NOTEBOOK FEATURE: From roads to rivers: How state agencies are tackling salmon-killing tire pollution

Researchers are uncovering the hidden dangers lurking in car and truck tires, a complex source of pollution that affects both air and water quality. Tires contain a hazardous mix of chemicals, microplastics, and heavy metals, with experts warning that emissions from tire wear pose significant risks to human health and wildlife. Given the enormous number of vehicles on the road and miles traveled daily, tires have become a widespread and pervasive source of pollution.

One particularly concerning pollutant is 6PPD, a chemical added to tires as an antidegradant. Over time, as tires wear down, tiny particles containing 6PPD are released into the environment. When it rains, stormwater carries these particles from roads and paved surfaces into nearby rivers and streams, where they contaminate aquatic ecosystems. Even more troubling, a 2021 study published in Science revealed that 6PPD reacts with ozone in the air to form 6PPD-Quinone (6PPD-Q), a highly toxic byproduct, which was directly linked to the mortality of coho salmon.

Since these findings, California state agencies have begun taking action to address the issue. On December 4, the State Water Resources Control Board, in coordination with the Department of Toxic Substances Control and CalTrans, presented their efforts to tackle 6PPD and 6PPD-Q contamination in the state’s waterways.

BACKGROUND

Dr. Manoela Romano de Orte, Research Scientist III at the State Water Board, began with background information.  6PPD is a chemical used in tires as an anti-degradant to prevent premature weathering of the rubber from sunlight, oxygen, and ozone.  It is the primary anti-degradant used in all tires.  6PPD-Q is one of the several transformation byproducts of 6PPD formed by the reaction of 6PPD and ozone.

6PPD has been used in tires since the mid-1960s, while 6PPD-Q was only identified in stormwater samples in late 2020 or early 2021 as the causal toxicant for the urban runoff mortality syndrome. It was observed in Washington state that when coho salmon would return to the rivers and streams to spawn, they would die before laying their eggs. This was observed for decades, and the causal toxicant was a mystery, but it was observed that this always happened after rain events and in waterways near roads. So they suspected the mortality was probably correlated with stormwater runoff from roads, which is why it’s called urban runoff mortality syndrome.

A group of scientists collaborated and applied advanced techniques and finally identified 6PPD-Q, a transformation product of 6PPD. They published their findings in the groundbreaking science paper, A ubiquitous tire rubber-derived chemical induces acute mortality in coho salmon.  After that, the scientific community undertook a significant effort to understand more about the sources, pathways, and toxicity of 6PPD/6PPD-Q to aquatic organisms.

This presentation will focus on both 6PPD and 6PPD-Q, but only 6PPD-Q is responsible for the urban runoff mortality syndrome.

SOURCES AND PATHWAYS TO RECEIVING WATERS

Tires are the primary source of 6PPD and 6PPD-Q to the environment. There are sources, but their contribution is negligent compared to tires.

The tire industry is a complex environmental problem. It is a big industry producing over 3.1 billion new tires every year. As tires roll over road surfaces, the friction between the rubber and the road produces tiny particles called tire wear particles, which end up in the environment.  Globally, tires may be one of the top sources of microplastics to the environment.

In addition to tire particles, there are over 1000 chemicals and compounds in tires which include heavy metals, polycyclic aromatic hydrocarbons, and other compounds that can be toxic alone or as a chemical cocktail.

Stormwater runoff is a significant pathway for transporting tire wear particles from roads to waterways, but other particle pathways exist.  The graphic on the slide is a conceptual diagram showing the sources and pathways to the environment, people, plants, and animals.  These particles are present in the aquatic environment, soils, and the atmosphere.

“Our main focus is on receiving water and aquatic organisms at the moment, but our monitoring management strategy will track all of the relevant pathways and sources,” said Dr. Romano d Orte.

TOXICITY TO AQUATIC ORGANISMS

Evaluating the toxicity of a chemical to an aquatic organism can be done in several ways, but the most common measure is the mortality rate, or more specifically, the concentration of a substance that can cause death in 50% of a test population within a specific time frame.  Hence, the term LC50 for Lethal Concentration of 50%.

The concentration is determined by building a dose-response curve, as shown on the slide.  The x-axis shows the 6PPD-Q concentration in nanograms per liter and the y-axis is the observed mortality in percentage.  The point on the curve that produces mortality in 50% of the population is the LC 50; in this case, it is 95 nanograms per liter.

“It’s important to highlight here that this is a very low LC50,” said Dr. Romano de Orte.  “In fact, it is one of the lowest LC 50s ever reported for any chemical. We have just a few chemicals with numbers lower than that, so this is a really toxic chemical, specifically for coho salmon.”

The table shows the LC50 for other species for comparison.  The table includes other salmonid species.  Sensitive species include brook trout, lake trout, and wide spotted chard; rainbow trout has a slightly higher LC50 but is considered moderately sensitive with relevant concentrations in the environment.

“The species that we can find in California are coho salmon, mainly found in Northern California, and rainbow trout, which is widely distributed in California, including in areas of really high traffic volume in Southern California where you would expect to see more 6PPD-Q,” she said.  “In contrast, another salmonid species found in California is Chinook salmon, but its LC50 is really high, so it’s not a sensitive species at a realistic environmental concentration.”

Dr. Romano de Orte noted that this is not a comprehensive table; LC50s have been developed for numerous species; the table illustrates that some salmonid species exhibit significantly greater sensitivity than other aquatic species tested.

The science is developing fast with new information coming every month.  In June, the Environmental Protection Agency published acute screening values for 6PPD-Q and 6PPD as guidance for states and tribes for their environmental protection program.  “This is supposed to be protective of aquatic life, including the sensitive species such as coho salmon,” she said.  “So the numbers are really protective and really conservative at 11 nanograms per liter for 6PPD-Q and 8900 for 6PPD.”

OCCURRENCE IN THE ENVIRONMENT

The figure shows data from ten different scientific papers on the concentration of 6PPD-Q in various environments such as groundwater, roadway puddles, roadway runoff, stormwater, surface water, and the influent and effluent of wastewater treatment plants.  The figure shows just the maximum and minimum concentration to make the visualization easier.

The x-axis shows the various source waters, and the y-axis is the concentration of 6PPD-Q in nanograms per liter. The EPA screening level, the coho salmon LC50, and the rainbow trout LC 50 are shown on the graph in red dashed lines.  The surface samples circled are from the Bay Area. 

“For roadway runoff and stormwater, the maximum values exceed all of the criteria included,” said Dr. Romano de Orte.  “On the other hand, for road puddles, runoff, surface water, and wastewater treatment plant influent, the maximum values exceed EPA screening and the coho salmon’s LC50. And the only matrix that does not exceed any criteria is groundwater with maximum concentrations of one nanogram per liter.”

She noted that 6PPD-Q and 6PPD have been detected in soil, sediments, air, indoor dust, roadway dust, and human urine. The effects of this contaminant on human health are an ongoing area of research.

DEPARTMENT OF TOXIC SUBSTANCES CONTROL AND THE SAFER CONSUMER PRODUCTS PROGRAM

Kelly Grant, Research Scientist III at the Department of Toxic Substances Control, then presented on the Department’s efforts to regulate 6PPD-Q through the Safer Consumer Products program. 

The goal of California’s Safer Consumer Products Program, founded in October 2013, is to advance the design, development, and use of products that are chemically safer for people and the environment.  The ultimate goal with most contaminants is source control or preventing the release of that hazardous substance into the environment, and the best way to achieve that is to find safer alternatives that can be used.

“The regulations allow us to be precautionary rather than wait for a risk assessment, which can take years to develop and publish,” said Dr. Grant. “This precautionary approach allowed us to move quickly on this emerging contaminant. So just eight months after 6PPD-Q was discovered, we proposed rulemaking.”

The criteria for regulation was to demonstrate the potential for exposure and the potential for that exposure to result in adverse impacts for 6PPD, which included impacts to endangered and threatened coho as well as downstream effects on the California Native American Tribes that rely on coho and other salmonids.

The regulations recognize that product manufacturers have the technical expertise to understand what chemistry will work in their product; therefore, the onus is on the product manufacturers to identify feasible alternatives and carefully evaluate the hazard traits of those potential alternatives. So manufacturers must conduct a comprehensive Alternatives Analysis to determine if the Chemical of Concern is necessary in the product and if any safer alternatives are available. 

The standard Alternatives Analysis (AA) is conducted in two stages. In the preliminary AA, the manufacturers screen a wide range of alternatives and select promising ones to evaluate in the final AA. That final AA contains a much more thorough comparison between the priority product and those selected alternatives.

“The AA process is intentionally long,” said Dr. Grant.  “It’s designed to allow time to develop alternatives and to critically evaluate the safety of those alternatives.”

The time factor is critical when studying alternatives for 6PPD as it has been used in tires for decades and is essential in ensuring safety. However, due to the absence of active research and development by tire companies in this area, no immediate replacements are currently available. This makes the Alternatives Analysis particularly well-suited to address this situation.

The Alternatives Analysis process aims to prevent regrettable substitutes by providing a holistic framework to compare 6PPD to its alternatives. The framework requires manufacturers to assess product performance, as well as a variety of adverse impacts across the life cycle of the product.  Regrettable substitutes for 6PPD would include alternatives that would reduce the durability of tires because scrap tires are an environmental disposal problem or alternatives that increase rolling resistance, which would reduce fuel efficiency, and chemicals with similar or worse hazards.

“The AA must evaluate hazards well beyond fish toxicity because we know there will be widespread exposure to the alternative,” said Dr. Grant.  “The fundamentals of tire wear and the generation of tire wear particles are unlikely to be changed by an alternative. So, based on this, we know that any alternative will be pervasive in the environment, leading to broad environmental and human exposure to it, as well as to its transformation products. So this is why it’s so critical that alternatives are carefully evaluated for both ecological and human toxicity before they are deployed in tires.”

The regulations went into effect in October 2023, and in March 2024, tire manufacturers submitted their preliminary alternative analyses.  Seventy-five global tire manufacturers responded to the regulations; most worked together in a consortium.  Out of 19 unique AAs analyzed, two were compliant, including the one submitted by the US Tire Manufacturers Association Consortium, which alone represents over 90% of the tire market in California, so she noted the percentage of the market that is in compliance is actually quite high.

The manufacturers have collectively selected 17 potential alternatives to evaluate in greater detail in the final AA.  Those alternatives range from fairly well-characterized chemicals to those that have very little information.   

“Several PPD derivatives are considered,” said Dr. Grant.  “These chemicals do have very similar structures to 6PPD and are also known to form quinones. However, several studies have indicated that 6PPD-Q might be uniquely toxic among the PPD-Qs, so that these listed alternatives may, in fact, be safer alternatives.”

In addition, Dr. Grant noted that the US EPA has initiated rulemaking under the Toxic Substances Control Act based on a petition filed by three tribal nations in the Pacific Northwest, including the Yurok tribe in California. Through their rulemaking, they are soliciting information about 6PPD alternatives so that these two regulatory processes may synergize to ensure that a safer anti-degradant is used in tires.

The final Alternatives Analysis is due in August 2026.  Beyond that, it will take several years to implement an alternative if it’s available at scale; it may take much longer if sufficient quantities are unavailable to the tire industry.

“This long timeline highlights the importance of the efforts of the Water Board and Caltrans to mitigate the adverse impacts of 6PPD-Q while we work towards source control,” said Dr. Grant.

For more information, click here for DTSC’s webpage on 6PPD.

CAL TRANS

Hardeep Takhar, Assistant Division Chief at the California Department of Transportation (CalTrans), then presented a summary of CalTrans’ strategy for monitoring 6PPD, including an overview of data, current status, and next steps.

Caltrans has implemented a stormwater management program for over 30 years, which includes a rapid response to emerging pollutants of concern.  Within two years of 6PPD becoming a known concern, Cal Trans implemented a program and began collecting 6PPD-Q samples around the state.  Over the past six months, CalTrans has developed a white paper report that integrates findings from a scientific literature review with results from Caltrans 6PPD-Q characterization and BMP effectiveness monitoring, the summary of which is the subject of this presentation.

Monitoring results

The Caltrans monitoring program collected 6PPD-Q data from 18 stations between January 2023 and October 2024.  Most monitoring sites were selected based on their proximity to salmonid habitat. Overall, the monitoring data shows that 6PPD-Q concentrations can be projected from Average Annual Daily Traffic (or AADT), receiving waters sampled are below 6PPD-Q levels of concern for coho salmon, and Cal Trans treatment BMPs are effective in reducing 6PPD-Q.

 

The monitoring shows that 6PPD-Q consistently increases with increasing Annual Average Daily Traffic, or AADT.  This graph groups Caltrans statewide 6PPD-Q results into three groups: low-traffic roadways with less than 1000 vehicles per day, medium-traffic roadways with 1000-10,000 vehicles per day, and high-traffic roadways with more than 10,000 vehicles per day.  The boxes represent the spread of data.  The horizontal dashed lines indicate the EPA 6PPD-Q screening level of 11 nanograms per liter, the lethal level for coho salmon at 95 nanograms per liter, and the lethal level for rainbow trout and steelhead at 1000 nanograms per liter at the top.

“The good news is that 6PPD-Q concentrations at each monitoring location are below levels lethal to rainbow trout,” said Mr. Takhar.  “More good news is that 6PPD-Q concentrations in stormwater runoff from low traffic roadways are consistently below the lethal level for coho salmon and are often below the 11 nanograms per liter EPA screening level. However, 6PPD-Q concentrations are projected to exceed coho salmon lethal level more often than not in medium traffic roadways and consistently in high traffic roadways.”

The monitoring results can be used to project 6PPD-Q concentrations for roadways without data.  Those projections can help identify priorities for receiving water monitoring and aid in water quality evaluations for planned CalTrans projects. The projections can be refined over time as more data is collected and more is learned about this emerging pollutant of concern.

The map on the lower left shows the location of the receiving water monitoring results from Fish Rock Gulch near Gualala, a medium AADT area on the Mendocino coast, south of Fort Bragg. The data was collected as part of a risk evaluation for a specific Caltrans project.

“The monitoring results showed that stormwater runoff 6PPD-Q concentrations of 180 to 240 nanograms per liter did not impact receiving water 6PPD-Q concentrations,” said Mr. Takhar.  “Samples were collected during the most intense rainfall period of an early season storm after .36 inches of rain fell over two hours.  Receiving water concentrations were two nanograms per liter upstream of our stormwater runoff discharge point and five nanograms per liter downstream of our discharge point. The low upstream background concentration is likely due to small county roads in the drainage watershed. There was no discernible increase in 6PPD-Q from the Cal transit runoff discharges; 6PPD-Q concentrations in receiving water remained below the 11 nanograms per liter EPA screening level.”

He acknowledged these are preliminary results from one storm.  CalTrans will be monitoring another storm at this location and other projects in the area as well; however, the first look at the receiving water shows that attenuation and dilution mitigate 6PPD-Q in stormwater discharges to below levels of concern.

Effectiveness of CalTrans BMPs

Open Graded Friction Course (OGFC)

Open Graded Friction Course (or OGFC) is a permeable pavement overlay.  The example is from a monitoring point in the Central Valley near Santa Nella, a high AADT roadway.  CalTrans was already monitoring other pollutants for removal effectiveness, so 6PPD-Q was added to the ongoing study.

OGFC reduces standing water on the freeway by allowing stormwater to flow through gaps created by large-grade gravel in the porous asphalt layer, as shown in the diagram. The benefits of OGFC include reducing standing water and splashing to improve visibility and hydroplaning risk to improve traction in rainy conditions. It also significantly reduces noise impacts from traffic, which can exceed the benefits of sound walls. Many studies have shown that stormwater treatment with OGFC reduces suspended sediment concentrations.

The monitoring results found that without OGFC treatment, the location had about 400 nanograms per liter of 6PPD-Q concentrations, about four times higher than the level lethal to coho salmon. However, treatment by OGFC reduces 6PPD-Q concentrations in stormwater discharges to below the lethal level for coho salmon.  These preliminary findings are consistent with the recent study of porous pavement overlays conducted by the city of Gresham in Oregon. 

“OGFC has potentially broad applicability in California roadways, so this finding is helpful to treatment BMP strategies for reducing 6PPD-Q risk from stormwater discharges,” said Mr. Takhar.

Media filter

The second BMP is a media filter at the Carquinez Bridge Toll Plaza on Interstate 80 near Vallejo.  Monitoring for 6PPD-Q was an addition to an ongoing study.

The media filter treats stormwater from eight freeway lanes and toll Plaza traffic. The inflowing stormwater has 200 nanograms per liter of 6PPD-Q concentrations, about double the lethal level for coho salmon. The data indicates that removing particles from media filtration also removes 6PPD-Q. The resulting 6PPD-Q concentrations discharged to the San Francisco Bay are well below the lethal level for coho salmon.

Biofiltration on the North Coast

The bioswale on State Route 299 flows into a bioretention basin that infiltrated all stormwater during the storm events sampled.  They sampled at the end of the bioswale before the flow reached the bioretention basin to allow characterization of 6PPD-Q removal through the bioswale alone.

The results showed that 6PPD-Q concentrations in stormwater from a moderate AADT roadway were reduced to below the coho salmon lethal levels. The treated water flowed into the bioretention basin, where all the water was infiltrated during the storms monitored.”

At the Circle Point curve on SR 299, stormwater flows through a bioswale towards a drainage culvert. The bioswale infiltrated 100% of the stormwater, resulting in no discharge, even after about .6″ of rain fell over 24 hours.

State Route 271, the Avenue of the Giants, is a low AADT roadway in Mendocino County.  At this location, 6PPD-Q concentrations in stormwater runoff at this locations were low or non-detect and consistently below the 11 nanograms per liter EPA screening level.

“This roadway provides an example of our low traffic, low 60 PDQ roadways,” said Mr. Takhar.  “This is another example of a bioswale infiltrating 100% of the stormwater result. And no discharge at this location, even after two and a half inches of rain fell over 24 hours during site reconnaissance.”

Among the next steps, the strategy for managing 6PPD-Q is to continue implementing the stormwater management program as required in their NPDES permit.  CalTrans will continue ongoing monitoring to improve the understanding of 6PPD-Q distribution on roadways and to verify the effectiveness of approved stormwater treatment BMPs.  They are evaluating water quality risks for Caltrans projects in sensitive coho salmon habitats.

“We will continue to share information and are open to feedback from our partnering agencies and the public. We will also continue to treat stormwater as required in our NPDES permit using all available and appropriate stormwater treatment BMPs. Our processes prioritize infiltration BMPs such as bioswales, and in settings where treatment is infeasible, using alternatives such as Open Graded Friction Course and the natural environment as treatment areas to provide effective treatment for 6PPD-Q, where possible.”

STATE WATER BOARD’S CEC PROGRAM

The State Water Board’s Constituents of Emerging Concern (CEC) Program is relatively new, having been created through a successful budget change proposal to support California’s water resilience portfolio. The program serves the entire water board as a central hub to help address CECs in all aspects of the Water Board’s programs.  The CEC program actions are informed by past science advisory panels and guided by executive management through a steering committee.

In 2023, the Science Advisory Panel on Constituents of Emerging Concern published a report on monitoring strategies for constituents of emerging concern in California’s aquatic ecosystem, and 6PPD-Q was identified as a high priority.  In response to the report, the CEC program included a 6PPD-Q in a pilot monitoring program where water and sediment samples were collected and measured for different constituents of emerging concern, including 6PPD-Q.

“We had detection of 6PPD-Q in a very small number of samples,” said Dr. Romano de Orte. “But it’s important to highlight here that this was focused on all CECs, so it was non-stormwater conditions, and it’s known that 6PPD-Q stays only a matter of hours in the water. So, to have a more realistic view of the situation here in California, we would have to repeat the sample in stormwater conditions.”

The CEC program is also building an areas of concern map for 6PPD-Q, which will focus on prioritizing sites for monitoring.  The map will include areas of high traffic volume, salmon populations, and stormwater discharges.  Lastly, the CEC program is involved in a project with the Environmental Protection Agency, Office of Research and Development, regional EPA offices, and Tribal partners to leverage what is known about 6PPD-Q and develop a strategy for the state.

“The main goal of this project is to enhance multi-agency collaboration with tribal partners and make sure that our efforts related to 6PPD-Q are efficient, minimizing the time and efforts from the tribal partners,” said Dr. Romano de Orte.

Currently, the program is focused on building foundational aspects, such as a CEC strategic plan and other documents to improve data quality and data management processes that will support a transition to using next-generation monitoring tools to proactively identify CEC for management and accountability.

“6PPD and 6PPD-Q are emerging contaminants, and the role of the program is to track information and knowledge of these contaminants, from awareness of sources and pathways to the environment, toxicity to human and ecological health, and to coordinate with relevant programs within the water boards and external agencies and partners to identify management actions that will eliminate or reduce releases to the environment,” said Erica Kalve, senior engineering geologist at the State Water Board.

The plan for the program is to engage with government, Tribal partners, and interested parties to help achieve the goals and objectives of the CEC program and ultimately develop collaborative solutions to address CEC using a cradle-to-grave approach.  The program has been working with other partners and agencies besides DTSC and CalTrans.

For example, the CEC program has been working with CalRecycle regarding their charge to support tire recycling efforts and the need to reuse rather than landfill tires, and their work characterizing water quality concerns associated with the various reuse products they’ve identified.  The CEC program has been working with OEHHA on the risk assessment of recycled waste tires and various products under evaluation.  They are also engaged with the Ocean Protection Council and their work leading California’s efforts to develop a microplastics monitoring plan and strategy that includes evaluating impacts from tire wear particles, a significant source of microplastics, to the environment.

The CEC program is also helping to shape a strategic approach for CEC monitoring and management that embraces a more holistic approach rather than a compound-by-compound approach.

“6PPD is an example of a CEC that moved rapidly from discovery to source control actions, and so it provides a really amazing case study for us to use to gain the experience we need to try to apply this same sort of rapid response to future emerging contaminants,” said Ms. Kalve.  “This will help us not only address 6PPD-Q but also help us to manage CECs more broadly.

“The science on 6PPD and 6PPD-Q is advancing rapidly, and significant knowledge gaps remain regarding the risk of these contaminants posed to ecosystems and human health,” she continued.  “Current research indicates that 6PPD-Q, particularly, is impacting certain salmonid populations with substantial ecological consequences. Additionally, salmonid mortality driven by 6PPD-Q and other factors such as climate change disproportionately affects tribal nations in North America; it threatens access to traditional foods and the cultural and economic well-being of Indigenous communities.”

DISCUSSION PERIOD

It is not very often that the discussion and public comment period are wholeheartedly positive, but that indeed was the case, with the Board members and California Coastkeeper Alliance representatives all impressed by how proactive the state agencies have been in addressing the 6PPD-Q issue.  Although not present, the US Tire Manufacturers Consortium was given high marks for its response and, by all accounts, has been helpful and collaborative.

Praise for agencies / proactive actions to continue

“It’s been a real hallmark of this effort, in so far as really being able to use our different authorities and work to be able to address something that just came on the scene, and the rapidness of the response,” said Chair Joaquin Esquivel.  “The amount of work that’s gone into this is impressive.”

Board member Laurel Firestone acknowledged the data gaps but noted, “This highlights the importance of the stormwater practices.  Those are huge investments, and the regulatory program that we’ve put in place over the past few decades is able to capture not only things that we knew about but now things that we are learning about.  I’m glad to see some good news from that so far.”

Board member Nichole Morgan was likewise impressed.  “We really are working through this, and we’re sharing what we know, where we’re going, what we’re learning, and what we don’t know. … the BMPs are working. We’re keeping it out of our streams; while we may not know the full impact on humans, we’re keeping it out. We are being protective. We’re doing what we can do.”

Sean Bothwell, executive director for California Coastkeeper Alliance, was pleased to see that the state agencies have taken this issue seriously and acted proactively.  He noted that they had introduced legislation in the last session that was not successful that would have ordered Cal Trans to do the work that they have done.  “So we really appreciate that that Caltrans took the initiative and did that work that we thought was so important.”

Mr. Bothwell acknowledged that finding an alternative to 6PPD-Q will be a long process, so mitigation now is essential.  “We’ve always felt that bioretention and stormwater capture is the direction to go.  The research has shown it’s the way to address 6PPD, and it addresses other things like microplastics and other common pollutants that we deal with on a day-to-day basis for stormwater.”

Board member Sean Maguire said the discussion feels a lot like PFAS and the risks throughout the environment, its ubiquity, and how it can impact humans and other aquatic species.

“It’s hard to peel back all the layers of the onion because there are so many different versions of PFAS. That’s not quite the situation we’re in today, but I fear it could become that if we’re not careful and mindful of what we’re using in our products. So I appreciate the DTSC’s efforts in looking at alternatives because I believe that source control is where this should all begin.”

“A lot of this is a risk process, and thinking about how do we prioritize these efforts,” he continued.  “So I’m looking forward to the maps. I’m looking forward to seeing and better understanding that integration of where coho salmon are most at risk, where they have been most impacted from these types of influences, where we can have the biggest impact, and where can Caltrans or other partners lean in on treating stormwater in the areas that are most sensitive. Because we don’t have another 25 years to sit around. We should act now and proactively.”

Fate of 6PPD/6PPD-Q

Chair Joaquin Esquivel asked about the fate of 6PPD.  Does it degrade ultimately in the environment? Do we know if it accumulates in sediments or falls out of the water column after stormwater events? 

Dr. Manoela Romano de Orte said that is not yet known.  We do know 6PPD transforms quickly to 6PPD-Q in water.  What happens in the sediment is a bigger mystery.  The EPA is currently studying that.

“There’s been one study in China looking at 6PPD-Q and sediment across a river delta estuary and then out into the deep sea,” said Dr. Kelly Grant.  “They did find a gradient of decreasing amounts of 6PPD-Q in that, but the fact that they found it in deep-sea sediment suggests that it may persist longer in sediment than it does in the water column, but it’s still a major data gap.”

Alternatives for 6PPD-Q

Chair Joaquin Esquivel asked if there were any frontrunners or particularly hopeful alternatives for 6PPD-Q.

Dr. Kelly Grant said in terms of ease of substitution, the other PPD derivatives would probably be the easiest for the tire industry.  Some of these other PPD derivatives have been tested, and the do seem much safer than 6PPD-Qin Washington state.  “Because we don’t understand the mechanism of action of 6PPD-Q, it’s a little worrisome to think about other PPD-Qs being out there. There’s certainly been a fair amount of biomonitoring studies on other PPD-Qs, and they’re pretty broad in the environment at this point and human urine. So I think the jury is still out, and we’ll have to wait until the alternatives analysis is complete to understand that.”

Cody Phillips said he was happy to see California taking the initiative to find alternatives that are effective at replacing 6PPD.  “There really is nothing available to replace it now, and it is essential to tire safety. Not only that, but studies have found, at least in the Bay Area, that half of the microplastics in the Bay are composed of tire particles.”

Mr. Phillips noted that some of the prioritized alternatives for further study are toxic to aquatic organisms.  “For example, 7PPD, according to a study that came out of Washington, is a chemical of high concern that should be avoided. That’s not the quinone version, that’s just 7PPD on its own. It was assigned a very high score of acute toxicity based on multiple fish studies the same way California does its toxicity.”

Cost and maintenance of BMPs

Vice Chair DeDe D’Adamo asked about the costs and maintenance for the different BMPs.  What would it take to deploy some of these treatment methods?

“The strategy that we’re looking at right now is trying to better understand the sources pathways and the fate of 6PPD-Q, which will also help us inform where the priority areas are that Caltrans needs to focus on implementing stormwater treatment measures,” said Mr. Takhar.  “Then, as the projects come along and we identify opportunities to implement stormwater treatment BMPs, we look at feasibility in terms of habitat, right of way, and safety concerns – those are all factors that go into the decision-making process of whether we can implement treatment BMPs or not, or is the natural environment that’s in place able to attenuate 6PPD-Q.  A lot of those evaluations happen early on in the project development phase and the environmental phase, where technical studies are performed. So we can make decisions about what type of treatments are appropriate for that location to address the environmental impacts associated with that specific project.”

“We’re also looking at putting together a retrofit program starting June 2025, which is essentially going to require us to retrofit treatment BMPs,” Mr. Takhar continued.  “That is basically to reconstruct treatment BMPs that have completed their life cycle. So, we don’t have a lot of information in terms of at what point the treatment BMPs are experiencing a breakthrough where the pollutants are no longer being removed by the BMP.  A lot of those studies are part of the strategy that we are developing right now that’s going to inform some of the decisions we’re going to make and at what point, whether it’s 10 years or 20 years, is the life cycle of these treatment BMPs that we’re going to put in the ground, at what point they will need to be replaced.”

Other sources of 6PPD-Q to the environment besides road pollution

Cody Phillips, California Coastkeeper Alliance, pointed out that Caltrans is not the only source of tire rubber pollution. Samples taken from parking lots at Cal Poly Humboldt were well above the lethal concentration.  “So while focusing on Caltrans is absolutely important as that is where most of these pollutants are concentrated, we have to be aware that there are other things that we need to focus on to more holistically deal with this issue.”

Board member Firestone pointed out that the Los Angeles region is currently working on stormwater permits, so the issue of parking lots points to the need to work with the regional boards to communicate information on emerging contaminants for their permitting processes.  Board member Sean Maguire agreed, noting that the parking lot issue brings up the need to consider other potential sources of stormwater discharges for prioritizing BMPs. 

Near term actions

Board member Laurel Firestone asked what near-term actions Cal Trans has planned.  Mr. Takhar said they are expanding their monitoring program which will inform where the priority areas are.  They are also in consultation with the National Marine Fisheries Service on an ongoing basis.

“We are in the process of generating data, looking at project level impacts, and then seeing what we can do in terms of implementing stormwater treatment BMPs because we may find that stormwater treatment BMPs are not feasible at some of these locations because we have endangered species habitat right on the roadside,” said Mr. Takhar.  “We have a narrow right away, and we have safety concerns. So there may be reasons we may not be able to implement treatment BMPs, but we may find that the natural environment may be attenuating 6PPD-Q as well at the same time. So, a lot of that data collection effort is ongoing and will inform the programmatic approaches we will take. We don’t have an answer yet.”

Tire manufacturers helpful

Board Member DeDe D’Adamo asked how cooperative the tire manufacturers have been.

“I’m surprised they didn’t present today because they were super helpful before the adoption hearing in 2022, helping us think through the legislation,” said Sean Bothwell, California Coastkeeper.  “They were the ones that actually brought up the parallel to the road issue that I hadn’t even thought about.  They were also the ones that brought up things other BMPs that might be more cost effective. And so I think, at least from where I’m sitting from the stormwater capture side, they were really helpful in providing me with information about the most cost-effective BMPs they’ve seen out their research in Washington and things like that.”

“In my role as co-lead of the Interstate Technology Regulatory Council that worked on 6PPD, we did have a lot of input from industry, and they were generally pretty helpful there,” said Dr. Grant.

“The US Tire Manufacturers Association has been extremely collaborative with DTSC,” said Jen Jackson, Environmental Program Manager for the Department of Toxic Substances Control.  “They came to us asking us to regulate 6PPD into motor vehicle tires. I think, probably from their perspective, it’s a good way for them to have some time to find that alternative so that, hopefully, they won’t have a regrettable substitute. So, you know, we’ve been very pleased with the work that they’ve done. The consortium they’ve built, they have some 33 different tire manufacturers who are part of their consortium, so we’ll continue to work with them, and really appreciate that they’ve been so collaborative with us.”

Sean Bothwell acknowledged that finding an alternative to 6PPD-Q will be a long process, so mitigation now is essential.  “We’ve always felt that bioretention and stormwater capture is really the direction to go.  The research has shown it’s the way to address 6PPD and it addresses other things like microplastics, and other common pollutants that we deal with on a day to day basis for stormwater.”

More coverage of 6PPD and tire pollution on Maven’s Notebook …

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