Project Description:

The overall goal of this project is to identify transport and mitigation of tire and road wear microparticles in stormwater runoff from highways.  Expulsion of microplastics (MP), metals, and other organic contaminants occurs in heavy traffic areas. Microplastic (size range 1–5000 μm) is solid, insoluble in water, and composed of synthetic polymers having thermoplastic/thermoset properties. Tire wear particles (TWP) that fall within the microplastic definition constitute the principal part of MP. As styrene-butadiene rubber, natural rubber, carbon black, and additives are the main tire components, TWP is generated during the interaction between the tire and the road surface creating black and elongated encrustations of pavement. Many factors control the TWP size, such as vehicle speed, driving patterns, weather conditions, road structure, road maintenance, sampling procedures. 
We will particularly focus on tire wear microplastics containing 6-PPD.  The composition of the tire is a chemically complex and heterogeneous mixture having about 5–10% additives required for its stability and durability. Therefore, a tire can contain as many as 200 various additives that can be leached from tire and road wear particles (TRWPs) into the aquatic region. Although predicting material leachability from tires and their subsequent potential risk was indicated in the early nineties, very few studies have reported so far investigating TRWP leachates. Very recently, Tian et al. (2021) have published an oxidation product of tire antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-pphenylenediamine (6-PPD), 6-PPD-quinone, as the lethal cause of coho salmon mortality.  6PPD quinone (the transformation product of 6PPD), has been reported in roadway runoff, tire rubber leachates, and road dust. Although very little information exists regarding the occurrence and fate of 6PPD quinone, this chemical is assumed to be present in surface waters globally because of the widespread use of the parent compound. The toxic impact of 6PPD quinone could be a significant threat globally to aquatic species and may not be limited to coho salmon only. 

We will accomplish the goal of this project through three tasks:

1. Literature Review: An extensive literature review will be conducted on tire and road wear microparticles primarily in stormwater runoff from Pacific Northwest highways.
2. Measurement of Tire and Road Wear Microparticles in Stormwater: We will collect stormwater from several Pacific Northwest sites including Seattle, Spokane and Pullman. We will also collect tires from junkyard and generate microparticles from the tires in the lab through grinding to test leaching of 6PPD-quinone from tires.
3. Modeling and Mitigation Approaches: The characterization results from Task 2 are critical for estimating filtration rates in sand filters, infiltration galleries, swales, and other Best Management Practices (BMPs). We will adapt modeling technology following our prior work to understand the transport of tire and road wear microparticles in stormwater from highways and in BMPs. We will also conduct packed-bed column filtration studies to collect data on the transport of tire and road wear microparticles through soil, to test estimated filtration rates and to develop mitigation approaches. Columns will be packed with highway shoulder and BMP soil materials, and stormwater will flow through the packed-bed columns. 

_{Figure 1. Overall project approach, outputs and outcomes.}

Outputs: 

Our technical deliverable will make recommendations concerning the types of best management practices (BMPs) and mitigation measures needed to remove tire and road wear microparticles containing 6-PPD as well as the resulting oxidized 6PPD-quinone potentially present during stormwater flows. 

Outcomes/Impacts: 

The results from this project will be helpful for developing mitigation approaches for tire and road wear microparticles transport in stormwater runoff from highways. Potential stakeholders are stormwater facilities, Department of Transportation, Department of Ecology, and local populations. For technology transfer, we will develop a webinar on “Transport and Mitigation of Tire and Road Wear Microparticles in Stormwater Runoff from Highways.”