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Title
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Evaluating Tire Road Wear Particles distribution in Thurston County, Washington
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Creator
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Melchert, Marlene
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Identifier
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Thesis_MES_2023_MelchertM
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extracted text
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EVALUATING TIRE ROAD WEAR PARTICLES IN THURSTON COUNTY,
WASHINGTON: IMPLICATIONS TO STREAMS
By
Marlene L. Melchert
A Thesis
Submitted in partial fulfillment
Of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
June 2023
By Marlene L. Melchert. All rights reserved.
This Thesis for the Master of Environmental Studies Degree
by
Marlene L. Melchert
has been approved for
The Evergreen State College
by
Erin Martin, Ph.D.
Member of Faculty
Date
ABSTRACT
Evaluating Tire Road Wear Particles in Thurston County, Washington
Marlene L. Melchert
Tire road wear particles (TRWP) are made through the abrasion of tires on the road surface. Tire
wear particles conglomerate and stick to other road particles and toxins once released into the
roadway, where they may be weathered, aged, and biofouled from toxic chemicals and heavy
metals (Lou, 2021). TRWPs are transporters of co-contaminants like 2-((4-Methylpentan-2yl)amino)-5-(phenylamino)cyclohexa-2,5-diene-1,4-dione
(6PPD-quinone),
poly-aromatic
hydrocarbons (PAHs), and heavy metals to urban streams (Baensch-Baltruschat, 2020). This study
provides a spatial analysis of TRWP count and size based on four road types (primary, secondary,
residential, and rural) in Thurston County, WA. Most TRWPs released to roadways are coarse,
with sizes ranging from 1μm2 to 175 μm2. TRWP counts were assessed by road type, and primary
roads contained between112 to 1225 particles, with a mean of 475 ± 441; Secondary roads had
particle counts that ranged from 298 to 4200, with a mean of 1493 ± 1741; Residential roads
ranged from 87 to 707, with a mean of 349 ± 264; Rural TRWP count ranged from 45 to 408,
with a mean of 205 ± 173. The results showed that secondary roads had the most recorded TRWPs
recovered from road dust samples but were not statistically different from the other road types.
The highest TRWP counts recovered for road dust samples were found on two secondary roads:
Eastside Street and College Street. These streets posed the most increased risks to urban streams
for becoming biofouled with TRWP pollution, and targeted efforts to reduce particle loading from
these roads into urban streams are recommended.
Table of Contents
List of Figures .............................................................................................................................................. vi
List of Tables .............................................................................................................................................. vii
Acknowledgments...................................................................................................................................... viii
Introduction ................................................................................................................................................... 1
Tire Road Wear Particles (TRWP) Significance ................................................................................. 2
Research Questions ................................................................................................................................. 4
Literature Review.......................................................................................................................................... 5
Introduction ............................................................................................................................................. 5
Roadmap Connecting Tire Road wear particle (TRWP) Emissions and Urban Runoff Mortality
Syndrome (URMS).................................................................................................................................. 6
Environmental regulations to monitor chemicals exposures of point and non-point toxins ......................... 8
History of Tire Production and Research .................................................................................................... 10
Tire Rubber Physical and Physicochemical Characteristics ............................................................ 14
Tire Rubber Microplastics Sources to the Environment................................................................... 16
Primary sources of Tire Rubber microplastics in the environment ..................................................... 17
Tire Road wear particle (TRWP) transport, fate, and flow in the environment .................................. 18
How TRWPs are Estimated ................................................................................................................ 20
Tire Road wear particle (TRWP) sizes and densities in environmental samples............................ 21
TRWP found in Soils .......................................................................................................................... 22
TRWP found in Road Dust ................................................................................................................. 23
TRWP found in sediments and surface water ..................................................................................... 23
Urbanization of Streams: Connecting TRWP as a Physical and Non-point Source of Pollution to Urban
Streams........................................................................................................................................................ 25
6PPD-quinones Environmental Production from TRWP ................................................................. 27
Point and Non-point Pollution to Urban Streams .............................................................................. 28
Environmental Fate and Transport of 6PPD-quinone ......................................................................... 29
Susceptible Species to Urban Runoff Mortality Syndrome............................................................... 29
Human exposure to 6PPD-quinone ..................................................................................................... 31
Importance of Salmon and Landscape Connectivity ......................................................................... 32
Tire Rubber Pollution in Washington State Aquatic Ecosystems ....................................................... 34
Green Infrastructure (GI) .................................................................................................................... 35
Conclusions ................................................................................................................................................. 38
Methods ...................................................................................................................................................... 40
iv
Location of Study: Thurston County, WA ......................................................................................... 41
Road Dust Sample Collection Protocols.............................................................................................. 42
Road Dust Sample Preparation and Separation of TRWPs ............................................................. 43
Microscopic Identification of TRWP from processed replicate samples ......................................... 45
Image J analysis of TRWP particle sizes and distribution ................................................................ 47
Statistical Analysis of TRWP Measurements (area μm2) .................................................................. 49
Results ......................................................................................................................................................... 50
Microscopy Identification and Image J analysis of Tire Road Wear Particles (TRWPs) .............. 50
TRWP Count Results ........................................................................................................................... 51
TRWP Measurement Results .............................................................................................................. 55
Discussion ................................................................................................................................................... 58
Thurston County’s TRWP’s Differences Among Roads ................................................................... 58
TRWP Morphologies ............................................................................................................................ 59
Image J for particle analysis ................................................................................................................ 61
TRWP distribution Implications to streams: Identifying regions with increased TRWP emissions
................................................................................................................................................................ 62
Mitigation of TRWP’s for Conservation and Rehabilitation of Susceptible Species ...................... 63
Limitations of this study ....................................................................................................................... 66
Conclusion .................................................................................................................................................. 68
References ................................................................................................................................................... 70
v
List of Figures
FIGURE 1.
13
FIGURE 2
14
FIGURE 3.
16
FIGURE 4.
19
FIGURE 5.
40
FIGURE 6.
42
FIGURE 7
45
FIGURE 8.
46
FIGURE 9
48
FIGURE 10.
50
FIGURE 11.
51
FIGURE 12.
52
FIGURE 13
54
FIGURE 14.
61
vi
List of Tables
TABLE 1.
14
TABLE 2.
20
TABLE 3.
45
TABLE 4.
52
TABLE 5
54
vii
Acknowledgments
I want to thank the Evergreen State College and the MES faculty for making this project possible.
Thank you to Jenna Nelson, a Science Inquiry Technician for MES, who helped affirm the
methodologies and helped acquire the lab space needed. Thank you to Erin Martin for
collaborating with me on this project. Also, thank you to my husband Vincent Riggs for helping
me with my sample collection and all the little things he has done to help make this project easier,
whether by allowing late nights and taking care of the kid or just giving the best pep talks. Lastly,
I want to acknowledge my parents and in-laws for their help and encouragement.
viii
Introduction
Tire road wear particles (TRWP) are considered the second largest global contributor of
microplastics to the environment (Verschoor et al., 2016; Lou et al., 2021). In the United States, it
is estimated that over 1,120,000 tons of tire wear particles are released per year from roadways
(Lou et al., 2021). Tire wear particles are emitted to the roadway through the abrasion of tires and
road surfaces, where they are weathered, aged, and biofouled (Lou et al., 2021). TRWP forms
primarily in conglomerates of tire and road materials in various sizes (Jekel et al., 2019). The size
and density of TRWP relate to what it forms with on the road and can be a factor in TRWP toxicity
and transportation (Jung et al., 202). The amount of TRWP emitted to the environment varies by
region due to the many different land use practices and the number of roads. For example, research
shows that driving habitats such as quick braking or accelerating and aggressive maneuvering,
along with traffic network structures, heavily impact how much TRWPs are emitted to the roadway
(Jekel et al., 2019). Densely populated urban areas with increased use of personal vehicles,
transportation of goods, and higher concentrations of impervious surfaces would have increased
rates of TRWP emissions (Charbouillot et al., 2023).
Tires are a globally significant pollution issue for both chemical and physical waste. Tires
also represent only one facet of motor vehicle pollution, and, unfortunately, because of their
immense importance to modern daily life, they are largely ignored as a major source of
microplastic pollution. Furthermore, as we focus on utilizing low-carbon vehicles, we
inadvertently increase the emission of tire road wear particles (TRWP) due to the increased weight
of hybrid and electric vehicles (Charbouillot et al., 2023). Heavier vehicles equate to higher trends
in tire and brake wear. The daily generation of microplastic-sized TRWP collects along the
roadside, where it accumulates in road dust and is transported to streams via stormwater/roadway
1
runoff (Charbouillot et al., 2023). It’s necessary to better quantify the amount of TRWPs found on
roadways as they have important ecological impacts on urban streams.
Tire Road Wear Particles (TRWP) Significance
The fate and transport of TRWPs are significant issues of urban stream pollution because
TRWPs potentially act as co-contaminant carriers of toxic chemicals and heavy metals that
negatively affect aquatic environments and human health (Mennekes et al., 2022; Wagner et al.,
2018). Researchers have confirmed TRWP chemical byproducts in various environmental samples
such as air, soil, stormwater, surface waters, and road dust samples (Mennekes et al., 2022; Lou et
al., 2021; Baensch-Baltruschat et al., 2020). However, few studies measure TRWP size and count
from road dust. TRWP size and count represent two factors that increase their risks to streams.
For example, the size of TRWP represents how readily the particles are transported and the ability
to absorb/leach toxins they carry from the roadway to urban streams (Lou et al., 2021). Whereas
the count of TRWP found on the roadway represents the number of TRWP pollutants transported
to urban streams (Baensch Baltruschat et al., 2020). These factors determine the toxicity, transport,
and fate of TRWP in the environment. In order to determine urban streams with increased risk, it
is essential to identify how many TRWPs are sitting on the road waiting to be washed out to
streams via roadway/stormwater runoff. By identifying regions of increased TRWP inputs, we can
assess areas that need targeted green infrastructure for stormwater/roadway runoff mitigation.
Significant
TRWP
pollutants
like
2-((4-Methylpentan-2-yl)amino)-5-
(phenylamino)cyclohexa-2,5-diene-1,4-dione (6PPD-quinone) have been profiled as casual
toxicants that induce Urban runoff mortality syndrome (URMS) and Coho mortality syndrome
(CMS), which are fish die-off events observed in Puget Sound (Peters et al., 2022). As TRWP
accumulates along the roadside, 6PPD, a tire additive used to maintain rubber stability, undergoes
2
environmental transformations with ground-level ozone and creates the lethally toxic chemical
6PPD-quinone (Tian et al., 2020; Järlskog et al., 2022). Toxic stormwater flushes refer to heavy
rain or snow melt, which transports large amounts of toxins from roadways into aquatic ecosystems
(Peter et al., 2022). These stormwater flushes have been related to the observed URMS and CMS,
which refer to the mortality events of aquatic species from exposure to toxic stormwater and
roadway runoff (Peter et al., 2022; French et al., 2022). Before discovering 6PDD-quinone as the
casual toxicant to induce CMS and URMS, studies showed that densely populated urban areas
with high impervious land coverage were directly related to fish mortality events (Fiest et al.,
2018). Expanding impervious surfaces increases the negative impacts of land use practices, human
activities, and urban growth on stream ecosystems (Fiest et al., 2018). The increased impervious
land coverage and dwindling freshwater resources put people and salmon at risk. A geospatial
analysis of the distribution of TRWP among various road types can help identify areas that require
targeted green infrastructure due to TRWP presence and abundance. Urban streams with
susceptible species of URMS / CMS, along with increased TRWP inputs, downplay the success
of significant ecological restoration goals, like the fish passage barrier projects in Washington
State.
Since TRWPs transport roadway toxins like 6PPD-quinone to urban streams, which are the
culprit of salmon mortality in Puget Sound, understanding the sum of tire microplastics (TRWP)
found on the roadway is vital to determine whether certain roads or road types are hotspots for
TRWP production. Few studies have quantified TRWP from environmental samples, most of
which are located outside of the United States; however, virtually none exists in the Pacific
Northwest at the time of writing this thesis. Since Puget Sound is an urban hotspot with crucial
salmon runs under threat, doing this work in Washington State is essential. Furthermore, because
3
the surface area of the particles can affect the sorption of pollutants onto particles, it is essential to
quantify surface area simultaneously.
Research Questions
How does tire road wear particle (TRWP) size distribution vary among road types (primary,
secondary, residential, and rural) in Thurston County, Washington? The second research question:
Does the annual average daily traffic impact the average mean TRWP particle size and count?
These research questions help provide insight into the spatial distribution of TRWP found on
roadways, indicating regions that need to target stormwater and roadway runoff mitigations based
on high emission rates.
4
Literature Review
Introduction
Tire road wears particle presence, and abundance are highly unknown, and only recently
have research developments made progress in detailing the presence of TRWP emissions. The
urbanization of streams poses one of the most significant threats to healthy freshwater ecosystems
(Scholz et al., 2 011). Urbanization represents an area no longer in its natural state with increased
impervious surfaces and various land uses that pollute the air, water, and soil (Scholz et al., 2011).
Road development is one of the most significant factors that pollute the environment with petrol
chemicals and heavy metals. Road networks act as the lifeblood of a communities economy and
trade (TRCP, 2022). As the population grows, so does the variety of land use practices and road
development that negatively impact stream water quality and biological function (Peter et al.,
2022). One of the most harmful land use practices to urban streams is using paved roads and motor
vehicles (Scholz et al., 2011). For example, tires emit toxins and physical debris into the roadway
contributing to air, water, and soil pollution. Studying and monitoring densely populated areas
with increased traffic for TRWP emissions is necessary to mitigate the loss of suitable stream
habitats for economic and culturally iconic species like Pacific salmon (Lou et al., 2021).
Washington's urbanization of streams led to the decimation of salmon populations in Puget Sound
and resulted in the urgency to restore and conserve stream habitats (Sobocinski et al., 2021; Scholz
et al., 2011). In order to aid the effort to conserve salmon species, more research on the presence
and abundance of TRWP found on roads is necessary.
This literature review seeks to connect Tire Road Wear Particles (TRWP) as a significant
source of non-point pollution in stormwater/roadway runoff which acts as physical and chemical
waste to urban streams. Since TRWP are transporters of toxins from the road, they act as a non-
5
point source of chemical pollution in urban streams which diminishes vital salmon habitats.
TRWP’s toxins from the roadway have been found to increase streams’ toxicity, resulting in
induced urban runoff mortality syndrome (French et al., 2022). In order to assess the extent of
damage TRWPs have on streams, it is vital to get an accurate measurement of TRWP contents in
road dust. For instance, determining the presence and abundance of TRWP on roadways can help
determine which roads have increased risks for stormwater /roadway runoff pollution effects on
urban streams.
Roadmap Connecting Tire Road wear particle (TRWP) Emissions and Urban Runoff
Mortality Syndrome (URMS)
The literature review for this study covers various topics to provide a comprehensive
viewpoint of tire road wear particle waste effects on the environment, specifically urban streams.
Reviewing relevant legislation established to protect the environment from manufacturing and
using toxic chemicals in consumer products gives the reader an understanding of how we can assert
these policies to mitigate and monitor TRWP pollution. Following the legislative review, the
literature review provides the global significance of TRWP pollution with production estimates.
Then a review of tire compositions’ physical and physicochemical characteristics helps to
understand the fate and transport of TRWPs. The fate and transport of TRWP are related to the tire
particle's size and density from environmental samples, providing the connection of TRWP
pollution to urban stream degradation. The literature review then turns toward the effects of stream
urbanization in relation to traffic and motor vehicle pollution, specifically tires. The review also
covers the production, fate, and transport of 6PPD and its relation to Urban Runoff Mortality
Syndrome in urban streams and Coho Mortality Syndrome. Once the relationship between the
URMS and TRWP pollution is established, the literature review covers salmon habitat connectivity
6
and the issues of TRWP pollution in reopened urban streams to sensitive species. Finally, the
review covers some of Washington’s most notable tire pollution-related events, followed by the
best green infrastructure mitigation options for lessening TRWP pollution in urban streams.
7
Environmental regulations to monitor chemicals exposures of point and non-point
toxins
Environmental regulation and policy provide consumer and environmental protections
from manufacturers who readily pollute the air and water during production. Environmental
regulations are the number one way we fund the monitoring and mitigation of environmental
pollution created by manufacturing and product uses. Therefore, it plays an essential role in
addressing tire rubber pollution to the environment. In our modern chemical-laden world, many
environmental toxins contribute to increased rates of communities diagnosed with neurological
illnesses and metabolic disorders (MacKendricks, 2018).
The Toxic Chemical Act of 1974 and the Clean Water Act of 1972 implore federal, state,
and counties to act on the protection of our communities from point and non-point toxic chemical
pollution that adversely impacts human health and the environment (IDEM, 2022). The Clean
Water Act introduced the National Pollution Discharge System (NPDES) program, which enables
the government to reduce the allowed discharges of point source pollutants into water systems
(IDEM, 2022). However, the success of the NPDES is limited in its effects on mitigating point
source pollution, whereas non-point source pollutants are still a significant issue in aquatic
management (IDEM, 2022). "Washington State's water quality standards for toxic substances code
WAC 173-201A-040[5] is defined by human health-based water quality criteria which references
40 CFR 131.36, also known as the National Toxics Rule" (DOE, 2022). These codes of regulation
and restriction only apply to a set list of toxins designated as dangerous to human health and have
a defined action plan to address chemical contamination. Tire wear particles are co-contaminant
carriers and creators of harmful, persistent organic chemicals such as PAHs, benzene-thiols, and
heavy metals listed in the chemical action plan (DOE, 2022). The Pollution Prevention for Our
8
Future Act SSB 5135 was signed into law by Governor Jay Inslee on May 8, 2019 (Toxic-Free
Future, 2019). This bill furthered Washington's progressive leadership in Environmental and
Public Health preventative action against environmental injustice.
The investigation and prevention of toxic chemical exposure by the Washington State
Department of Ecology (DOE) is through SSB 5135. DOE developed a Chemical Action Plan
(CAP), which identifies, and phases out toxic chemicals used in consumer products (DOE, 2022).
The law requires the agency to act on and seek out products containing any of the five classes of
chemicals listed as a priority. According to SSB5135, a new group of chemicals will undergo this
investigation every five years. Persistent toxic chemicals are compounds that do not readily break
down, or they have even more poisonous metabolites which cause harm to the environment
(Stohler, 2019). The policy contains five classes of toxic chemicals: PAHs, organohalogens, flame
retardants, biphenyls, and PCBs (Stohler, 2019). Policy SSB5135 allows state agencies to ban
chemicals and requires the disclosure of chemicals used in consumer product production. In the
last fifty years, over 80,000 new synthetic chemicals have been invented and used in consumer
and commercial products (Stohler, 2019). Tires are among this group of consumer products. Tire
production has increased, and the need for chemical additives to meet consumer needs and safety
regulations set by federal and state governments has also increased (UTMS, 2022). The list of
chemicals for Washington’s chemical action plan may be expanded to include 6PPD. This tire
rubber additive stabilizes rubber to increase its safety and performance. Tires additive 6PPD
undergo an environmental transformation, creating a highly toxic byproduct, 6PDD-quinone,
which presents eco-toxicity and human health risks.
9
History of Tire Production and Research
The 1950s began the global production of thermoplastics industrial manufacturing, and by
2013 approximately 322 million tons/year of thermoplastics had been produced (Kole et al., 2017).
In 2016 the global market sold 12.3 million tons of natural rubber and 14.6 million tons of synthetic
rubber (Kole et al., 2017). Tire rubber is considered one of two types of thermoplastics due to its
elastomer content that enables the tire to withstand temperature and pressure fluctuations (Kole et
al., 2017). Unfortunately, tire rubber is produced annually at rates the environment cannot support.
Establishing the abundance and prevalence of environmental toxins like 6PPD-quinone, a
chemical byproduct of 6PPD, a tire rubber additive, is essential in addressing the health of urban
streams important for salmon life cycles. TRWP impacts urban streams and damages Washington's
communities' health, economy, and culture. By constantly expanding our urban boundaries, we
have poisoned the water, resulting in diminished vital habitat for culturally and economically
important resources like Coho Salmon (O. Kisutch) (Scholz et al., 2011).
The Michelin Brothers created the first inflatable tire in 1895, and in the 1950s, the radial
tire used today was produced (Halle et al., 2020). As innovations in tire production increase, so
does the available research on tire rubber. In 1966 the first research paper on tire rubber particles
and dust was published (Halle et al., 2020). In the 1970s, tire rubber research began to characterize
tire rubber emissions on toxicity, size of particles, and abrasion patterns (Johannesen et al., 2021;
Halle et al., 2020). In the 1990s, studies of TRWP turned toward the environmental impacts of
tire pollution and began categorizing the chemical hazards from roadway runoff to water systems
(Johannesen et al., 2021; Halle et al., 2020). Finally, in the early 2000s, environmental scientists
began identifying and quantifying specific tire rubber wear chemical groups and heavy metals like
polycyclic aromatic hydrocarbons, benzothiazoles, and Zinc (Zn) (Johannesen et al., 2021; Halle
10
et al., 2020). Each decade in tire production has raised questions and concerns about the
environmental pollution factors of tires’ physical and chemical waste and how it may compromise
human health.
In 2016 public concerns over the use of recycled tire crumb infill used for sports fields
enacted a federal response from the Environmental Protection Agency (EPA), Centers for Disease
Control (CDC), and the US Consumer Product Safety Commission (CPSC) (EPA, 2019). These
agencies developed an action plan for the current research status and knowledge gaps of synthetic
turfs’ human health and environmental hazards. The technical report generated from the federal
review noted that TRWP chemical leachates and heavy metals have multiple human exposure
pathways, such as digestion, inhalation, and dermal contact, which may adversely impact
developing bodies (EPA, 2019). As of 2020, we now recognize tire rubber as the second most
prominent source of global microplastics that leach chemicals and heavy metals into the
environment (Lou et al., 2021). In addition, tire tread wear in the early 2000s began to surface as
a concern in heavy-traffic urban areas due to its presence in particulate matter (Kreider et al.,
2010). Measuring urban air particulates has been linked to many developmental, reproductive, and
increased cancer rates due to inhalation of poor air quality (Kreider et al., 2010).
Tires represent a standard technology that has been innovated to provide consumers with
better performance measures and safety for drivers. Post-WWII represents this modernization era
where consumer goods became the new gold rush (MacKendricks, 2018). In such a competitive
manufacturer and consumer world, tire manufacturers aim to provide their consumers with safe,
high-performing products. For example, using 6PPD as an antioxidant helps lessen thermal
degradation and protects tires in constant and dynamic conditions (USTMA, 2020). The National
Highway Traffic Safety Administration regulates new tire manufacturing by the Motor Vehicle
11
Safety Act, 49 USC §§ 30103-30105 et seq. (NHTSA, 2008). Since discovering 6PPD-quinone’s
toxic effects on Coho salmon, tire manufacturers have responded to governmental inquiries
regarding using and replacing 6PPD in tires. However, the US Tire Manufacturers Association
affirmed that without 6PPD, tires would not meet the Motor Vehicle Safety Act standards
(USTMA, 2020), preventing phasing 6PPD-quinone out of use in tire manufacturing.
Vehicle and tire production are essential indications of the total sum of tire rubber
microplastics that enter the environment (Wagner et al., 2018; Lou et al., 2021). The global
production of tires increased from 4.08 million in 2010 to 17.85 million in 2019 (Lou et al., 2021).
As global vehicle production grows, so does the amount of tire wear particles, as shown in Figure
1, Box A (Lou, 2021). Figure 1, Box B indicates that the US has the highest production of tire
wear microplastics emitted to the environment. One reason for this is related to the immensity of
the United States transportation network, which contains many roads/lengths of roads, equating to
high levels of tire wear products (Lou et al., 2021). Expanding our
transportation network increases the production of TWP, which raises its toxicological effects in
urban and rural streams via urban expansion.
12
Figure 1.
Global tire production with the global tire wear production.
Note: a) Global production and consumption of TWPs from 2010-2019. (b) shows TWP
production for several countries (Lou et al., 2022).
13
Tire Rubber Physical and Physicochemical Characteristics
Tires manufacturing comes with various materials and layers with different compositions.
Manufacturing tires involves mixing synthetic and natural elements under high heat and pressure,
then rolling them into long sheets of rubber (EPA, 2019). The seven significant parts of tires
include the outermost layer of tread, the sidewalls covering the body piles, the inflatable inner
liner, steel belts, bead filler, and beads (US tire.org 2022; EPA, 2019). Tire production depends on
natural resources to produce the yearly global demand for new tires. Therefore, synthetic rubber
and elastomers are used alternatively to natural rubber in tire production (EPA, 2019). Tires
composition includes "supportive filler materials; curatives like vulcanizing agents, activators,
accelerators, antioxidants, antiozonants, inhibitors, and retarders; extender oils and softeners;
phenolic resins and plasticizers; metal wire; polyester or nylon fabrics; and bonding agents" (EPA,
2019). The tire assembly occurs in layers of rubber and coated materials, and a steal belt is fastened
to the tire tread, after which the tire is cured at temperatures ranging from 150 º F to 180º F to
crosslink the tires' polymer chains (EPA, 2019). The composition of tires
Figure 2
Tire Composition
14
Note: Tire Composition for passenger cars and trucks (US tires.org., 2022).
depends on the type of vehicle, as shown in Figure 2; note that there is a higher percentage of
antioxidants in passenger cars than in trucks (US tires.org, 2022). These chemical additives
increase the tire's performance, and the tire lifespan of passenger cars is higher than trucks.
Today, tires are chemically enhanced by innovative technology to provide consumers with
top-quality safety and performance measures. Table 1 provides an overview of the physiochemical
properties of tire rubber to understand how TRWP impacts the environment. A thorough review
of the physicochemical properties is vital when calculating the environmental fate of TRWP
chemical leachates.
Table 1
Physiochemical properties of tire rubber microplastics
Parameter
Materials
Description
“Polybutadiene, styrene-butadiene,
chloroprene-isoprene, polysulfide, carbon
black, and silica”
Additives
“Mineral oils, thiazole, organic peroxides,
nitro compounds, halogenated cyanoacrylate
alkanes, amine, phenol, calcium oxide,
aromatic and aliphatic esters, peptizer, zinc
oxide, ZnO, sulfur, selenium, tellurium” and
6PPD.
Fluoranthene, Chrysene, anthracene,
Environmental polycyclic-aromatics, benzothiazole, 1Tire Pollutants indanone, aluminum, 1-octanethiol,
found in
phenanthrene, anthracene, and aluminum,
Stormwater
Zinc, Copper, Cadmium,
6PPD, DPG, and HMMM.
and roadway
runoff
15
Reference
Lou 2021; EPA 2019;
Guzman 2013
Lou 2021; Gonzalez 2012;
Johannessen 2022
Muller 2020Halle 2021;
Johannessen 2022; Lou
2021; Järlskog 2022
Tire Rubber Microplastics Sources to the Environment
Tire rubber microplastics enter the environment via its primary source of tire wear particles
discharged to aquatic ecosystems via stormwater and roadway runoff. The secondary source of tire
rubber microplastics to the environment comes from recycled tire crumb and tire repair-polished
debris. Figure 3 displays the potential sources of primary and secondary tire rubber microplastic
sources and their environmental fate once discharged. The recycled sources of tires are depicted
in the top portion, and the primary ones are displayed at the bottom of
Figure 3.
Sources of Tire Microplastics to the Environment.
Note: illustrates TRWP sources, transport, and environmental sinks. Tire wear particles (TWP),
recycled tire crumb (RTC), and tire repair-polished debris (TRD) (Lou et al., 2021).
Recycled tire sources manufacture tire crumb rubber for reuse as mulch, playground
materials, and synthetic turf for sports fields (EPA, 2019). The two methods used to make tire
crumb rubber are ambient and cryogenic (EPA, 2019). These processes create tire rubber particle
16
sizes that range between 0.84 to 2.0 mm (EPA, 2019). In addition, the tire crumb rubber
decomposes into particles with consistent measurements, whereas primary sources like tire road
wear particles have a wide variety of particle sizes (Lou et al., 2021). For example, tire wear
particles size ranges from a few nanometers to 265μm (Lou et al., 2021; Wagner et al., 2018).
Recycled tire rubber crumb particles have consistent size ranges manufactured for specific
purposes. Recycled tire rubber particles in the environment range from ≤0.063 to >4.75 mm (EPA,
2019). The smaller the tire rubber particle increases the surface area for leaching and absorption
reactions, which is why the infill poses high human health concerns (Pennell et al., 2002).
According to the EPA, as of 2019, there were 12,000-13,000 synthetic turf fields in the US and
estimated that 12,00 to 1,500 new turf installations occur yearly. (EPA.org 2019). Recycled tire
rubber pollution is an overlooked source of microplastics that continuously leach toxic organic
chemicals and heavy metals into the air, soil, and water (Halle et al., 2020). Tire rubber crumb
infill used in artificial turf is a crucial source of micronized tire rubber microplastics to the
environment (Redondo-Hasselerharm et al., 2018). With that said, recycled tire crumb is a steady
source of microplastic-sized tires to the environment.
Primary sources of Tire Rubber microplastics in the environment
The friction between the tire rubber and the road creates tire road wear particles (TRWP)
and is the primary source of tire rubber microplastics in the environment (Mennekes et al., 2022;
Lou et al., 2021; Wagner et al., 2018). TRWP concentrations accumulate on roadways and are
dispersed to streams via stormwater runoff (Hiki et al., 2022). As we increase our global demand
for personal vehicle use, the need for new tire production rises, thereby increasing the issue of
TRWP accumulation in roadway dust (Lou et al., 2021). The number of roadways can indicate the
amount of TRWP deposited to nearby streams based on roadway dust accumulation since over
17
40% of TRWP deposited to roadways end up in surface waters (Hiki et al., 2022). The global
production of TRWP annual release is estimated to be 6.15 × 106 tons per year using a world
population of 7.59 billion (Mennekes et al., 2022; Lou et al., 2021; Wagner et al., 2018). The
United States has the highest TRWP emissions per capita, estimated at 4.70 x 106 tons per year,
due to our extensive transportation network with the most vehicles (Lou et al., 2021; Kole et al.,
2017). Tire wear particles' environmental pathways are heavily dependent on localized factors
such as road types and what types of regional stormwater infrastructure are in place. (Kole et al.,
2017).
Tire Road wear particle (TRWP) transport, fate, and flow in the environment
The road environment is full of particles deposited and suspended by traffic, forming
heterogeneous road dust (Järlskog et al., 2022). Non-exhaust emissions are qualified by the wear
of the vehicle’s tires, brake pads, motor fluids, and road surfaces as a significant source of roadway
pollution (Klockner et al., 2020; Järlskog et al., 2022). These non-exhaust emissions contribute 35
- 55% of total trafficking emissions (Klockner et al., 2020). Tire road wear particles (TRWP)
account for up to 30% of these non-exhaust emissions (Klockner et al., 2020). TRWP are often
hetero-agglomerates made-up of tire particles and other roadway particles such as minerals,
chemical pollutants, and heavy metals (Klockner et al., 2020). The fate and transport of TRWP
depend on the size of the particle. For example, the fine fraction TRWPs are easily suspended in
the ambient air where inhalation poses human health hazards (Wagner et al., 2018). On the other
hand, the coarse fraction of TRWP on the roadway is transported to nearby soils and surface waters
(Wagner et al., 2018). The coarse fraction of TRWP poses eco-toxicity and human health
challenges that are magnified through the food chain. Figure 5 shows the fate and transport of
TRWP; once released to the roadways, approximately 90-95% of TRWP released to the road will
18
be transported to nearby soils, streams, and water treatment facilities (Wagner et al., 2018;
Baensch-Baltruschat et al., 2020).
Figure 4.
Fate and Transport of TRWP in the Environment
Note: illustrates the transport and fate of TRWP once emitted into the environment (Wagner 2018).
A study from Germany found that approximately 40% of TRWP are considered coarse
particles that are transported from the roadway to nearby soils and streams (Järlskog et al., 2022).
Furthermore, road dust total particle mass analysis has shown that samples collected adjacent to
curbs contain up to 400 – 1240 g /m2 (Järlskog et al., 2022). Whereas industrial sites in Korea
found areas with high traffic volumes reported TRWP in road dust to be 334-1669 g/m2 (Jeong et
al., 2020; Järlskog et al., 2022). These sums are immense and require further investigation into the
best way to monitor and measure TRWP pollution in the environment.
The size of the TRWP determines the transport. The smaller, finer particulate are more
readily transported since they can be suspended in the air and remain suspended in surface waters
19
(Wagner et al., 2018). In contrast, the coarse larger TRWP are not transported as far and settle in
soils and sediments where they degrade and release chemical leachates into their surrounding
environment (Wagner et al., 2018). Road dust is a mixture of TRWP sizes with finer TRWP than
roadside soils (Järlskog et al., 2022). Therefore, it is safe to say that TRWP found in road dust will
have higher rates of co-contaminants due to TRWP’s mixed composition of fine and coarse
particles that undergo environmental reactions.
How TRWPs are Estimated
Almost all studies that estimate TRWP in the environment come from lab-simulated tests
of tire wear (Mennekes et al., 2022). There are few real-world estimates of TRWP from the
environment, so the current estimates are likely underestimated. Current studies on global
estimates used a probability material flow analysis of tire rubber microplastic emissions to
calculate tire wear particle emissions; this is done by multiplying the emissions factor of TWP in
mg by the vehicle kilometers traveled (Lou et al., 2021; Verschoor et al., 2016). Traffic-related
factors control the total amount of pollutants on the road. These traffic-related factors include
transportation network structure, velocity, and the number of vehicles on the roadway (Markiewicz
et al., 2017). The method used to estimate TWPs production goes as follows: (E) is E = Emissions
Factor × Vehicle Km Travelled × number of the vehicle. EF assumes 100 mg/vehicle-km (Lou et
al., 2021; Kole et al., 2017). The above equation can be adapted to estimate TWP emissions on a
more regional scale to estimate the TWP emissions to stormwater. Other methods that identify tire
wear microplastics include visual identification and a confirmation of the sample using gas
chromatography-mass
spectrometry
(GC-MS)
analysis
looking
for
N-cyclohexyl-2-
benzothiazolamine (NCBA) (Knight et al., 2020). This method is reliable in identifying the
emissions of tire rubber microplastics to the environment. However, visual identification is
20
essential in TRWP identification because the size of the tire rubber microplastics will determine
the leaching capabilities of its known chemical contaminants (Knight et al., 2020).
Tire Road wear particle (TRWP) sizes and densities in environmental samples
TRWP particle size and distribution research has examined TRWP in environmental
samples such as air, soil, sediment, water, and road dust (Baensch-Baltruschat et al., 2022).
However, quantitative analysis of TRWP particle sizes and densities from ecological samples is
still being determined (Wagner et al., 2018; Klockner et al., 2020). The primary quantitative
analysis for tire wear contents in environmental samples estimates specific tire chemical or
elemental markers (Klockner et al., 2020). These tire makers are degradation products of rubber
polymers or elemental markers like zinc; other tire makers are organic constituents from chemical
additives in tire production (Klockner et al., 2020). TRWP elastomer and chemical leachate
contents provide reliable measurements for TRWP environmental release factors of cryogrind tire
leachates. Wagner et al. (2018) compared three size ranges of TRWP to investigate tire wear
particles aging of tire tread wear particles of milled tire particles, TRWPs from a road simulator,
and road dust samples. Wagner et al. (2018) findings are listed in Table 3 below and detail the
differences and commonalities in size, shape, density, and composition found among the different
sample types from milled tire particles, road simulators, and road dust TRWPs.
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Table 2.
Differences between milled TWP and TRWPs were generated from road simulation studies
(Wagner et al., 2018).
Tire Rubber Particle
Density fraction
Composition
Cryogenically
milled Tire Particles
were Micro-sized
and irregular shapes
1.2 g/cm3
“No inorganic particle constituents from road dust, no
asphalt components, 100% tire (tread) rubber.
Chemical composition will depend on the age of the tires
used for milling. Only tire tread should be milled”
(Wagner et al., 2018).
“TRWPs contain inorganic particles from the road or
road dust and organic TPs (tire tread only particles). The
varying ratio of tire tread rubber and mineral particles is
50–80% tire tread rubber and 20–50% mineral dust from
road surface). Contains transformation products formed
by tire aging.” (Wagner et al., 2018)
Simulator TRWP’s
1.3-1.9 g/cm3
and Environmental
samples TRWP were
Nano-sized to
Micro-sized and
elongated, with
some irregular
shapes
TRWP found in Soils
Thomas et al. (2023) examined soil samples near the roadside and found TRWP size ranges
of 5 mm to less than 1 mm in Ohio and Kansas. They estimated TRWP content to range between
800 to 1300 µg/g in Ohio; in Kansas, the TRWP content was estimated between 1,200 to 3,100
µg/g (Thomas et al., 2023). For the size range less than 1 mm, researchers found that in both Ohio
and Kansas roadside soil samples, TRWP weight percentage ranges from 0.15 to 2.1%, and TRWP
content is determined to depend on the locations and traffic (Thomas et al., 2023). Therefore, the
presence and abundance of TRWP are dependent on traffic and urbanization rates as they indicate
road use. Once TRWPs are transported to the soil, they degrade and leach out heavy metals and
other co-contaminants from the roadway (Wagner et al., 2018). Therefore, soils tend to have larger
TRWP than other environmental media, such as dust and water samples.
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TRWP found in Road Dust
Road dust samples for TRWP and their chemical leachates have been more recently
investigated due to 6PPD-quinone. Road dust comprises a variety of organic and non-organic
particulates and is generally heterogenic in particle size composition (Baensch-Baltruschat et al.,
2020). Kreider et al. (2010) assessed the interactions of car tires on concrete pavement, where they
found a range of TRWP sizes between 4 and 350 µm, with most of the particles ranging from 5
µm and 25 µm. Aatmeeyata et al. (2009) studied air samples and found TRWP sizes that range ≥
10 µm. Dahl (2010) used a road simulator to collect tire wear and tear from the wheel well and
found particle sizes that ranged from 15 to 700 nanometers. Dahls et al. (2010) TRWP had a mean
particle size diameter of 75 nm. Matthiessen et al. (2011) measured airborne particle
concentrations from the vehicle's wheel well and found size ranges of 6 to 562 nanometers while
driving. The same study highlighted that breaking produced a variation in the tire where particle
emission sizes were between 30 and 60 nanometers (Matthiessen et al., 2011). The common theme
in these studies' collection and particle sizes is that each collection method was by air sampling on
and near tires on the roadway. Other studies analyzed TRWP from road dust samples and found
that these particles on the roadway tend to be larger than the portion located in air samples and
sediment samples (Klockner et al., 2020). Since dust can be collected into clumps off the roadway
and dispersed into the air, TRWPs found in road dust pose a significant threat to human health and
requires further research to understand its damages fully.
TRWP found in sediments and surface water
Water and sediment samples of TRWP are much scarcer in the literature. A few studies
looked at TRWP when researching microplastic inputs to the environment. For example,
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Charleston Estuary, South Carolina, USA study found TRWP in subtidal and intertidal sediments
and documented that TRWPs were abundant and widely distributed among sediment samples
(Leads et al., 2019). The subtidal and intertidal samples contained TRWP size ranges between
63-149 μm (198 particles), 150-499 μm (94 particles), and size fraction ≥ 500 μm (19 particles)
(Leads et al. 2019). The study highlights the fate of larger TRWP transported from roadways into
marine and freshwater ecosystems that will end up in sediments. A review of urbanization
drivers that directly impact the amount of TRWP entering urban streams is needed to understand
how to mitigate the mounting issue of TRWP pollution in urban streams.
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Urbanization of Streams: Connecting TRWP as a Physical and Non-point Source
of Pollution to Urban Streams
Urbanization introduces diverse alterations in watersheds and presents many disruptions to
streams' chemical, physical, and biological processes, negatively affecting stream biota (Booth et
al., 2005). The mechanism that enacts these disruptions is replacing porous natural ground
coverage with impervious surfaces like roads and buildings (Booth et al., 2005). The change of
water inputs from stormwater and roadway runoff changes the velocity and abundance of water
entering urban streams, resulting in stream channel erosion and degraded water quality (Booth et
al., 2005). Point and non-point source contaminants impact urban streams' biodiversity, habitat
function, and water quality (Booth et al., 2005; DOE, 2022). Tire rubber microplastics are a source
of physical waste and non-point pollutants with known toxicological impacts on aquatic
ecosystems (Muller et al., 2020). TRWPs carry co-contaminants of persistent organic pollutants
like PAHs and benzothiazole derivatives, as well as heavy metals like zinc (Zn) and copper (Cu)
(Klockner et al., 2021; Wagner et al., 2018). TRWPs also undergo photochemical degradation and
environmental reactions with ground-level ozone, which emits harmful chemicals, leachates, and
heavy metals into the air, soil, and water (Lou et al., 2021; Johannesen et al., 2022; Muller et al.,
2020). One of these reactions occurs with tire additive 6PPD which pushes to the surface of the
TRWP and reacts with ground-level ozone to create acutely toxic N-(1,3-dimethyl butyl)-N′phenyl-p-phenylenediamine-quinone (6PPD-quinone), the casual toxicant for urban mortality
syndrome observed in Coho salmon since the 1990s (Tian et al., 2021). TRWP emissions are a
mounting problem that affects the health of our communities and aquatic environments.
Tire Road Wear Particles (TWRP's) toxicological effects are not limited to the
environmental production of 6PPD-quinone. Instead, TRWP leaches and absorbs various
25
persistent organic pollutants and heavy metals (Johannesen et al., 2021; Järlskog et al., 2022).
TRWPs function as co-contaminant carriers of chemical toxins and biotic (pathogens, i.e., viruses
and bacteria) from terrestrial environments to aquatic ecosystems (Lou et al., 2021). The transport
of TRWPs into streams poses a significant threat to stream ecosystems because of co-contaminants
carried from the roadway and stormwater runoff. The risks of URMS exponentially increase to
90% mortality for Coho salmon as the percentage of impervious land coverage reaches above 40%
(Fiest et al., 2018). The expansion of the transportation network directly increases the inputs of
motor vehicle pollutants like tire wear particles from roadways to streams where it toxifies urban
streams (Fiest et al., 2018; Wagner et al., 2020). In Thurston County, WA, the population is
expected to increase by 100,000 people by 2040 (TRCP, 2022). This means Thurston County has
to make considerable efforts to ensure its stream ecosystems are not overrun by roadway runoff
and motor vehicle pollution.
Urbanization drivers like stormwater runoff create stressors on stream ecosystems by
altering environmental features that trigger a biological response such as URMS. Roadway and
stormwater runoff is the main route of tire road wear particles (TRWP) transport to urban streams
and surface waters (Wagner et al., 2018; Lou et al., 2021). The prevalence and abundance of TRWP
emissions in urban streams make it necessary to measure TRWP emissions. To evaluate whether
there was variation in the toxicity of TRWP, Halle et al. (2021) studied the effects of new versus
old tires on the Hyalella azteca amphipod crustacean species. The result of the study determined
that newer tires had higher toxicity effects than older tire particulates (Halle et al., 2021). The lethal
impact of new tires on Hyalella azteca was estimated to be LC50 to be 364 ± 64 particles (0.19 ±
0.03 g L− 1) (Halle et al., 2021). Aged tire wear particles’ lethal concentration was estimated at
3073 ± 211 particles (0.91 ± 0.06 g L− 1) (Halle et al., 2021). These study results provide a few
26
insights; first, some species will take up TRWP as food; second, these animals can only take up so
many particles before a lethal response; and third, TRWP degrades, and they have the potential to
become less toxic. Tires becoming less toxic over time is not true of TRWP found in road dust and
other environmental samples. For example, Järlskog et al. (2022) determined that road dust TRWP
are highly absorbent polymers laden with roadway pollutants washed out into streams via runoff.
6PPD-quinones Environmental Production from TRWP
Identifying regions with increased TRWP inputs can highlight areas that require
stormwater mitigation efforts to alleviate the toxic effects of 6PPD-quinone in urban streams. The
spatial distribution of TRWP can indicate the potential abundance of 6PPD-quinone in the
environment (DOE, 2022). The flow of TRWP once emitted to road environments has shown that
TRWP is found in measurable quantities in air, soil, road dust, and surface water samples (Wagner
et al., 2018). The chemical additives' reaction mechanism goes as follows: 6PPD within the tire
particle pushes to the surface, reacting with ground-level ozone producing a byproduct called
6PDD-quinone (Tian et al., 2022). This reaction occurs at the interface of the particle surface due
to 6PPD's chemical behavior, so over time, as the particle ages, its concentration of 6PPD
decreases simultaneously as the particle increases 6PPD-quinone concentrations (Järlskog et al.,
2022). Tire particles readily absorb toxins and minerals while on the roadway, where they become
co-contaminate carriers.
The mechanism of the 6PPD surface reactively suggests that as the TRWP becomes
smaller, it increases the available surface area for 6PPD to react and generate 6PPD-quinone.
Studies also show that 6PDD-quinone can accumulate in tire rubber particles, enhancing tire
rubber microplastic toxicity as it ages (French et al., 2022; DOE, 2022). This toxicity increase
means that as tire rubber on roadways accumulates, it can become saturated in 6PPD-quinone
27
because 6PPD-quinone sorbs to particles as it transfers through the environment. As TWP
accumulates along the roadside, the particles become loaded with 6PPD-quinone and, during
storm events, are washed into urban streams (DOE, 2022; French et al., 2022; Peter et al., 2022).
Point and Non-point Pollution to Urban Streams
Urban streams are subject to both point and non-point pollutants that change the water
quality and introduce chemicals and heavy metals that negatively impact aquatic organisms. The
point source of pollution in urban streams is the direct input of contaminants to marine systems
via wastewater and stormwater effluent (Mallick et al., 2017). The same goes for tire rubber
microplastics acting as an indirect or non-point source of chemical pollutants in urban streams.
Urbanized regions tend to have higher ozone levels because of the number of vehicles and varying
land use practices (Hiki et al., 2022). In addition, road dust samples show that 6PPD-quinone
concentrations are higher on arterial roads than on roads with lower traffic volumes (Hiki et al.,
2022). Therefore, identifying all the arterial roadways and their proximity to streams may indicate
the amount of 6PPD-quinone contamination. Other factors that alter the effects of TRWP on urban
streams include rain events and snow melts, which tend to carry large amounts of tire rubber
microplastic chemical byproducts into streams (Peters et al., 2022; Johannessen et al., 2021). Road
dust has been shown to have the highest TRWP chemical byproduct levels due to photochemical
and ozone degradation (Hiki et al., 2022).
In the spring and summer, ozone levels naturally increase due to increased human activity,
and the road dust accumulating in tire rubber microplastics increases the toxicity of 6PPD-quinone
(Hiki et al., 2022). For example, Hiki et al. (2022) collected dust from arterial and residential roads
in Tokyo, Japan, between May and October and recorded the highest levels of 6PPD-quinone were
28
found between May to June, with 6PPD-quinone concentrations ranging from 35 to 47 ppb (Hiki
et al., 2022).
Environmental Fate and Transport of 6PPD-quinone
Stormwater from roadways mobilizes TRWP chemical compounds and transports them
into aquatic ecosystems (Johannesen et al., 2021). The lethal concentration of 6PPD-quinone
needed to induce acute coho mortality response is 0.8 +- 0.16 µg/L (Tian et al., 2020). Stormwater
runoff is a vector of toxic organic chemicals from TRWP into urban receiving waters.
Environmental concentrations were estimated retrospectively and show that 6PPD-quinone is
widespread, ranging from 0.3 to 19 µg/L (Tian et al., 2020). The widespread occurrence of 6PPDquinone makes it toxicologically relevant and needs further investigation in quantifying surface
water concentrations. The difficulty of establishing a lethal concentration for 6PPD-quinone comes
from the fact that it is not commercially produced but made through the oxidation of a stabilizing
rubber additive (Tian et al., 2020; Johannesen et al., 2021). Each year Pacific salmon journey to
their natal streams; they travel through diverse waters impacted by various land practices and
stormwater/roadway runoff inputs (French et al., 2022). Monitoring the environmental levels of
6PPD-quinone in Coho transit streams can provide vital parameters for the population modeling
of Coho salmon.
Susceptible Species to Urban Runoff Mortality Syndrome
There is a stark difference in the susceptibility of Coho salmon relative to other salmon
species for both 6PPD-quinone and stormwater/ roadway runoff. For example, a study tested the
lethal tolerance of a commercial 6PPD-quinone on Coho and Chum salmon; the finding showed
that Chum went unaffected, whereas the Coho salmon exhibited 100% mortality within 4-6 hours
29
after exposure (McIntyre et al., 2021). Another study tested the susceptibility from collected
stormwater runoff on four salmon species: Coho, Steelhead, Chinook, and Sockeye. The study
found that stormwater/roadway runoff brings a caveat of toxins into freshwater streams and rivers
that have acute mortality effects on Coho salmon (O. kisutch) and steelhead trout (O. mykiss)
(French et al., 2022). Coho and steelhead trout show unique sensitivity to stormwater and roadway
runoff; however, Coho showed the highest sensitivity displaying mortality with a 5% stormwater
concentration exposure limit (French et al., 2022). Since tire rubber is the source of the 6PPD, the
parent product of 6PPD-quinone, it is crucial to account for all tire rubber-related sources to gain
insight into the prevalence and abundance of TRWP toxic effects on the environment.
TRWP can impact the food web through trophic interactions of tire particles and
bioaccumulative chemical leachates (Ji et al., 2022). Both 6PPD and 6PPD-quinone have a
bioaccumulative capacity and were found in the tissues of two fish species, the snakehead weever
and the Spanish mackerel (Ji et al., 2022). Other research has shown that some benthic organisms
will indiscriminately ingest TRWP, such as Gammarus pulex (Redondo-Hasselerharm et al.,
2018). The estimated trophic transfers of tire particles for G. pulex were 4.7 x 10-9 for
G. pulex (Redondo-Hasselerharm et al., 2018). This transfer implies that tire rubber may damage
the food web and negatively affect human health. Furthermore, the ability of 6PPD-quinone to be
retained in fish tissues means it accumulates and poses a risk for biomagnification (Ji et al., 2022).
More research is needed to determine the 6PPD-quinones biomagnification factor to understand
its environmental fate truly.
30
Human exposure to 6PPD-quinone
The risk of 6PDD-quinone is not limited to the issues of trophic interactions and URMS
but also has implications for human health. For example, the main human pathway of exposure to
6PPD-quinone occurs via inhalation and dermal exposure (Zhang et al., 2022). As we know, 6PPD
readily reacts with ozone to create 6PPD-quinone, which means people who live, work, and eat
near regions of heavy vehicle use are at risk for inhalation exposure to 6PPD-quinone. At the same
time, those who work in tire manufacturing, tire shops, and tire recycling centers are also at risk
for exposure. Research shows that 6PPD-quinone tends to accumulate in coarse tire rubber
particles, with the highest reported concentrations of 6PPD-quinone at 7.78-23.2 pg. m-3 (Zhang,
2022). The highest concentrations of 6PPD-quinone were found in areas with heavy vehicle or tire
rubber-related locations (Zhang et al., 2022).
Human health exposures via inhalation have shown various distributions of 6PPD-quinone
within the airway systems (Zhang et al., 2022). For example, the headspace airways have been
shown to absorb approximately 89-91%; whereas the tracheobronchial absorbs approximately 3.2
-3.8%; and the Pulmonary alveoli has shown to take in up to 6.0- 6.9% (Zhang et al., 2022). More
research is needed to understand the toxicological effects of 6PPD-quinone on humans fully.
However, the human body takes up 6PPD-quinone via particle-bound inhalation, which has been
traced through the airway systems; and has also been found to have trophic interactions. This
means that 6PPD-quinone impacts may be more significant than expected since human pathways
are mainly through touch, inhalation, and ingestion. This increases the need to mitigate TRWP
from roadways before they are transported to streams and devise safety measures for workers in
tire manufacturing, tire shops, and recycling centers. Very little is known about the consequences
of human exposure, and more research is needed to understand how these multiple human
31
pathways for exposure can limit human health. Since tire chemicals like 6PPD have been
manufactured at grand scales, evaluating the potentially significant environmental and
occupational injustices to those working in tire manufacturing and recycling centers is necessary.
The environmental injustices of 6PPD-quinones impacts are not limited to workers but also tribes
who depend on the salmon for their livelihood and cultural customs.
Importance of Salmon and Landscape Connectivity
Environmental injustice takes many forms. For example, salmon in Washington State is
vital to the Native American tribes, their customs, and their culture. Unfortunately, over the last
century, the salmon in Washington have faced unprecedented losses to their population from
overfishing, barring of natal stream habitats, and chemical contamination of our fresh and saltwater
ecosystems (Scholz et al., 2011). Washington has begun to prioritize salmon habitat rehabilitation
to address these injustices and to provide reparations to the tribes of Washington. For instance, the
fish passage barriers removal project in Washington works to remove/replace culverts and bridges
that cut salmon off from the natal streams (State of Salmon, 2022). This project hopes to provide
more spawning and rearing habitat for salmon. The project was asserted in 2007 the U.S. District
Court tasked Washington State to order State to begin remediating the issue of fish passage barriers
throughout Puget Sound; this was to provide salmon with their historic spawning grounds and
safeguard the treaties made with the Native Americans at the time of Washington's statehood was
established (U.S vs. WA 2013). Washington State and the Native American Tribes of Washington
have long recognized that the rate of urbanization in Puget Sound has led to salmon habitat
fragmentation (State of Salmon, 2022).
However, despite this acknowledgment, Washington State needed to do more to address
the issue of fish passage barriers. This led the tribes to take further legal action with the United
32
States Superior Court, where they appealed the 2007 decision in 2013 for the timely removal of
fish passage barriers (U.S. vs. WA 2013). First, Washington State needed to fully consider the
urgency of saving salmon habitat due to limited funding for the project (U.S. vs. WA 2013). In the
cross-motion judgment, the language references "the right to fish in accustom places" secured by
Steven's treaties as a reason to grant the tribe's appeal for the timely removal of fish passage
barriers (U.S. vs. WA 2013). Washington State and its residents are responsible for installing
culvert/fish passage barriers. They, therefore, are responsible for providing mitigation and
reparations to tribes for severing their native food supply of salmon. The United States vs. the State
of Washington culvert injunction order required the State to assess, prioritize, and correct up to
90% of culverts/barriers within the injunction site by 2030 (U.S. vs. WA 2013). The injunction
order is unique in that it requires habitat management as a way of complying with treaties and will
likely function as a foundation for future court decisions. These two judgments are necessary for
the State to allocate funds to connectivity projects, evidenced by the reluctance to fund projects
before the injunction and the ticking clock set by the federal government.
Landscape connectivity for anadromous species like salmon is essential, and the
fragmentation of its habitats results in the loss of biodiversity and decline of many species within
Puget Sound (Cederholm et al., 2000). Salmon inhabit many environments within Puget Sound,
from streams, rivers, and deltas, to the Salish Sea and the open ocean (Scholz et al., 2011). This
means salmon pass through areas with varying aquatic ecosystems with various land use practices
that negatively impact water quality. Land use management in Washington is now required to
determine if the road crossings are necessary and, if so, choose a fish passage-friendly culvert
design specific to the location (Barnard et al., 2013). With so much on the line for removing fish
33
passage barriers, adapting green infrastructure in regions with roads that have increased TRWP
inputs will provide safe urban habitats once the fish passages become available.
Tire Rubber Pollution in Washington State Aquatic Ecosystems
In Washington State, the most critical source of water pollution is stormwater and road
runoff, with less than a third of our freshwater resources deemed fit for human use (DOE, 2022).
Freshwater resources require stronger regulations of waste management practices and stormwater
mitigation if we hope to reverse the many aquatics pollution events and daily loading of TRWP to
Puget Sound and its tributaries. For example, in the 1970s, the Washington state department of
Natural Resources placed over 500,000 waste tires in Puget Sound to create artificial reef habitats
(Vargas, 2022). The project had hoped to consolidate the issues of mounting tire waste and aquatic
habitat losses by using tires to create it. Unfortunately, over the last 50 years, these tire reefs have
sprawled across the Puget Sound floor. The most recently highlighted tire chemical byproduct,
6PPD-quinone, is a chemical of emerging concern due to its widespread and poisonous products
on marine life. With the discovery of 6PPD-quinone, the SCUBA Alliance group is addressing the
failed restoration project through the location and removal of the 500,000 tires thrown in Puget
Sound (Vargas, 2022). The removal process will take time to locate and coordinate the drives to
remove the tire debris once found, but at least the issue is being addressed.
In 1990, Seattle initiated restoration projects to support Coho salmon by removing physical
barriers to known spawning habitats, resulting in the observed Coho mortality syndrome (Lohse
et al., 2008; Scholz et al., 2011). Ironically, these restored streams turned into ecological traps due
to the removal of physical barriers that had prohibited Coho salmon from entering lethal urban
streams (Scholz et al., 2011). As we move towards an increasingly urbanized Puget Sound,
resource managers require new policies, processes, and technologies to mitigate the effect of
34
stormwater runoff on urban streams. It is important to remember that even when a project to
rehabilitate an ecosystem fails, many things can be learned from those failures. For example, we
now know that tires are toxic to aquatic life, so using them to build habitats is not viable. Also, if
we reopen streams for salmon in highly trafficked urban regions, there may be mortality responses.
Sometimes projects prioritize the costs and timeliness ahead of environmental concerns or
permitting. Unfortunately, these business practices often come at the price of the ecosystem and
human health. For example, in 2020, a significant tire rubber pollution event occurred on the
Puyallup River resulting in a fish kill event for 7 ESA listed species (AGO, 2022). In the summer
of 2020, the Electron Hydro dam illegally used synthetic field turf to make a temporary bypass
channel during construction, placing approximately 2,400 sq. yards of artificial turf, which had up
to sixteen to eighteen cubic yards of crumb rubber, into Puyallup River (AGO, 2022).
Subsequently, the river overtook synthetic turf and polluted up to fourteen miles downstream with
tire crumb rubber (AGO, 2022). Although the pollution event will have stretched much farther
than the fourteen miles of the river, considering the chemical contamination from the tire crumb
particulate has trophic factors, and the ability to remove tire crumb rubber from sediments is
limited. The Dam owners now face millions in fines and cleanup costs but are also at risk of jail
time for the illegal installation of synthetic turf (AGO, 2022). Each tire pollution-related event has
taken an unmeasured toll on those affected aquatic ecosystems that have far-reaching impacts on
the people who live and depend on these aquatic ecosystems.
Green Infrastructure (GI)
Although Washington State faces many challenges of tire rubber-related pollution, more
efforts are being made to detail and understand how to minimize TRWP effects on aquatic
ecosystems. The fate and transport of TRWPs determined that an estimated 21,600 tons per year
35
go to wastewater treatment and surface waters, where it poses ecotoxicological and human health
risks (Wagner et al., 2018). Research has shown that the green infrastructure approach is the best
way to mitigate TRWP pollution in streams (French et al., 2022)
Green infrastructure should be a standard for all future land use planning to mitigate TRWP
impacts on streams. GI provides an intermediate stage where stormwater is passed through to pull
out unwanted roadway contaminants before its released into the receiving waters. The health of
Puget Sound's watersheds, streams, lakes, and rivers are all at risk from TRWP, physical pollution,
and chemical degradation. Globally in urban areas, stormwater and roadway runoff are the number
one source of pollution in our freshwater and marine habitats (Wagner et al., 2018). Therefore,
innovative green infrastructure is the only possible measure to help reduce the amount of TRWP
entering urban streams through stormwater and roadway mitigation. Thurston County,
Washington, has a variety of green infrastructure methods to reduce the amount of toxic
stormwater and roadway inputs (TRCP, 2023). For example, researchers suggest catchment scale
bio-retention systems could effectively remediate motor vehicle pollutants impacts in stormwater
receiving streams (McIntyre et al., 2015).
Several types of green infrastructure are used in urban areas to prevent roadway and
stormwater runoffs and adverse effects on streams. For example, curb cuts are spaces cut into
parking lots to allow stormwater to move through a porous surface (IDEM, 2022). Porous
pavements send stormwater and roadway runoff through layers of organic materials to mitigate
toxins before entering streams such as rain gardens and retention ponds (IDEM 2022). Swales are
GI similar to rain gardens but are usually much more extensive and treat significant amounts of
stormwater (IDEM 2022). Finally, street sweeping mitigates TRWP and other road pollutants from
entering urban streams (McIntyre et al., 2015). Street sweeping and bio retentions systems are the
36
most effective GI for TRWPs and other motor vehicle pollutants in stormwater and roadway
runoff.
37
Conclusions
Tire road wear particles are a significant source of microplastics to the environment and
are a global issue that has yet to receive the attention it needs to be addressed. In Washington State,
considerable efforts have been made to save salmon, costing the residents millions in restoring and
remediating salmon habitats. However, land use practices like agriculture, timber harvesting, and
urban development contribute to the loss and degradation of salmon habitats' water quality and
negatively affect salmon life cycles (Fresh et al., 2004). These land-use changes alter the quality
and quantity of available nutrients, water, sediment, and debris entering stream habitats (Fresh et
al., 2004). Water and sediment quality directly impacts the longevity of a juvenile salmon's
progression into adulthood (Chittenden et al., 2017). As we know, tire rubber is a significant source
of chemical contamination to urban streams posing significant risks specifically to Washington's
Coho salmon populations. Each stage of a salmon's life cycle is essential in Puget Sound's trophic
food web; approximately 138 species depend on salmon's presence and abundance in Puget Sound
(Cederholm et al., 2000). The rapid urbanization of Puget Sound and its tributaries and the growing
dependence on its ecosystem services presents various environmental issues. In Puget Sound,
Chinook, Steelhead, and Coho salmon are listed on the Endangered Species Act as endangered
and threatened (State of Salmon 2022). A continuum of aquatic habitats, such as freshwater
streams/tributaries, nearshores, estuaries, and the ocean, is vital to completing salmon lifecycles
(Muir et al., 2012). In Washington state, road-crossing fish passages have increased with urban
expansion, and, with that, the rates of habitat degradation from contaminants like TRWP have led
to the loss of reliable salmon populations in Puget Sound.
To support more significant ecological goals for salmon in Washington, assessing the risk
of TRWP in the streams is crucial. Especially those reopened for salmon to ensure the project's
38
objective of providing more salmon spawning and rearing habitats are free of TRWP
contamination. TRWP alone concerns the community's general health and the environment due to
the many co-contaminate they absorb and transport. Unfortunately, in Washington, we have yet to
fully address TRWP as a source of chemical contamination in our urban streams. By identifying
regions most at risk for increased TRWP concentrations, we can adapt green infrastructure where
it is most needed and limit the toxic effect of TRWP in aquatic ecosystems.
Identifying and quantifying tire road wear particles (TRWP) can indicate urban streams at
risk for urban runoff mortality syndrome (DOE, 2022). 6PPD-quinone has one of the highest
aquatic life toxicity ratings, with a lethal concentration of 95 ng/L (Tian et al., 2021). Several
factors increase 6PPD-quinone inputs to urban streams, such as increased impervious land
coverage, tire road wear particle concentration on the roadway, snow melts, and rain/storm events
(French et al., 2022; Tian et al., 2021). Washington's Department of Ecology listed the evaluation
of TRWP distribution size and quantities from roadways as a current knowledge gap in assessing
and mitigating 6PPD-quinone (DOE, 2022). Therefore, the identification of the prevalence and
abundance of TRWP emissions sources can function as an indicator of streams at risk for 6PPDquinones toxicological effects. In addition, evaluating the spatial distribution of TRWP can
provide insight into areas that need priority stormwater and roadway runoff mitigation. Several
known factors influence TRWP emissions on roadways, such as speed limits, the topographical
design of the road, and the ratio of new to used tires driving on the road (Gehrke et al., 2020).
Therefore, understanding TRWP size and count can help researchers understand the extent of
TRWP pollution.
39
Methods
Research questions: How does tire road wear particle (TRWP) size and count vary among
road types (primary, secondary, residential, and rural) in Thurston County, Washington? The
second question: Does the annual average daily traffic (AADT) count affect TRWP’s count and
average size? For both questions, the dependent variables are TRWPs count and size (area μm 2).
The independent variables are road type for the first question and annual average daily traffic for
the second research question. The research questions seek to understand if TRWP counts and sizes
are similar among road types or dissimilar and if AADT plays a role in the distribution of TRWP
sizes and counts. Figure 5 provides the methodology overview used in the study of TRWP in
Thurston, county Washington.
Figure 5.
Overview of Methods
Studying tire road wear particles (TRWP), distribution in road dust requires an
interdisciplinary approach utilizing environmental analysis methodologies, GIS, and image
analyzing software, Image J. The above diagram shows an overview of the methods used to
evaluate the distribution of TRWP in Thurston County, Washington. To assess TRWP particle
40
sizes and distribution, road dust samples were collected from nineteen roads. The Thurston County
Planning Office website provided the other datasets, road frequency, and zoning. The distinction
of road type (primary, secondary, residential, and rural) will answer whether road type determines
TRWPs’ size and count. Four sample sites were chosen per road type to establish the distribution
of TRWP in Thurston County. The four road types are categorized as follows:
•
Primary roads are connected directly to the state’s transportation system (I-5 and
highways).
•
Secondary roads are located in commercial and urban areas which connect to primary and
residential roads.
•
Residential roads are, by definition, residential, meaning regions where people live.
•
Rural roads are classified as not urban and tend to have moderate to high speeds, with low
residential and commercial areas (WDSOT, 2022).
A GIS map of Thurston County’s zoning, road networks, road frequency, and urban regions was
developed to determine suitable sample locations for each road type. The study also used ArcGIS
survey 123 to develop field assessment surveys for the safety protocols and the sample collection,
which is elaborated on in the fieldwork section below.
Location of Study: Thurston County, WA
Thurston County, Washington, is located at the southern point of the Puget Sound. It contains six
cities/towns (Olympia, Tumwater, Lacey, Yelm, Rainier, and Tenino). According to the Thurston
County Planning Office, seventeen thousand traffic control signs and over one thousand
streetlights have stormwater drainage and retention systems (2023). The transportation network is
often considered the lifeblood of the local economy that connects Thurston County’s
approximately three-hundred-thousand residents with employment, services, and the ability to
41
transverse in and out of the county (ThurstonCountywa.gov, 2023). Figure 6 shows the map of
Thurston County, which displays the county’s boundaries, State transportation network,
urbanization, and traffic count. Four replicates of each road category were taken to represent
Thurston County’s TRWP distribution.
Figure 6.
Map of Thurston County Road Dust Sampling Sites
Note: Map of Thurston County zoning county green), state traffic network (blue lines), traffic
count (purple dots), and urbanized areas (highlighted in pink). The red dot represents the sample
locations for the study.
Road Dust Sample Collection Protocols
A risk assessment for safe sample collection was performed for each sample location. Factors
considered in the risk assessment were road safety features that included: crosswalks, bike lanes,
42
sidewalks, round-about, traffic lights, and school zones. These features were standard in urban
areas, whereas the rural locations had little to no safety features. Road dust sampling protocols
were similar to those the Environmental Protection Agency designed for collecting road dust near
construction sites (EPA, 2020). Upon finding a safe location, a one-mile swath of road was
analyzed at each site, collecting samples every half mile. The initial sample was collected at the
zero-mile mark, moving approximately half-mile marks the second collection, and the one-mile
mark was the third. All three samples were placed into one jar; road dust was collected via
sweeping with an eighteen-inch width broom and dustpan; the collection was done on each side of
the road, starting from the middle of the lane to the curb or edge of the road. As such, each road
studied spans a distance of one mile with three collection sites.
Allowing a period of dry days before sample collection was necessary for road dust to
accumulate on the roadway. Due to this requirement, sampling occurred on the 10th and 11th
consecutive dry days, 1/30/2023 and 1/31/23, from 9 AM-3 PM, the lowest traffic times in
Thurston County. The location was recorded at each sample site with the prepared ArcGIS 123
survey field assessments completed for each road collection.
Road Dust Sample Preparation and Separation of TRWPs
Evaluating tire road wear particles from road dust required homogenizing the sample,
which includes drying the sample and mechanical size fractionation with a one mm custom-made
wire sieve. The mechanical size fractionation was performed to help homogenize the sample for
TRWP extraction from road dust. The road dust of less than one mm TRWPs was separated by
Wet Peroxide Oxidation (WPO) digestion and density separation using NaCl which was adapted
from NOAA (2015) protocol on microplastic separation from beach sand. Microplastics from
environmental samples are measured between less than one µm to five mm (Kovochich, 2021)—
43
the range of TRWPs recovered by Jung ( 2022) measures consistently as less than one millimeter.
Therefore, the TRWP size range evaluated for this study is between one mm and thirty micrometers
(μm); the lower limit was arbitrarily set by the smallest mesh sieve size available. However,
TRWPs may likely be smaller than the thirty μm due to their ability to cling to other larger road
particles (Jung, 2022). After the mechanical size fractionation (i.e., the 1 mm sieve) of the road
dust samples, three one-gram replicates were utilized to extract TRWPs from road dust samples.
The extraction process consisted of digestion and density fractionation for each road.
Sample preparation process: Road dust samples were dried for twenty-four hours at 65 ºC
to eliminate any water content and passed through a one mm sieve to remove large rocks or other
debris (that could disproportionately affect the sample weight). Then, the content that passed
through the sieve was directly transferred into beakers, which were weighed and stored for further
analysis. Finally, the portion of TRWP collected from the above one mm fraction weight was
recorded and then discarded.
The extraction and identification of TRWPs (i.e., material < 1 mm) had a three-step
process; digestion to eliminate organic matter, density separation to separate TRWPs from road
dust, and microscopic identification of TRWPs. Each replicate had one round of digestion using
a Wet Peroxide Oxidation (WPO) reaction. The WPO reaction combined equal parts 0.5 M Fe(II)
solution and thirty percent hydrogen peroxide with one gram of road dust for digestion (NOAA,
2015). The WPO reaction uses a heavy oxidizing agent to denature and dissolve organic materials
within the sample (Rodrigues, 2019).
The density separation uses NaCl to increase the WPO solutions density and separates
TRWPs from the materials left after digestion. Rodrigues (2019) suggested up to two doses of the
WPO for TRWPs; however, since the replicates were weighed out at one gram, only one WPO
44
dose was performed. The density fractionation step immediately follows the WPO. It uses ~ seven
g of NaCl per 20 mL of the WPO solution left to increase the density (see Figure 6). The sample
was left in the solution overnight to separate TRWPs. The calculated ranges of the replicate’s
density fractionation were between 1.4 and 1.5 g/cm3.
The float portion of each density
fractionation was recovered by rinsing the sample with deionized water and pouring it through a
custom-made thirty μm mesh sieve. The samples were then transferred from the top of the sieve
using a metal spatula and rinsed into a labeled plastic cup with a lid for microscopic analysis.
Leaving the sample suspended in a small amount of water for up to forty-eight hours made
transferring sample contents from plastic cups to a viewing plate easier and avoided the loss of
fine particles.
Figure 7
Digestion and Density Separation Lab Samples
Note: On the left is a picture of road dust samples in the cooling stage of digestion. The right
shows samples in the density separation stage and heating the solution to dissolve salt (NaCl) to
increase the density of the WPO solution.
Microscopic Identification of TRWP from processed replicate samples
The primary methods for TRWP’s identification used visual, texture, shape, and color
verifications as a guide. In addition, a dissecting microscope was used to identify TRWPs and then
45
transferred to a glass slide, which was further examined with a compound microscope. The
identification of TRWP is based on the morphologies in Figure 7 and listed in Table 3 below.
Figure 8.
Morphologies of TRWP (Jung, 2022)
Table 3.
TRWP morphology and description (Jung, 2022; Leads, 2019).
TRWP morphology
Color
Texture
Shape
Description
Black or dark in color, sometimes brownish
Encrustation, rough surface, rubbery consistency, and malleable
when manipulated with fine tip forceps.
Elongated or cylindrical
In total, nineteen roads were assessed for TRWPs, with three replicates performed for the
TRWP extraction process (for 57 subsequent extractions). To identify the suspected TRWPs, a
30X dissecting microscope (Olympus SZ40) was used with a directed light and fine-tip dissecting
46
tweezers to pick out suspected TRWPs then further verified using a 40X compound microscope
(Omax DV5V). For each sample, a few particles were photographed with a compound microscope.
After the sample TRWPs were verified using a compound microscope, the slide was placed onto
a white piece of paper and photographed (Google Pixel phone) to use image analyzing software
(Image J) to estimate particle count and size (μm2) using Image J.
The analysis of road dust samples utilizes the weight-by-analysis method to identify
TRWP; first, by isolating TRWPs through digestion and density separation; second, by visual
identifiers confirmed by reference materials descriptions of TRWP’s characteristics and
morphology. The TRWPs were classified by density selection (1.4-1.5 g/cm2) and visual markers.
Density separation helped isolate the TRWPs from other particulates in road dust samples.
However, various other particles were found in each sample, such as glass beads, paint particles,
dirt conglomerates, some plant particles, minerals, and asphalt particles.
Image J analysis of TRWP particle sizes and distribution
Particle size distribution analysis is a highly desirable method in investigating TRWP
particle sizes from road dust samples. Image J provides particle analysis by identifying the shape
and dimension measurements and expressing them in pixel measurements (Igathinathan, 2008).
To my knowledge, utilizing Image J software for analyzing environmental particles such as road
dust is a new approach in TRWP analysis. Image J, a free-to-public software, has proven
reliable, user-friendly, and used for various image analysis methods. Before beginning Image J
analysis, the global scale must be set to measure photo particles based on the user’s
measurement (Rueden, 2017). For this step, the scale was calibrated by taking a picture of a
47
known one μm measurement. The image J particle analysis method is based on the software
“watershed analysis” in Rishis (2018). The photo of TRWPs was uploaded to ImageJ with the
image settings adjusted to 8-bit; images were calibrated to estimate particle measurements in
pixels/μm and report particle area sizes in micrometers squared. Finally, image J does some
statistical analysis and assesses the sample distribution. It also saves a CSV file with all the
particle analysis data generated by image J which was used in the statistical analysis. The
TRWPs obtained from the extraction process were photographed and analyzed with Image J.
Note: displays one road replicate processed with image J for particle analysis. The left shows the
stage after the photo is prepped and ready for particle analysis. The right shows the image
produced post-particle analysis with each particle outlined, numbered, and measured.
Figure 9
Image J particle analysis (left) before particle analysis (right) post particle analysis
numbered and outlined.
48
Statistical Analysis of TRWP Measurements (area μm2)
The statistical analysis for this study used the data values generated by Image J and the
annual average daily traffic (AADT) per road retrieved from the Thurston County Planning Office.
The values given by image J were the recovered TRWPs count and the measured area for each
particle (μm2) for all TRWPs recovered from digestion and density separation. All roads were
assessed for normality among TRWP measurements. A data transformation of the TRWP
measurements was done using a natural log transformation. However, it was still not normally
distributed. Nonparametric tests were run due to the small sample size. These non-parametric tests
consist of a Wilcoxon/ Kruskal-Wallis with a post-hoc Dunn method ranks of all sums assessment.
The Wilcoxon Kruskal-Wallis test determines if two or more populations are statistically different
by ranking all values across each population. A post hoc test using the Dunn method to rank all
sums for each road type was used to evaluate if there is a statistical relationship between the two
or more populations or road types. These tests help indicate whether road types’ TRWP counts are
similar or dissimilar. A correlation test was done using a non-parametric Spearman’s rho test to
assess if AADT, TRWP count, and the average TRWP measurement per road showed any
correlation.
49
Results
Microscopy Identification and Image J analysis of Tire Road Wear Particles (TRWPs)
TRWPs were identified from all sampled roads in all replicate samples. Figure 5 shows the
morphologies of the recovered TRWPs; some of the particles show how they are found as single
free-floating particles or stuck in conglomerates with other road particulates. The tire pieces were
distinct when looking at them under the microscope and shared the same features listed in previous
studies (Jung, 2022; Leads, 2019).
Figure 10.
TRWPs recovered from road dust samples, and photos are identified by the name of the street
from which they were sampled.
50
TRWP Count Results
When analyzing the particle count for each road, urban roads (primary, secondary, and
Figure 11.
Tire Road Wear Particles Counts by Road Type
residential) consistently had more TRWPs identified than rural roads (see Table 2). The range
particle count for rural roads was from 45 to 408, with an average of 205 ± 173 mean average
and standard deviation, respectively. The particle count for primary roads ranged from 112 to
1225, with a mean average count of 475 ± 441. For secondary roads, the range of particle counts
is 298 to 4200; the reported mean TRWP count was 1493 ± 1741. Finally, the residential road
TRWP count ranged between 87 to 707, with an average mean of 349 and a standard deviation ±
264. Figure 10 displays the distribution of TRWP count by road type and shows that secondary
roads have the highest TRWP count compared to the other road types.
51
Figure 11 displays the particle count per road sampled. The figure shows Eastside Street,
Olympia, contains the most TRWP, with 4200 tire particles collected from the three replicates
processed and found mainly in conglomerates. The second road with the highest was College
Street, Lacey, with 2309 TRWPs counted. Both roads are considered secondary roads and
contribute to the highest TRWP counts in Thurston County.
Figure 12.
Distribution of TRWP count by sample site
Sample Site
'Sample Site': Eastside St has noticeably higher 'TRWP
Count'.
Eastside St
College St
Tumwater Blvd
Israel Rd
Martin Way
Mud Bay
507 Rte. Rainier
Mullen Rd
Capitol Blvd
Yelm Hwy
507 Rte. Yelm
North St
Marvin Rd
Case Rd
Deschutes Pkwy
1st Street
507 Rte. Tenino
110th
0
500
1000
1500
2000
2500
3000
3500
4000
4500
TRWP Count
Table 4 displays the analyses of TRWP count; it gives the TRWP count per road, the estimated
total TRWP counts in road dust samples, and the estimated TRWP per gram total dry weight of
52
road dust. The yellow highlighted roads show the highest TRWP contributor to Thurston County
roadways, and the white indicates the low TRWP counts and estimates.
Table 4
TRWP count data, and TRWP estimated counts per road sampled and TRWPs by grand dry
weight (g).
Road
Type
Primary
Primary
Primary
Primary
Primary
Secondary
Secondary
Secondary
Secondary
Secondary
Residential
Residential
Residential
Residential
Rural
Rural
Rural
Rural
Rural
Sample Site
Mud Bay
Tumwater
Blvd
Martin Way
Marvin Rd
Deschutes
Pkwy
Capitol Blvd
Eastside St
College St
Yelm Hwy
507 Rte.
Yelm
Israel Rd
North St
Mullen Rd
1st Street
507 Rte.
Rainier
507 Rte.
Tenino
Case Rd
110th
Delphi
Road Dust dry
weight (g)
Total estimate
TRWP per Road
sampled.
TRWP
Count
38
408
5168
43
1225
17558
122
86
432
196
17568
5619
64
112
2389
36
22
15
64
334
4200
2309
324
4008
30800
11545
6912
51
298
5066
82
103
37
42
707
237
364
87
19325
8137
4489
1218
144
366
17569
31
46
475
49
16
42
159
45
408
2597
240
5712
TRWPs /
gdw.
10.74
28.49
3.54
2.28
1.75
9.28
190.91
153.93
5.06
5.84
8.63
2.3
9.84
2.07
2.54
1.48
3.24
2.81
9.71
ADDT and TRWP particle count were weakly associated and insignificant (rho = 0.2115, p =
0.385). Furthermore, the correlation between ADDT and TRWP/gdw. was weak and not
significant (rho = 0.1150, p = 0.6393). Finally, a nonparametric Kruskal-Wallis test with the
53
TRWP /gdw. and road type showed no significant differences among road types (Chi-squared =
4.2458, p = 0.2361). The conclusion of these tests all determined no significant relationship
between ADDT and TRWP/ gdw. or TRWP count. There was no notable difference among road
types for TRWP per gdw.
To further examine the relationship between AADT by road type, Figure 12 shows that Primary
roads have the highest AADT counts per hour (PM), followed by secondary roads. AADT
frequency is opposite to TRWP counts regarding primary and secondary roads, however, there was
no statistical difference between road types.
Figure 13
Annual Average Daily Traffic (AADT) counts per hour (PM) by road
type.
54
TRWP Measurement Results
The reported TRWP size range for each road type varied among road types and within
replicate samples. For example, the range of TRWP sizes for rural roads varied from 1 μm 2 to
175 μm 2, whereas for primary, secondary, and residential roads, values ranged from. 1 μm 2 to
165 μm 2 , 1 μm 2 to 176 μm 2 and from 1 μm 2 to 175 μm 2, respectively. The residential roads’
average TRWPs had the largest average mean size of the road types. Table 5 below reports the
average TRWP size (μm2) per road sample and the sampled roads’ annual average daily road
count.
55
Table 5.
Overview of Sample Sites in Thurston County, WA: Road type, Annual Average Daily Traffic
(AADT), Average TRWP size, Standard deviation, Coefficient Variant, and TRWP count per
road.
Road
Type
Primary
Road ADDT per
Hour (afternoon
peak)
Average Area of
TRWP (μm2)
found in a g of
road dust
Standard
deviation
CV %
1029
4
10
7
8
12
13
15
16
62
19
325
21
10
38
10
9
111
200
633
250
150
Primary
1001
Primary
2915
Primary
2934
Primary
555
150
229
775
158
Secondary
1739
Secondary
365
Secondary
1993
Secondary
816
Secondary
1045
19
5
6
4
6
Residential
244
9
16
Residential
614
Residential
736
Residential
816
13
9
18
22
19
30
178
169
211
167
Rural
494
Rural
608
Rural
347
Rural
112
Rural
461
6
85
20
27
7
14
17
37
33
16
233
20
185
122
229
Sample Site
location
Mud Bay
Tumwater Blvd
Martin Way
Marvin Rd
Deschutes Pkwy
Capitol Blvd
Eastside St
College St
Yelm Hwy
507 Rte. Yelm
Israel Rd
North St
Mullen Rd
1st Street
507 Rte. Rainier
507 Rte. Tenino
Case Rd
110th
Delphi Rd
Unfortunately, due to the large CVs for each sample, no further analysis was conducted to assess
the difference between mean particle size by road type or ADDT.
To conclude, the results of this study found that all roads contained TRWPs whose size ranges
varied considerably for each road. By road type, the TRWP count showed that secondary roads
had the most recovered TRWPs compared to the other road types. Spearman’s rho correlation
56
results showed no significant correlation between AADT, TRWP count, and TRWP/ gdw. These
analyses provide limited answers to the research question of whether TRWPs count varies among
road types. The answer is yes with uncertainty; TRWP counts vary amongst road types, but we
cannot indicate what makes these differences occur. Furthermore, the study could not answer the
second research question on whether AADT affects TRWP measurement due to the heterogenic
nature of TRWP measurements from road dust samples. The results showed no significant
correlation between TRWP count and AADT.
57
Discussion
Thurston County’s TRWP’s Differences Among Roads
Road types provide several factors that increase TRWP emissions to a roadway. For
instance, primary and secondary road types have more congested areas and have higher rates of
quick braking and aggressive driving, increasing tire wear and tear (Mian, 2022). At the time of
sample collection, each road was assessed for stormwater drains and noted that all the urban roads
had storm drains with straight-to streams marked on the gutters. RCW 35.78 details that
Washington state cities and towns must adopt a uniform standard of road definitions (MRSC,
2023). This helped determine the differences between state, county, and city road types. Thurston
County defines its road types based on functionality (TRCP, 2023(b)). Functionality refers to a
road’s usage needs and its urban or rural setting. For this study, road types were determined based
on functionality and urbanization. Primary, secondary, and residential roads comprise urban road
types, whereas rural roads are defined by low population and commercial zones (TRCP, 2022).
The daily annual road counts are higher in urban areas when compared to their rural counterparts
(WSDOT, 2022). More vehicles on roadways result in higher estimates of TRWP counts. Another
aspect of the increased TRWPs to roadways is electric vehicles (EV), the increased vehicle weight
results in more wear and tear on tires (Baensch-Baltruschat, 2020; Charbouillot, 2023). Thurston
County has 2,264 registered EVs as of Spring 2021, and the estimate increases daily as more
Thurston County citizens move toward EVs (TRCP, 2023(c))
Urban roads typically are more heavily trafficked and located in cities and townships
(WSDOT, 2022). TRWP counts from secondary roads like Eastside Street and College Street
contained the highest contributions of TRWP from all the road tested in Thurston County. From
this study’s field observations and mapping, these roads are located in dense urban regions and act
58
as connectors routes from residential areas to the state’s transportation network. Based on TRWP
counts, secondary and primary roads are the most significant contributor of TRWPs on the
roadway and require targeted mitigation of TRWP.
The factors contributing to increased TRWP on secondary roads are the number of
connector roads to secondary roads versus Primary roads (TRCP, 2023(a)). Also, secondary road
traffic networks had more of a variety of road pavement materials, traffic patterns with multiple
speed changes and school zones, and heavy traffic controls like lights, stop signs, round-about,
and multi-lane roads (TRCP, 2023(a)). These factors are likely reasons for the increased rate of
TRWP count on both College Street and Eastside Street, as these factors were present on both
roads. The study shows that road type may be a determinant of TRWP count. However, further
research is needed to examine features of the transportation network and their interactions with
TRWP on roadways. For instance, secondary roads had a high variability in the TRWP counts,
and the non-parametric tests showed road types with TRWP by gram dry weight had no significant
differences among road types. This means that statistically, we cannot say road types strictly
determine the abundance of TRWP distributions. However, with such a small sample size, this
may not indicate the reality of TRWP abundance on the roadways when analyzing the relationship
between road type and TRWP counts.
TRWP Morphologies
Environmental samples of TRWP have rarely been shown to be pure tire wear particles
and are generally found in mixed compositions of tire wear particles and other road particles
(Baensch-Baltruschat, 2020). The observations of TRWP from the microscopy analysis in this
study showed that the dark elongated particles had very fine mineralization over the surface. The
brownish/black TRWPs often had more extensive mineralization and stuck to other road dust
59
particulates (glass beads, paint particles, plant particles, asphalt particles, etc.). They were similar
to Jungs’ (2022) and Leads’ (2019) findings. A further chemical analysis would be needed to
determine if the color difference in TRWP is a variation in the types of tires on the road or if aging
/ weathering plays a role in the decolorization of tire wear particles. Much TRWP comprises
coarser heterogeneous particles released to the road’s surface, soils, and aquatic environment
(Baensch-Baltruschat, 2020).
From the results of this study, it is apparent that TRWPs exist in very fine ( 1 μm2) to
coarse TRWP ( > 100 μm2) on all Thurston County roads sampled. TRWP’s limited information
on real-world measurements has uniformly concluded that a better way to measure TRWP is
needed for TRWP estimates (Mennekes, 2022). Approximately 90-95 % TRWPs are not air-borne
particles (Mennekes, 2022; Baensch-Baltruschat, 2020; Klockner, 2019; Panko, 2013). The
remaining 5-10% of TRWP emissions are emitted into the air and contribute to urban air pollution
(Mennekes, 2022; Baensch-Baltruschat, 2020; Klockner, 2019; Panko, 2013). The transport and
fate of TRWP are determined by the size of the particulate when emitted (Wagner, 2018). For
example, fine particulates are released into the ambient air on roadways, and coarse particles
remain on the roadway and are washed out with stormwater/roadway runoff. The morphology of
TRWP makes them susceptible to absorbing other road contaminants, and as particles become
smaller, they are more likely to hold contaminants like 6PPD-quinone (French, 2022).
60
Figure 14.
TRWP Measurement Range
Note: Tire Road wear particle size ranges from environmental samples and lab-simulated tire
wear (Wagner, 2018; Kreider, 2010).
The size ranges in Figure 14 were established by lab-simulated tires' wear and tear, where
researchers simulated road conditions and collected TRWP by collecting environmental samples
of the air, soil, and water near roadsides (Wagner, 2018). The coarse fraction of TRWP transported
to surface waters poses ecotoxicity and human health risks (Baensch-Baltruschat, 2020). The
findings of this study TRWP measurement ranges were between the fine and microplastic fractions
(1μm2 to > 100 μm2). This means all sizes reported in this study pose ecotoxicity and human health
risks to airborne and surface waters of nearby streams or rivers.
Image J for particle analysis
For this study, finding a way to count and measure all TRWP recovered from the extraction
process was vital. Microscopy identifies TRWP morphologies, and to perform the particle analysis,
the study utilized an image processing software (Image J) to count and measure TRWPs found in
road dust samples. Image J has been used widely for various scientific studies, such as medical
and agricultural research (Igathinathan, 2008; Rueden, 2017; Rishi, 2018). Image J was initially
61
created in 1997 as an image processing software intended for the National Institutes of Health (
Rueden, 2017). Since that time, image J has been vital in scientific study, especially in the life
sciences (Arena, 2017). Other uses for image J include powder particles and cell counting, which
is essential for many medical sciences (Igathinathan, 2008; Rishi, 2018). Other applications for
Image J include seed counting and mineral counting for the American Agriculture Association
(Rishi, 2018). In addition, the software has proven a helpful alternative in particle analysis when
research expenses are low and require innovative approaches.
Image J is used in this study’s particle analysis as an experimental technique for measuring
and counting TRWPs. For this study, each image sample replicate of recovered TRWPs was
analyzed three times to ensure that image J reported a consistent value. This ensured that the roads
with high TRWPs, like Eastside Street and College Street, consistently reported the same values
for each sample replicate processed. Another approach used in this study to ensure the values’
validity was running more than one image of the sample replicate to ensure consistency in the data
reported. Both strategies reported consistent values for each sample replicate processed and helped
affirm that image J was viable for particle measurement and count.
TRWP distribution Implications to streams: Identifying regions with increased TRWP
emissions
TRWP increases the risks of chemical and heavy metal loading to urban streams due to the
particles’ increased surface area and absorbent nature (Wagner, 2018; Lou, 2021). Furthermore,
many streams in urbanized areas are at increased risk for urban runoff mortality syndrome due to
higher rates of impervious surfaces within those regions (Mallick, 2017; McIntyre, 2015). In
addition to the issue of impervious surface percentage is the emission of TRWP in congested
traffic, which increases the dangers of poor stream habitat quality on susceptible species like coho
62
salmon (French, 2022). TRWP implications for streams are heightened with the co-contaminants
transported from the roadways to streams (Lou, 2021; French, 2022). The study aimed to see how
road type affects TRWP input into urban streams by examining the quantity and measurements of
TRWP on the roadways. These distinctions between road types can help determine streams’
implications without chemically testing the water. However, further investigation of stormwater
discharged into streams for TRWPs is necessary to follow up work, especially near roads with
very high particle counts.
This study has identified roads with increased rates of TRWP counts by road type, which
adds essential data to a highly underdeveloped area of research. In addition, TRWPs are annually
released on scales that go unnoticed due to their microplastic nature (Mennekes, 2022; BaenschBaltruschat, 2020). How do we move forward with habitat rehabilitation for susceptible species
without providing environmental measurements of TRWPs found directly on the road? These
measurements can be used to determine areas that need targeted mitigation of urban streams. Since
TRWPs are retained easily within soils, bioretention systems make the particles inert (French,
2022). The other potential best form of mitigation is diluting stormwater and roadway runoff to
avoid urban mortality response (French, 2022). TRWP’s significant damage comes from the cocontaminants they carry to urban streams that foul the water and degrade viable habitats for salmon
(Mennekes, 2022; French, 2022). This is why removing TRWP before it enters streams can reduce
the number of toxins from the roadway that enter and degrade viable stream habitats.
Mitigation of TRWP’s for Conservation and Rehabilitation of Susceptible Species
Stormwater mortality events have been mainly studied in Coho salmon regarding its lethal
effects (French, 2022), and in the Thurston County Deschutes River, Coho runs are under threat
(TRCP, 2023(b)). Recent studies have shown a variation in the susceptibility of urban runoff
63
mortality syndrome and that dilution of stormwater effectively limits mortality response (French,
2022). Coho and Steelhead are highly sensitive to stormwater runoff, whereas Chinook, Chum,
and Sockeye showed a stark difference in susceptibility compared to Coho and Steelhead (French,
2022). In Thurston County, the Coho salmon are considered vital markers of habitat health and
function of the Deschutes River (TRCP, 2023(b)). Currently, three Coho salmon runs are located
on the Deschutes River, all of which have faced exponential losses (TRCP, 2023(b)). The TRWP
count from Deschutes Parkway was 112 particles, and when compared to other roads is not the
greatest. However, other roads in the area tested, like Capitol Blvd and North Street, had much
higher quantities of TRWP, and each had straight-to-stream drains on the roads tested. Therefore,
the Deschutes River and its tributaries in Thurston County may likely be at risk for TRWP cocontaminant pollution.
The life cycle of Coho salmon in the Deschutes River is three years of migration runs; the
brood year represents the year eggs are laid by adult spawners (TRCP, 2023(b)). Coho smolts
migrate from the Deschutes River approximately two years after brood year (TRCP, 2023(b)). This
means the most susceptible species to urban runoff mortality syndrome spend up to three years in
the Deschutes River before migrating to the ocean. Unfortunately, juvenile coho salmon have
shown the same susceptibility to 6PPD-quinone and stormwater runoff as the adult spawners for
the syndrome were initially recognized (McIntyre, 2015; French, 2022).
Addressing the issue of green infrastructure not adequately placed in frequently trafficked
areas presents an issue for habitat conservation and rehabilitation of susceptible species like Coho
salmon. The U.S. Superior Court’s injunction order of 2008 determined that providing adequate
and timely transitional habitats for salmon is necessary for the rehabilitation and conservation of
the species (United States v. Washington, 2013). The same logic invokes WA states responsibility
64
to provide historical salmon habitats mitigation of TRWP’s toxicological effects on urban streams
in many reopened urban streams. Without this consideration, we have little hope for rehabilitating
susceptible species like Coho salmon and Steelhead trout populations. The Puget Sound and its
tributaries comprise the injunction area in the fish passage project and are also the most urbanized
areas of Washington State (United States v. Washington, 2013; State of Salmon, 2022). Streams
reopened for salmon are only assessed for fish passage, not for toxins that induce URMS. This
poses an issue for these restoration and conservation efforts because these streams may not have
adequate water quality or high toxic inputs that would result in urban runoff mortality syndrome
events or Coho mortality syndrome.
From the results of this study, it is apparent that urban areas with heavy traffic also have
more significant average TRWP emissions. The increased counts of TRWP have been attributed
to aggressive driving; sudden stops and acceleration increase the friction of the wear and tear of
the tire (Peters, 2023; Järlskog, 2020). Road types can function as a way to identify hot spots for
TRWPs and the co-contaminants they carry to streams based on TRWP counts. It is daunting to
approach the global toxic issue of tire rubber microplastic pollution, especially at the annual rate
the environment currently endures. However, if we started our approach by turning road
environments into semi-closed systems, where roadway and stormwater runoff goes through a
transitional stage to catch or pull-out major road polluters like TRWP’s, there is hope. Tires are an
essential function of everyday life; this study aims not to demonize tire companies for meeting
consumers’ needs and federal regulation parameters needed for tire safety and performance but to
provide awareness and real-world TRWP measurements and counts. Moreover, this study adds to
a developing area of research that has become highly underdeveloped regarding the methodology
and environmental measurements of TRWPs.
65
Limitations of this study
The major limitation of this study is the methodology, sample size, and no instrumental
chemical identification was performed. The methods used for this study were created to find a way
to estimate and measure TRWP distribution on the road. This study devised a method based on
road dust collection near construction sites and extraction methods for microplastics from beach
sand. The use of Image J for particle analysis has yet to be done for road dust before. However,
the lack of conventional TRWP collection, identification, and measurement methods limited the
studies’ ability to homogenize the sample replicates processed. It may account for the wide
variation in TRWP distribution per road. To evaluate the road for TRWP, this study recommends
keeping each sample from the road separate instead of placing the whole one-mile road sample
into one jar. Each replicate process showed high variation in the average mean for particle area,
indicating the samples were not homogenized. This may be accounted for by three separate
locations in the one-mile road being sampled. The road dust collections were placed into the same
jar to represent the one mile sampled; however, from the study’s results, this may not be the best
way to represent TRWP on the roadway. Furthermore, the future methodology should assess how
to homogenize the road dust after collection best so that the aliquot selected for future analysis is
representative of the sample from which it was collected.
The sample size is another study limitation when determining a large region like Thurston
County. Nineteen samples are a great start, but more is always better in the spatial qualification
and quantification of environmental pollutants. The other limitation of this study is that no
instrumental chemical verification was performed due to time constraints, and future research is
highly recommended. The best form of chemical verification is hard to say since no routine
methods are found in the environmental assessments of TRWPs. The most common methods used
66
are the Inductively Coupled Ion Mass Spectrometer (ICPMS), Gas Chromatography-Mass
Spectrometer (GCMS), and Fourier-transform infrared spectroscopy (FT-IR). The most common
tire chemical markers researchers used to quantify TRWP’s are
polystyrene butadiene (Johannsen, 2022; Lou, 2021).
67
Zn, HMMM, 6PPD, and
Conclusion
Investigating the environmental impact of TRWP on streams requires an interdisciplinary
approach of tire science (manufacturing and chemical innovations), environmental chemistry,
stream ecology, transportation infrastructure, policy, current stream rehabilitation projects in
Washington State, and restoration conservation ecology. Each of these disciplines has a part in
monitoring and orienting the public to the issue of TRWP impacts on streams and illuminating the
public on the issue of tire rubbers’ contribution to the global emissions of microplastics. The tire
wear particles are emitted to the roadway through the abrasion of tires on the road, where they are
weathered, aged, and become biofouled (Lou, 2021). Road dust poses as transitional media that
transports TRWPs that become carriers of co-contaminates like 6PPD-quinone from roadways into
urban streams via runoff. To address the mounting issue of TRWP pollution of urban streams, we
would have to understand the transport and fate of TRWP. As well as how our transportation
system can help define regions with increased risks of TRWP contamination of urban streams.
The finding of this study showed higher counts of TRWPs in road dust from secondary
roads. Surface area μm2 and particle count indicate two factors that determine the toxicity inputs
of TRWPs to urban streams and require further investigation to define the distribution of TRWPs
on roadways truly. However, surface area varied widely within each road, so additional analyses
could not be conducted to see how road type or AADT affected surface area. Targeted green
infrastructure approaches to TRWP’s mitigation from streams are the best option to support more
significant ecological restoration goals like the fish passage barriers removal project. Washington
State has the initiative and duty to maintain a continuance of suitable aquatic habitats for salmon.
The fish passage barriers project currently focuses on reopening stream habitats throughout Puget
Sound (State of Salmon, 2022). However, there is no implicit consideration of whether these
68
reopened streams are monitored for URMS-inducing toxins like 6PPD-quinone. This project
provides vital information in a severely lacking field in the study of tire road wear particles. The
study also provides vital information on TRWP’s presence and abundance on the roadways in
Thurston County. To conclude, secondary roads should be further assessed to mitigate TRWP
since they had the highest contributions of TRWP found on Thurston County roads.
69
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