In response to a significant decrease in salmonid fish populations in urban streams, including several listed under the Endangered Species Act (ESA), several West Coast organizations conducted studies to evaluate the cause. In 2020, a culprit was identified – a breakdown compound commonly found in tires: 6PPD.
6PPD (N-(1,3-Dimethylbutyl)-N’-phenyl-p-phenylenediamine; C18H24N2 CAS 793-24-8) is an additive in the manufacturing of both natural rubber and common synthetic rubbers such as butyl rubber and styrene-butadiene rubber. It is a highly effective anti-oxidant. In layman’s terms, it helps tires resist degradation caused by exposure to oxygen, ozone, and fluctuating temperatures. According to the US Tire Manufacturers Association (USTMA), all USTMA members use it (USTMA website, 15 Aug 2023).
Recent studies have shown that the reaction of 6PPD in rubber tires with the oxygen and ozone in the air generates transformation products, including storm (6PPD-q; 2-((4-Methylpentan-2-yl) amino)-5-(phenylamino) cyclohexa-2,5-diene-1,4-dione; C18H22N2O2; CAS 2754428-18-5).
As tires wear, tire wear particles (TWP) and fragments containing 6PPD-q can be carried by stormwater runoff from roadways and parking lots to aquatic environments, such as salmonid spawning streams.
For instance, recent studies indicate that 6PPD-q present in such waters may be acutely toxic to coho salmon, including juveniles. A study by Tian and others indicates that 6PPD-q toxicity to coho salmon ranks among the most toxic chemicals for which the US Environmental Protection Agency has established aquatic life criteria.
Subsequent studies indicate that 6PPD-q exhibits large differences in species sensitivity, with reduced toxicity to steelhead trout, Chinook salmon, rainbow trout, and brook trout. No mortality was observed with sockeye salmon, chum salmon, Atlantic salmon, brown trout arctic char, and white sturgeon. Regardless, the impacts on coho salmon and other species may be significant.
Many questions remain unanswered before we better understand the environmental and toxicological impacts of 6PPD-q and develop potential solutions. Ongoing efforts include the following:
Whether 6PPD-q contamination will become a significant issue outside spawning regions of coho salmon and other ESA-listed fish species remains to be seen. Bioaccumulation in higher-order animals may also be an issue; more research is underway.
About the Authors:
Jeff Marshall, PE in five states, is a vice president and the practice leader for the Environmental Services Practice for SCS offices on the eastern seaboard. He also serves as the SCS National Partner for Innovative Technologies and Emerging Contaminants. He has a diversified background in project engineering and management, with emphasis on the environmental chemistry and human health aspects of hazardous materials/waste management, site investigations, waste treatment, risk-based remediation and redevelopment, and environmental compliance/permitting issues.
Dr. Shane Latimer, CSE, is an SCS vice president. He is an environmental planner, ecologist, and toxicologist with three decades of experience in environmental assessment, planning, permitting, and implementation. His specialty is developing projects that challenge the interface between the built and natural environment, such as solid waste facilities, oil and gas infrastructure, mines, sewage treatment facilities, and similar developments. Solutions for these projects often require careful assessments of alternatives, impacts, and opportunities to successfully navigate the applicable public regulatory processes (e.g., NEPA, local land use, etc.) and ensure environmental integrity.