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A Nasty Salmon-Killing Tire Chemical Is in Bay Waterways. Can It Be Cleaned Up?

6PPD-quinone comes from a long-used chemical that will be hard to replace in tires. But ‘green infrastructure’ may help stop it from entering our waters.

March 2, 2023

When Heidi Petty began cleaning up Rodeo Creek nearly two decades ago, she pulled seventeen shopping carts out of the waterway, which runs from western Contra Costa County into San Pablo Bay. Since then, she has extracted mattresses, chairs, fake Christmas trees, a golf club, and a wig. But despite her efforts to clean up this creek-turned-dumping-ground, one particularly noxious type of trash keeps coming: tires. Parts of this urban watershed contain a mess of whole tires, mired in the mud like a monstrous serpent. 

“There seems to be a pretty solid theme of using creeks as a disposal for highly toxic things,” says Petty, the watershed program manager at the Contra Costa Resource Conservation District. “And tires are hard to get rid of.”

Trouble is, they’re not just eyesores but likely fish-killers. As tires break down, they release a cocktail of chemicals that leach into the water in creeks all over the Bay Area—which in turn empty into the broader San Francisco Bay. Most pollutants have more subtle impacts on wildlife. But now we have a new—or rather, newly identified—pollutant to worry about: 6PPD-quinone (aka 6PPD-q). This chemical is formed when 6PPD, a chemical long added to tires to prevent degradation, reacts with ozone in the atmosphere. In the past few years, researchers have learned it is lethal to steelhead trout, coho salmon, and chinook salmon; it kills cohos even at very low concentrations. Now, regulators and environmentalists are working to manage 6PPD-q pollution, to protect already imperiled local steelhead trout populations.

Many of the tires at Rodeo Creek’s mouth appear to have been there a while. Nearby is a relatively out-of-sight easy access point for those with items to dump. (Photo by Kate Golden)

A 20-year mystery, solved

Scientists published the first research identifying 6PPD-q in late 2020, but researchers at the National Oceanic and Atmospheric Administration (NOAA) have been observing its effects on coho salmon for decades. 6PPD, its precursor, was first used in tires in the mid-1960s and was in widespread use by the 1970s, according to the U.S. Tire Manufacturers Association.

In the early 2000s, Barb French, a NOAA research chemist, was surveying fish in Puget Sound streams. She observed that when salmon returned to their natal streams to spawn, they grew disoriented. “They’d swim in circles and sideways through the water,” French says. They splayed their fins and gaped their mouths, then died in a few hours. It shocked French. Having just returned from the ocean, these salmon were at a point in their life cycle when they should have been strong. 

“That then led to 20 years of research to figure out the needle in the haystack of what was killing these fish,” said French’s colleague Nat Scholz, NOAA’s ecotoxicology program manager. 

French, Scholz, and their team linked the fish kills to roadway runoff, but they did not know which chemical was responsible. Stormwater is a complicated soup—and many chemicals used in tires are proprietary secrets, guarded by manufacturers. “You can’t measure a lot of things in the stormwater if you don’t know what they are,” French says. Finally, at the end of 2020, another team led by Zhenyu Tian, then at the Tacoma, Washington-based Center for Urban Waters, separated urban stormwater into its components and identified the culprit of the fish kills: 6PPD-q. 

Once French knew what was killing the coho salmon, she and her team began investigating how the chemical affected other fish. She and her colleagues exposed different types of salmonids to undiluted stormwater containing 6PPD-q. Steelhead trout and chinook salmon were susceptible to the chemical, exhibiting the same suite of symptoms French had seen among the coho salmon in the Pacific Northwest, though they were not as acutely vulnerable as cohos: In French’s study, up to 42 percent of steelhead and 13 percent of the chinooks exposed to the stormwater died within two days, while over 90 percent of the cohos died.

A coho in its brilliant red spawning getup. Cohos haven’t been seen in the Bay since the mid-to-late 1980s, but they still have North Bay spawning sites, including in Lagunitas Creek and the Russian River. Cohos have proved to be the most vulnerable to 6PPD-q. (Bureau of Land Management, CC-BY-2.0 via Flickr)

6PPD-q is here

Since steelhead and chinooks are both native to Bay Area waterways, scientists at the San Francisco Estuary Institute began testing for the chemical in streams and watersheds in and around the San Francisco Bay in late 2018, as part of the research efforts led by Zhenyu Tian. 

“We’ve found quite high concentrations of 6PPD-quinone in our waterways,” says Ezra Miller, a scientist at the institute, an independent scientific nonprofit that assesses and improves the Bay’s health. 

In an area as heavily urbanized and car-centric as the Bay Area, 6PPD-q is likely ubiquitous. Rodeo Creek runs beneath a “hub of freeways, overpasses, and underpasses, (built) with very little concern for wildlife,” Petty says. In addition to the 6PPD-q that all those whole tires likely leach into the creek, the chemical also makes its way into watersheds via the small particles that tires slough off whenever they are rolling.

Rodeo Creek once teemed with fish, molluscs, and aquatic plants. But a series of construction projects destroyed the fish habitat: the creek was channelized, widened and lined with concrete, to prevent flooding in Rodeo; small dams called drop structures were added in an effort to slow erosion; and railroad construction diverted the creek’s path.

“We’re kind of working around bad infrastructure from a long time ago,” says Petty. Today, Rodeo Creek’s fish have completely disappeared. 

It’s a story that has played out for streams across the state.

Steelhead here ‘face extinction’

To distribute water to California’s Central Valley, the state built many dams, destroying salmon and trout habitat, as well as their migration routes. Climate change is warming waters and changing streamflows, making life harder for these coldwater fish. California steelhead trout and other salmonid populations have been declining for a long time, according to Peter Moyle, researcher and UC Davis fisheries professor emeritus. 

The Bay’s steelhead are part of a distinct population of fish that is federally listed as threatened and “face extinction in the next 100 years” unless there are “significant investments in monitoring, habitat restoration, and water management,” according to a 2017 report Moyle coauthored.

Pollution plays a role in the decline of these fish, explained Moyle, but the extent of its influence is hard to tease out.

The Center for Ecosystem Management and Restoration, an organization dedicated to rehabilitating California’s streams until it closed its doors at the end of 2016, documented steelhead populations in many creeks neighboring Rodeo, including Pinole, San Pablo, and Walnut Creeks in Contra Costa County, and Codornices, San Lorenzo, and Alameda Creeks in Alameda County. And, according to Miller, testing of area creeks has shown that concentrations of 6PPD-q sometimes exceed its LC50 for coho salmon. (LC50 is a term used by toxicologists—the “LC” means “lethal concentration”—that refers to the chemical concentration expected to kill half of exposed individuals in a group.)

Coho salmon appear to be more vulnerable to 6PPD-q than steelhead. But based on the data collected by the SFEI, Miller says Bay Area levels of 6PPD-q are potentially high enough to harm steelhead, too. 

Coastal rainbow trout (Oncorhynchus mykiss, subspecies irideus) like this one, caught south of San Jose in October 2022, are the same species as steelhead. Steelhead, like salmon, spend time at sea, while rainbow trout live their lives entirely in fresh water. (Photo by prickly_sculpin via iNaturalist, CC-BY-NC)

Human health effects? Unknown.

In places where 6PPD-q is present at not-quite-fatal levels, the chemical may still have nasty sublethal effects scientists are only beginning to investigate. “It doesn’t matter if they live, if they can’t reproduce, if they can’t eat, if they can’t swim away from predators,” says French. 

Researchers are also studying whether 6PPD-q can be passed up the food chain to other predatory fish. “This particular compound looks to be metabolized in fish,” says Scholz. That means that even if the compound harms or kills the fish, it most likely does not accumulate in their bodies, where it could harm other organisms that consume them—like people.

Humans are exposed to 6PPD and 6PPD-q, but scientists do not fully understand the mechanism through which 6PPD-q kills fish, let alone how the compound affects human health. 6PPD-q has not been found in drinking water, says Ezra Miller of the SFEI, as far as they know. “But that may be because we haven’t been looking for it,” they add. Scientists have found both 6PPD and 6PPD-q in the urine of adults and children in South China. Miller suspects the main exposure route in humans is via inhalation of very small tire particles, rather than through water, but more research is needed. 

An American avocet feeds avidly at the mouth of Rodeo Creek, working around one of the many tires mired in the muck. (Photo by Kate Golden)

Chemical alternatives are a ways off

In May of 2022, the California Department of Toxic Substances Control initiated the process of designating 6PPD as a “priority product,” which would force tire manufacturers to disclose their use of 6PPD and begin research to find safer alternatives for use in tires. 

PPDs, the class of chemicals including 6PPD, were first developed by the U.S. government. “​​One of the biggest pushes for an improvement in durability for rubber happened between World War II and the Korean War,” says Jamie McNutt, Director of Regulatory Science at the US Tire Manufacturers Association, or USTMA. US military officials found the tires on their combat vehicles had degraded since WWII, and in preparation for conflict in Korea, they developed PPD chemistry to fight tire degradation. 

The chemistry worked exceptionally well, and 6PPD is now used in every consumer tire McNutt is aware of. “If you look at a tire, you think it’s just one big black ball of rubber,” says McNutt, but in actuality “it’s a multiple-layered composite product,” she says. And every layer except the innermost lining contains 6PPD. The chemical is essential in preventing the outermost layer of tire from cracking, but it also protects inner layers from oxidation that could lead to tire failure. 

Tire manufacturers say they were unaware of the breakdown product 6PPD-quinone’s presence in the environment—and its impact on fish—until Zhenyu Tian’s research was published in 2020. Since then, the USTMA has advocated for analyses of alternative products to replace 6PPD, partnered with researchers to address data gaps around 6PPD-q, and invested in mitigation efforts to remove polluted stormwater, according to Sarah Amick, a USTMA senior vice president of environment, health and safety, and a senior counsel.

Tire manufacturing is complex. All the materials must work together and meet high standards for road safety. Finding an alternative for a chemical as essential and widely used as 6PPD represents a huge challenge that will likely take a long time. While tire manufacturers know how to test for performance, safety testing for toxicity is less well-defined. “Even if we had a material identified as an alternative, we do not know how to test it for safer toxicity,” says McNutt. 

Until there’s a safer chemical alternative, scientists and engineers say, we must treat the stormwater itself. And once car tire particles, and the 6PPD-q in them, enter the environment, they may be extremely difficult to remove. Tiny tire particles can travel far from the roads where they originated, and once they reach creeks and larger bodies of water, there are no effective methods of filtering them from the water. How long 6PPD-q sticks around is still unknown.

But there are solutions.

Trap it!

One solution is to collect the tire particles and their pollutants before they reach waterways.

Soil filtration columns are cylinders of dirt designed to trap pollutants contained in urban stormwater as it passes through so that only clean, filtered water enters the environment. These types of green infrastructure projects have effectively removed 6PPD-q from urban stormwater, in research conducted by French’s NOAA colleagues and other scientists. 

Some of the 6PPD-q problem could be mitigated by green infrastructure projects already under way in the Bay Area that were designed to deal with old foes like mercury, copper, and polychlorinated biphenyls. 

“We are steadily increasing the number of green stormwater infrastructure facilities, such as bioretention facilities,” also known as rain gardens, wrote Terry Fashing, Oakland’s watershed and stormwater program manager, in an email. 

In the Rodeo Creek watershed, Heidi Petty is planning to install a “living levee” in an effort to remove pollutants like 6PPD-q and prepare to bring fish back into the creek. Typical levees usually incorporate a steep slope toward the water, with concrete or other human-made walls that leave no habitat for aquatic life. Petty aims to build a gradual slope and repopulate the area with native plants like eelgrass. “We’re just basically recreating what should be there,” says Petty. This type of living levee also creates a wetland further upland that can also filter out pollutants like 6PPD-q. 

Eventually, Petty hopes to reintroduce steelhead back into the creek. It’s not just about salmon. If the water can be made clean enough for sensitive salmonids to live in it, other species will benefit, too.

Petty is hopeful. She imagines a future restored tidal marsh at Rodeo’s mouth similar to what existed centuries ago: The garbage is gone. Native aquatic plants like eelgrass attenuate the force of oncoming waves and prevent erosion. Oyster reefs filter out pollutants. The air is a soundscape of bird calls from clapper rails and terns, their stomachs free of plastic and pieces of tire. The steelhead return, their silvery bodies catching the glint of the sun. 

About the Author

Kate Raphael is a reporter based in Berkeley. She covers toxics, climate change, and health. She grew up in Indiana, then spent eight years in New England before moving to the Bay Area to attend the UC Berkeley School of Journalism, where she is a current student. Her favorite way to enjoy the nature around her is through trail running, and you can often find her in the Berkeley Hills.