Coho salmon make their way back to coastal rivers and lakes after a few years at sea in their final migration. Once they’re upstream it’s all about spawning; they put all their energy into reproduction, and then die once they’ve spawned. This effort to create the next generation of coho also fuels forest regrowth and provides food for bugs, birds, bears—and everything in between.
Or that’s how it’s supposed to go.
Since the 1990s, researchers in the Pacific Northwest have documented the impacts of stormwater toxicity on salmonids, including the impacts of urban runoff on coho pre-spawn mortality. As coho migrate upstream through waterways that receive stormwater discharges, they rise to the surface, gasp, and swim around in circles. They then die before getting a chance to spawn. Over the years, scientists pieced together data showing that these deaths occur where stormwater discharges were draining areas of vehicle traffic—roads, bridges, parking lots. Indeed, since 2011, the National Oceanic and Atmospheric Administration has been studying and confirming this “direct and highly consequential threat to salmon conservation.” Related research at the time also confirmed that running stormwater discharge through soil with plants, for example rain garden-type basins, before the discharge entered a stream, solved the problem and protected salmon.
In December 2020, a team of University of Washington Tacoma chemists isolated the specific chemical in stormwater that is killing coho: a transformation product called 6PPD-quinone (N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone) that is formed when a ubiquitous anti-degradant in tires (called 6PPD) is exposed to ozone. According to a review by the California Department of Toxic Substance Control, 6PPD makes up about 1 to 2 percent, by weight, of all tires, and has been in use for over 50 years.
Over the past two years, research into 6PPD-quinone has rapidly advanced. Data suggest that that 6PPD-quinone “is among the most toxic chemicals known for aquatic organisms.” With coho mortality occurring at ∼0.1 parts per billion, 6PPD-quinone is “among a very small group of pollutants, mostly organophosphate or organochlorine pesticides, with acute toxicity expectations at tens of nanograms per liter,” or incredibly small amounts. See Zhenyu Tian et al., 6PPD-Quinone: Revised Toxicity Assessment and Quantification with a Commercial Standard, Table 1. In addition to the chemical’s known toxic effects on coho, research is beginning to show that 6PPD-quinone adversely affects other fish species as well.
6PPD-quinone enters salmon streams from stormwater running off roads and recycled-tire infrastructure (e.g., playfields, recreational trails). Particles generated as tires physically wear down can also be deposited directly into streams or collected and concentrated through catch basins and storm drains. 6PPD-quinone has been found to be widespread in surface waters wherever it has been studied.
Addressing toxic 6PPD-quinone discharges involves activation of two well-understood pathways for pollution prevention: finding safer alternatives and managing stormwater.
The good news, which has been well understood for decades, is that bioinfiltration green infrastructure such as soil-based bioswales or rain gardens protects receiving waters and coho salmon from 6PPD-quinone. Municipal stormwater permittees must reduce the unfiltered discharge of street runoff and tire wear particles by installing and implementing green infrastructure in salmon waterways. Further, sources of 6PPD-quinone pollution such as tires used as bumpers along working waterfronts and recycled tires used in parking lots and drainage systems also need to be identified, and either removed, replaced with a non-toxic alternative, or runoff from these sources captured and treated.
A host of efforts are underway to find, test, and possibly deploy replacement tire preservatives. In Washington, the state legislature funded a survey in 2021 of potential 6PPD alternatives but concluded that “data gaps—particularly around transformation products and urban runoff mortality syndrome” make finding safer choices difficult. In May 2022, California’s Department of Toxic Substances Control proposed listing motor vehicle tires containing 6PPD as “Priority Products” under the Safer Consumer Products regulations. (As of the drafting of this article, the public comment period for this action was open until July 20, 2022. For updates on that rulemaking, see https://dtsc.ca.gov/scp/motor_vehicle_tires_containing_6ppd/.) If listed, California would not be banning 6PPD in tires, but instead would be requiring manufacturers of tires containing 6PPD to notify the state that they produce these products and either perform an alternatives analysis or pursue other options as identified in California Code of Regulations, Title 22, section 69505.
With years of data about how green infrastructure functions and can filter out chemicals found in stormwater, and after a legislatively mandated synthesis of the current knowledge of 6PPD and 6PPD-quinone, including physicochemical properties, sources, fate, and transport within the built environment, the Washington State Department of Ecology finalized new Stormwater Management Manual guidance in June 2022. This update included an assessment of the stormwater best management practices expected to reduce concentrations of 6PPD and 6PPD-quinone in runoff.
Beyond safer alternatives and stormwater management, a host of other systems play a part in protecting salmonids from 6PPD-quinone.
First, environmental impact reviews, solid waste management plans, recycling policies and credits, producer responsibility laws, and even tariffs will likely see some shifts given the damage to fish these sources of 6PPD can cause. In 2019, the United States generated an estimated 263.4 million scrap tires, and tires have been recycled for use in everything from artificial reefs to building roofs. See Product-Chemical Profile for Motor Vehicle Tires Containing 6PPD (March 2022), Table 5.
Second, most salmonids are endangered throughout most of their ranges and experience stormwater-driven mortality that can now be traced to one chemical, from one discrete source: 6PPD-quinone from tires. Moreover, salmonids are a critical food source for endangered orca populations. Agency consultations for projects affecting these species and habitats should be considering ways to prevent exposure to 6PPD-quinone. This is especially true for major infrastructure bills and other stimulus programs that tend to prioritize federal spending for new bridges, roads, and highways.
Third, coho, and other salmon and trout species we know are affected to varying degrees by 6PPD-quinone, have cultural and subsistence importance. As such, this 6PPD-quinone salmon mortality crisis must be a focal point in all our communities.
Whether through removal at the source or retrofitting streets with green infrastructure, coho and other salmonids will not survive until and unless we eliminate 6PPD-quinone from our waterways. Tire reformulation will need to occur, and stormwater permits, federal transportation projects, recycling and solid waste management, and fisheries management will need to account for this highly toxic chemical.
