can be incredibly toxic to the surrounding environment.

Study Results

The city collected water samples for the study twice from four locations around the county with a focus on key Coho salmon recovery streams: Zayante Creek, Branciforte Creek and the San Lorenzo River, downstream near downtown Santa Cruz and upstream near Felton. 

The first sampling event occurred on Nov. 8, 2022, following the initial onset of rains last year. The second on Feb. 3, 2023, following more intense flooding events, with the objective being to collect samples during high flows.

Results for three of the locations found that between November and February, the amount of tire chemicals in the water more than doubled.  

“We found, much to our surprise, that the level actually went up after it started raining,” says Chris Berry, Watershed Compliance Manager with the city’s water department. Berry had assumed that the initial flush of rain in November would wash off most of the particles from the roadways.

6PPD-quinone is measured in nanograms per liter (ng/l). One part per trillion (ng/l) is equivalent to one drop of water in 10 Olympic-sized swimming pools. While that doesn’t sound like much, 50% of adult Coho can die in the presence of 95 ng/l of 6PPD-quinone. According to more recent research, 50% of juveniles can die at 41 ng/l.

For both Felton and Zayante Creek, levels jumped by about 240%. While neither of these locations hit the 41 ng/l, or especially the 95 ng/l level required for 50% of a generation of Coho to die, they do indicate that the heavier rainfall in February was likely the cause of the increase. 

The chemical levels in Felton and Zayante Creek likely caused some fish fatalities. Some individual fish could be more sensitive and die from lower concentrations, says Miller.

Of the three locations where concentrations doubled, the highest levels were found in Branciforte Creek, which showed a roughly 230% increase. Berry hypothesizes that Branciforte Creek showed particularly high concentrations of the chemical due to its proximity to Highway 17, a roadway receiving a heavy influx of tire particles every day. 

Branciforte Creek’s most recent sampling shows chemical concentrations beyond the level that causes juveniles to die and is concerningly close to causing 50% of a generation of Coho adults to die.

The San Lorenzo River near downtown was the only location that went down, dropping from 6.2 ng/l to less than 2 ng/l. 

While research published in August, 2022, found that 6PPD-quinone is also toxic to other salmonids, such as Chinook and Steelhead, Coho salmon are unique in that they seem to be more susceptible to this toxin. All three of these species exist in Santa Cruz County, and furthermore Steelhead were listed as threatened under the federal Endangered Species Act in 2006. 

“Generally speaking, I’d be concerned about all aquatic species,” says Berry.

He adds that there may be longer term consequences, or cascading effects on other creatures that just haven’t been studied yet.

Tribal Consequences

Coho were also significant for the Amah Mutsun, the indigenous peoples of the south-San Francisco and north-Monterey Bay area, also collectively referred to as Ohlone. 

“We are a coastal tribe and salmon was a very important fish for our people. We saw salmon as a sacred gift to our tribe” says Chairman Val Lopez.

As an important dietary source for the Amah Mutsun and an important member of the surrounding ecosystem, it was the tribe’s responsibility to ensure that the population could meet the needs of other species, says Lopez. 

Due to their diminished population, the Amah Mutsun Land Trust have focused their efforts on Coho recovery. Some of these efforts include managing the mouth of the rivers and streams to ensure the fish could enter the freshwater from the ocean, cleaning the areas where they make their nests and ensuring they have nooks and crannies to rest in while migrating upstream. Part of this process also included releasing a dam in Davenport to allow for more freshwater access to migrating salmon in November of 2021. 

Even before Coho’s population dwindled, the peoples of the Amah Mutsun followed healthy fishing management practices that allowed the salmon to survive. As part of this practice, they waited to fish for a number of days while the salmon migrated to ensure the longevity of the species.

“I look forward to the day when I can fish for salmon guilt free, but I will just wait to fish until that time,” says Lopez.

Looking Ahead

“It’s always better to manage environmental contaminants closer to the source. It’s harder to clean them up once they’re already in the environment,” says Miller. 

In this case, to manage the source of the contaminant would be to manage the production of car tires. In May of 2022, the state of California’s Department of Toxic Substance Control (DTSC) began the process of listing motor vehicle tires that contain 6PPD as a Priority Product

That’s an official label declaring that a consumer product could harm people or the environment. It would mean tire sales would be regulated and the search for alternative tire preservatives would be prioritized. 

While this rulemaking hasn’t yet been approved, Miller confirms that scientists and the manufacturers alike are taking 6PPD very seriously.  

“This ruling provides strong incentives for the tire industry to find an alternative,” Miller says. 

Another solution is to treat the stormwater before it reaches local waterways. 

“This 6PPD issue I think further illustrates just how important treating stormwater is before it hits our waterways,” says Chris Berry. Berry points out that, due to our natural topography, many of our roadways were built parallel to rivers and streams.

“I think this issue really highlights how interconnected our transportation system is with our stream systems. It’s easy to think of them as two separate things, but that’s not how water flows,” says Kristen Kittleson with the Santa Cruz County Environmental Health Department. 

Berry recommends approaching our infrastructure, new and old, with an eye towards protecting waterways. In particular Berry advocates for building and maintaining more biofiltration green infrastructure such as riparian buffers or bioswales which act as natural treatment systems. 

Bioswales are vegetated depressions in the ground built along urban developments like parking lots and roadways, while riparian buffers are vegetated barriers or corridors constructed along waterways. Both are designed to naturally filter runoff before it hits downstream waterways by absorbing pollutants and other debris. They also improve landscapes aesthetics, provide habitat for wildlife and help cool surrounding areas. With a watershed of this size, it would be extremely difficult for the city to comprehensively capture stormwater runoff to treat, says Berry. 

“A lot of stormwater goes completely untreated before entering surface water bodies,” says Miller. “And green stormwater infrastructure is only treating a very, very small fraction of stormwater runoff.”

Moving forward, Berry and the city’s water department plan to continue testing for this chemical to get a more comprehensive picture of its presence year round. 

Since only the two sets of samples have been gathered, the results from the lab tests can’t offer conclusions on long term trends in Santa Cruz County. These initial results do fit trends discovered by researchers in the Pacific Northwest. Berry also hopes to involve larger agencies, like the Department of Fish and Wildlife and the Marine Fishery Service, who may be better equipped to pursue the large-scale investigation this watershed may need.


  1. Tire prices at name brand stores are higher than at discount tire outlets.
    They come with balancing and solid guarantees and warranties.

    The discounters buy new tires that have aged and out gassed some of their “new tire smell” becoming a little less flexible and more likely to pop when hitting a pothole or curb.
    They are sold with weaker guarantees and warranties.

    All tires have codes on the sidewall that reveals the date of manufacture.
    A ten year old tire, even with the whisker whatever’s still on it… is an old, untrustworthy tire, just because the preservative doesn’t last forever.

    BTW, I don’t see any mention of toxic chemicals from 1000 homes and cars burned during the CZU Fire of 2020…. Nor from orange fire retardant dropped from air tankers.

    Why did Kristen Kittleson not support either version of “The Cannonball Express”, a video about suburban forest fires, fire safety and the environment, submitted to the County Fish & Wildlife Commission four times in the past 30 years? A $5,000 grant, from the Commission was the cornerstone of a $19,000 grant package that would have completed this innovative half hour video, designed for adult homeowners.

    Now Kristen has retired and the ball is in the hands of the new liaison to this citizen’s advisory group. It remains to be seen whether Sean Abbey will encourage the Commissioners to do their homework before voting on the proposal to create a STUDENT version of the video… ie

    Read the Middle School Version of the script
    Read California Fish & Game Code section 13103(a)
    Watch the demo reel (adult-homeowner version)
    Read the “letters of interest” from educators and labor leaders

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  2. Thank you for highlighting this situation GT. Reading this highlights the urgency to get people walking, biking and on public transit!
    Yeah it’s convenient to have our own cars, but at such a high cost to the environment and our health. Do we really want to (again), support the plastics industry?
    It’s beyond time to make the hard changes, individually.

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Blaire Hobbs
I am freelance writer and editor with a degree in Cognitive Science from the University of California Santa Cruz and a professional background in media literacy, misinformation research, behavioral science, and teaching.
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