State of Knowledge on the Effects of Tire-Derived Aggregate (TDA) Used in Civil Engineering Projects on the Surrounding Aquatic Environment

Abstract

Tire-derived aggregate (TDA) is an entirely recycled material created by processing scrap tires, which are shredded into a fundamental geometric shape, typically measuring from 5 to 30 cm in size. TDA possesses desirable properties such as low earth pressure, improved drainage, and a lightweight structure, making it an ideal material for numerous civil engineering applications. Unfortunately, the environmental suitability of TDA use has previously been questioned. This article outlines that TDA does not release a significant amount of potentially toxic compounds, the leaching rate in surrounding water environments is low, and TDA can even be a medium to remove nutrients and toxic organic and inorganic compounds commonly found in agricultural land and urban runoff. This study aims to collect the most up-to-date scientific data on the environmental impact of scrap tires and evaluate the data specifically for TDA applications in civil and environmental engineering applications. TDA has been proven to be an environmentally safe, long-lasting, cost-effective, and sustainable resource with many potential applications in civil engineering. Guidelines should be developed for specific projects to achieve a circular economy for end-of-life tires in the form of TDA to avoid potential environmental issues and problems.

1. Introduction

Over 270 million tires were discarded in the United States (U.S.) in 2021 alone. However, despite an annual increase in scrap tire generation by close to 7%, the recycling market experienced a 25% decline between 2013 and 2021. This trend has prompted the U.S. Tire Manufacturers Association (USTMA) to urge recyclers, the industry, environmental groups, and academic partners to do more to achieve a circular economy with the help of state and federal regulators [1]. Most waste tires can be put to a safe, cost-effective use simply by being converted to TDA. In addition, TDA offers a variety of practical uses as alternatives for traditional gravel and lightweight fill.

While only 5~10% of end-of-life tires have been utilized in civil engineering projects from 2011 to 2021, misinterpretations of published laboratory and field data have prevented the widespread usage of tire-derived aggregate (TDA) in public and civil engineering applications [2]. The inadequate peer-review of technical reports and the citation of test data obtained under extreme conditions, which would not occur in most TDA usage, have hindered the widespread use of TDA.

Thus, an urgent need is to analyze published data and interpret them precisely for TDA applications in civil engineering applications. This study aims to (1) collect the most up-to-date scientific data on the environmental impact of scrap tires and (2) evaluate the data specifically for TDA use in civil and environmental engineering applications. Doing so can expand TDA usage to over 20 possible applications while ensuring a sustainable environment and achieving a circular economy. TDA was first applied for civil engineering in the U.S. in 1986 as a lightweight fill material for road construction [3]. Over time, TDA usage has significantly increased and expanded to various applications, including noise barriers, drainage systems, road constructions, and more. TDA is environmentally friendly as it repurposes waste tires, reduces the carbon footprint, and uses natural resources. Furthermore, TDA is durable and not biodegradable, making it suitable for construction in hostile conditions. It maintains its engineering properties over time, making it a safe and reliable resource at a fraction of the cost of traditional materials. Moreover, there is no proof suggesting that any leached nutrients, metals, or volatile organic compounds (VOCs) from the TDA could pose a risk to the quality of the water received in stormwater management systems [4,5].

This study evaluates the technical properties of TDA for use in civil and environmental engineering applications and the potential environmental impacts of TDA contrasted with scientific data obtained from various sizes and forms of scrap tires, including whole tires, ground tires, tire chips, etc. This study aims to promote the use of TDA in civil and environmental engineering projects as a sustainable alternative and help achieve a circular economy by providing the state of knowledge and information on TDA’s environmental properties.

2. Definition and Important Properties of Tire-Derived Aggregate (TDA)

2.1. Definition of TDA

TDA, entirely derived from recycled materials, is processed from discarded tires that are shredded into a basic geometric form, varying from 5 cm to 30.5 cm in size. It is designed for implementation in civil engineering projects and is considered one of the most sustainable aggregate materials [6]. TDA comes in two sizes: Type A, which is a maximum of 7.5 cm long, and Type B, which is a maximum of 30.5 cm long. Many manufacturers only produce Type A or Type B. Specific projects may be more suitable for using one or the other.

2.2. Beneficial Properties

Unique properties of TDA that create beneficial engineering value for civil engineering projects are as follows:

• Lightweight: TDA is 67% lighter than traditional aggregates, facilitating more cost-effective transportation and reducing the load on soft soils and sensitive structures [7,8].
• Thermal insulation: TDA is eight times more effective than gravel [8,9].
• High void space and porosity: TDA has a porosity of 0.5, which makes it an excellent material for drainage and filtration applications [10].
• High shear strength: TDA has a high friction coefficient, which provides increased structural stability, reduced deformation, increased durability, a heightened safety factor, and greater flexibility in design [11,12,13,14].
• Reduced lateral load: TDA exerts 50% of earth pressure, which reduces structural requirements, improves structural stability, enhances safety, increases durability, and provides more flexibility in design [8].
• Interlocking properties: TDA provides increased stability, load distribution, reduced maintenance, faster construction, less cracking, and faster construction due to easy installation [15].
• Freeze–thaw mitigation: TDA forms a non-capillary, hydrophobic layer, preventing moisture from entering foundations, enhancing durability, and reducing structural damage in cold climates [16,17,18].
• High permeability and drainage: TDA drains water ten times faster than gravel, reducing water damage, improving safety, enhancing durability, reducing standing water, improving water management, and lowering maintenance [8].
• Vibration seismic force mitigation: TDA has a more excellent vibration absorption property than earth backfill materials, which reduces noise, increases comfort, improves safety, enhances durability, reduces maintenance, and increases lifespan [8].
• Chemical adsorption and filtration capability: TDA contains nonpolar compounds, carbon, and steel, making it suitable for adsorbing nonpolar chemicals, precipitating metals, and removing halogenated hydrocarbons by iron and zinc present in end-of-life tires as reducing agents [19,20,21,22,23,24,25,26,27,28].
• Lower cost: TDA is 1.4~29.5 times cheaper than earth materials due to lower transportation costs and improved safety measures that reduce the risk of damage to roads, vehicles, and machinery [29].
• Sustainable construction material: TDA, composed of non-toxic recycled rubber, is durable, requires less maintenance, and can withstand harsh weather, making it suitable for construction projects in freezing winter conditions [30,31,32].

Overall, TDA is an economical, versatile, and durable material used in various applications, from drainage systems to road construction and noise barriers. Its sustainable nature and low environmental impact make it an attractive alternative to traditional aggregates. TDA has been implemented into the following applications:

Retaining wall backfills [7,29,33];

Embankment fill material [7,11,29,34];

Vibration dampening in railways and earthquakes [29,32];

Road landslide repair, lightweight fill for the road [11,29];

Slope and bank stabilization [29];

Capillary break/freeze–thaw mitigation barrier fill [35,36,37,38,39];

Thermal insulation fill [9];

Green-roof garden backfill material [40];

Floating foundation [29];

Drainage material in golf courses [27];

Backfill material in landfill leachate and gas collection systems [24,29,30,31];

Adsorption of pesticides released in the fairway and tee-ground elevation material in golf-course [27];

Leaching field distribution-layer backfill material [41,42,43,44];

Septic system absorption fields [43];

Rain garden backfill material [5,45];

Stormwater management system backfill material [5,43,45];

Filtration media for wastewater treatment and contaminant removal [33,40,42,43,44,45,46];

Removal of nutrients such as phosphorus and nitrogen in water [42,43,44,47];

The catalyst for hydrogen sulfide removal in biofilters [18];

Buffer strip around agricultural lands to remove contaminants from runoff and around drinking water sources where potential chemical spills are anticipated;

Free-draining fill;

Fill material for water storage, retention, and infiltration;

Compressible layer between integral and general abutments;

Backfill of above and below pipes;

Wastewater treatment filter and support media;

Recycled tires, mainly in the form of TDA, have been used as a sustainable aggregate in over 400 projects in Minnesota since 1986 [3,48] and have obtained numerous recycling and engineering awards. TDA has been used in various Vermont Agency of Transportation (VTrans) projects since 1990. The first TDA project in Vermont was the Dixon Landing Interchange Project in 2001 [49]. California’s Department of Resources Recycling and Recovery (CalRecycle) has become the leader in generating scientific and technical data on TDA, testing its effectiveness in many civil and environmental engineering projects, and promoting its further use. The recent trend of TDA usage is to use the chemical adsorption and filtration properties of TDA in stormwater management and wastewater treatment. The TDA applications are expanding further through creative and innovative ideas.

2.3. Tire Composition

The composition of a truck tire consists of fillers, natural rubber, synthetic polymers, steel, antioxidants, antiozonants, and curing systems, with percentages varying based on the truck classification, as shown in Figure 1. For a passenger/light truck, fillers and synthetic polymers make up half of the tire and are critical for performance. Fillers, including black carbon and silica, enhance tear, tensile strength, and abrasion, improving wearability and traction. Synthetic polymers, commonly butadiene and styrene-butadiene rubbers, control the individual tire components and performance qualities such as rolling resistance, wear, and traction. Another standard synthetic rubber, halo butyl rubber, creates an impermeable inner lining to ensure the tire stays inflated. Textiles add to the tire reinforcement to provide further load stability. With heavier load-bearing truck tires, natural rubber makes up one-third of the composition, acting strongly to prevent fatigue cracks and tearing. Steel wire is used in truck tire plies, belts, and beading to strengthen tire casting, enhance handling, and anchor the tire to the wheel. Antioxidants and antiozonants prevent the rubber from breaking down from temperature, oxygen, and ozone. Curing systems, including sulfur and zinc oxide, influence the length and quantity of crosslinks that form in the rubber matrix. A wage range of different materials and compounds are needed for a safe performance under various circumstances [50].

The chemical components are summarized in

Table 1. Chemical composition of a car tire [51].

Element/Component	Content	Unit
Carbon	Approx. 70	weight %
Iron	16	weight %
Hydrogen	7	weight %
Oxygen	4	weight %
Zinc oxide	1	weight %
Sulfur	1	weight %
Nitrogen	0.5	Weight %
Stearic acid	0.3	weight %
Halogens	0.1	weight %

. Carbon and iron account for approximately 70% and 16%, respectively [51]. While the elemental content of tires can vary depending on the type of tire and its composition, carbon tends to be abundant because of its role in rubber, a polymer composed of mainly carbon and hydrogen. Zinc oxide accounts for 1%. Copper components, cadmium, chromium, nickel, and lead are present at concentrations of 200, 10, 90, 80, and 50 mg/kg, respectively [51].

As shown in various studies [52,53,54,55,56], zinc has a significantly higher concentration range than the other metals in the tire rubber content (8380–13,490 mg/kg). Depending on the study, Na (610 mg/kg), Ca (113–562 mg/kg), and Al (82–420 mg/kg) had higher concentration ranges. Fe, Al, Ti, K, Pb, and Mg also had relatively high concentrations of 2.1–533, 81–420, 195, 160, 1–160, and 32–106 mg/kg, respectively. Ni, Se, Cu, Co, Cd, Ba, Sr, Li, Mn, As, V, and Ag had lower concentration ranges of 0.9–50, 20, 1.8–29.3, 0.9–24.8, 0.9–4.1, 1.2–3.2, 0.23–2.3, 2, 0.8, 1, and 0.08 mg/kg, respectively.

The tire manufacturing process relies heavily on zinc, which acts as an activator during the vulcanization process and is a critical component. Transforming the rubber from soft to sturdy enables the vehicle to stop safely and withstand the vehicle’s weight. Therefore, all USTMA members utilize zinc in the production of tires to ensure the vehicle’s safety; without it, the federal standards would not be met. In addition, USTMA asserts that reducing zinc would not make a difference to zinc levels in stormwater and that aquatic life poses little to no harm [50].

The most extensive compounds in tire wear particles are organic and elemental carbon, accounting for over 50% [57]. This is due to the rubber used in tires, largely composed of carbon. The wear of tires can produce particles containing many metals, as well as cationic and anionic compounds.

3. Adsorption Capability of TDA

Park J [19,20] found that organic compounds permeated through a gasket used in water distribution. The mechanisms were adsorption (partitioning), diffusion, and desorption (partitioning). Since the gasket composition is similar to that of a tire, Park postulated that the tire could be a good candidate for removing organic compounds. Park J [20] tested end-of-life tires’ organic compound sorption capacity. From the batch sorption isotherm experiments, Park J [20] found that tire chips had a sorption capacity of 1.4% to 5.6% of granular activated carbon on a volume basis. Sorption equilibrium was reached in 2 days for tire chips of 0.6, 1.3, and 2.5 cm sizes. The capacity of tire chips to adsorb organic compounds in a multi-solute system was similar to that in single-solute systems. However, 3.4% to 7.9% of the organic compounds sorbed in tire chips could be desorbed.

Motivated by the study of Park J [20], Crouthamel B [21] and Kershaw D [22] found that tire rubber had a capability of adsorbing aromatic hydrocarbons, such as ethyl benzene, toluene, and o-xylene, confirming the findings by Park J [19]. Since then, several studies [20,21,22,23,24,25,26,27,28,58] have explored the ability of end-of-life tires to sorb organic compounds and validated earlier study findings. In a study focused on a leach field, Finney B [26] observed that the chemical oxygen demand (COD) removal rate in a TDA leach field was greater than that in a rock leach field. The COD in the effluent from the TDA leach field was 10 to 30% less than in the influent. This reduction was credited to the biological growth that was encouraged by the TDA. Comparable outcomes were recorded for sulfate and total phosphate.

Park J [24] simulated landfill leachate in contact with tire chips in an 800-day laboratory batch experiment and found that tire chips could reduce mercury in landfill leachate by 43%. This finding was supported by Knocke W [59], who found that tire chips could remove arsenic, selenium, and sulfur from landfill leachate. They also found that the equilibrium sorption capacity of tire chips was comparable to that of powdered activated carbon (PAC), but the optimum pH range for sorption was 5~7.

In the second experiment, Park J [24] used test cells to simulate a landfill leachate collection system. The tire chips were found to be more effective at sorbing oil, grease, arsenic, cobalt, lead, and nickel than gravel. It was found that the concentrations of BTEX (benzene, toluene, ethylbenzene, and xylenes) were higher in the gravel-lined cell than in the tire chip-lined cell, suggesting that three of the four BTEX compounds were removed by tire chips. From these findings, Park J [24] concluded that tire chips could be used as a sorbent to remove contaminants at sites with high contamination levels. The use of tire chips as a landfill drainage medium was further explored by Edil T [25]. They found that tire chips could significantly reduce VOC concentrations in landfill leachate. This not only reduces the migration of these compounds from the landfill site, but it may also help to preserve the hydraulic conductivity of the clay liner of the landfill. Aydilek [60] also investigated the sorption of toluene in a landfill experiment. They found that tire chips could effectively remove toluene from landfill leachate.

In summary, the research on using tire chips as a sorbent for environmental cleanup is promising. Tire chips are effective at removing various contaminants from landfill leachate, including mercury, sulfur, arsenic, selenium, oil, grease, cobalt, lead, nickel, BTEX, and toluene. Further research is needed to optimize the use of tire chips for environmental cleanup. Still, the results of the studies to date suggest that tire chips have the potential to be a valuable tool for cleaning up contaminated sites.

Moreover, the rubber in TDA can remove pesticides, and the steel wire can remove phosphorus by precipitation. The required thickness of the TDA layer for mitigating the movement of pesticides and fertilizers through runoff and infiltration can be calculated using the following equations [27]:

$${d=1.00\times 10}^{-5}\times \frac{M_a}{(1-n){\cdot \rho }_t}$$

(1)

where

$M_a=$ required tire mass per unit area, kg/ha;

$n=$ porosity of the TDA layer; and

${\rho }_t=$ density of TDA, g/cm³.

$$M_a=\frac{f\cdot Q_r\cdot t_d\cdot {\rho }_t\times {10}^3}{(1-f)\cdot K_d}$$

(2)

$$K_d=f_{oc}\cdot K_{oc}$$

(3)

where

$f=$ fraction of a compound to be removed;

$Q_r=$ infiltration rate, m³/ha/yr;

$t_d=$ design life of the TDA layer, yrs;

$K_d=$ soil-compound partition coefficient, L/kg;

$f_{oc}=$ fraction of organic carbon in TDA; and

$K_{oc}=$ soil organic carbon-water partition coefficient, L/kg.

Given a rainfall rate of 1250 m³/ha/yr (mm/yr), a mass density of tire chips is 1.22 g/cm³, and the porosity of the tire chip layer is 0.4. The thickness of the tire chip layer needed to achieve a 90% removal of organic compounds over various design lives can be calculated using the given equations. The calculations for benzene, trichloroethylene (TCE), m-xylene, and pentachlorophenol are illustrated in Figure 2 [61]. Similar calculation methodologies could also be developed for removing heavy metals, halogenated hydrocarbons, and phosphorus.

When the landfill is engineered for a lifespan of 30 years, the necessary thicknesses of the tire layer for achieving the 90% elimination of VOCs such as benzene, TCE, m-xylene, and pentachlorophenol are roughly 50 cm, 28 cm, 15 cm, and 1 cm, respectively. Benzene and m-xylene readily biodegrade, requiring a thinner layer for adsorption. Polar compounds necessitate a thicker adsorption layer, yet their ready biodegradability suggests that a TDA adsorption layer is unnecessary. Generally, compounds with an aqueous solubility of over 8000 mg/L are easy to biodegrade. Since halogenated hydrocarbons are persistent but readily adsorbed to tire chips, anaerobic dehalogenation or adsorption are major removal mechanisms. Thus, a 30 cm thick leachate collection layer should be sufficient to remove toxic synthetic organic compounds (SOCs).

It was also found that TDA could effectively remove biological oxygen demand (BOD), chemical oxygen demand (COD), and essential nutrients such as nitrogen and phosphorus by providing a surface for microbial growth [26,39,40,47,48]. Scheels and Park [18] found that a ground tire-packed biofilter bed removed hydrogen sulfide even at −40 °C due to hydrophobic and thermal insulation properties.

Wanielista M [42] conducted a study to assess the feasibility of using waste tires for contaminant removal in stormwater management systems, drainfields, and water conservation systems in Florida. Three 11.5-inch inner diameter 5 ft tall septic columns were tested: (1) sand + tire crumb + paper (STP) and (2) sand + tire crumb + sawdust (STS). For the 4.5ft bottom sampling ports, the average removal efficiencies of STP and STS for about five months are summarized in

Table 2. Performance of septic columns mixed with sand and tire crumb with paper or sawdust—extracted from [42].

Contaminants	Sand + Tire Crumb + Paper (STP)	Sand + Tire Crumb + Sawdust (STS)
Ammonia-N	96.2	91.2
Nitrate-N	90.1	97.0
Organic-N	98.8	99.5
Total N	98.6	98.3
Ortho-P	97.8	98.8
Total P	99.9	99.9
BOD5	77.0	92.7

for nitrate, organic nitrogen, ammonia, total nitrogen, ortho-phosphorus, total phosphorus, and BOD₅.

Garcia-Perez A [41] tested a tire-chip-packed constructed wetland for treating high-strength wastewater from a bakery and cafeteria for three years.

Table 3. Treatment efficiency of the horizontal-flow constructed wetland—extracted from [42].

Water Quality Parameters	Subsurface Constructed Wetland, Mean ± SD		Efficiency, %
	Influent	Effluent
Fat, oil and grease	1100 ± 150 mg/L	5.3 ± 0.5 mg/L	99
Fecal coliforms	8 × 105 ± 13 × 105 MPN/100 mL	2 × 104 ± 3 × 104 PN/100 mL	97
BOD5	2373 ± 1445 mg/L	197 ± 610 mg/L	92
Ammonia-N	28.7 ± 11.8 mg/L	3.6 ± 4.4 mg/L	87
TSS	383 ± 245 mg/L	117 ± 69 mg/L	69
TP	5.2 ± 1.9 mg/L	1.8 ± 1.5 mg/L	65
TKN	57.3 ± 29.7 mg/L	24.9 ± 13.2 mg/L	57
TN	57.5 ± 29.6 mg/L	25.1 ± 10.9 mg/L	56
K	31.9 ± 8.9 mg/L	20.5 ± 9 mg/L	36
Nitrate-N	0.24 ± 0.29 mg/L	0.19 ± 0.27 mg/L	NA
DO	1.4 ± 0.9 mg/L	1.5 ± 1.3 mg/L	NA
pH	3.9~6.2	6.3~7.6	NA
Water temperature	23.6 ± 2.9 °C	17.4 ± 6.3 °C	NA

TSS: Total Suspended Solids; TKN Total Kjeldahl Nitrogen.

shows the treatment efficiency of the horizontal flow constructed wetland for various water quality parameters.

The fat, oil and grease removal efficiency was 99%, and BOD₅ removal efficiency was 92%, meaning it performed equal to or better than typically constructed wetlands. While horizontal-flow gravel-packed constructed wetlands removed 40~60% of phosphorus, tire-chip-packed ones removed 65%. Through efficient nitrification, ammonia-N, TKN, and TN were removed by 87% (from 29 to 4 mg/L), 57% (from 57 to 25 mg/L), and 56% (from 58 to 25 mg/L), respectively. The wetland’s ability to buffer pH effectively is noteworthy, as it met the typical wastewater discharge limit of pH 6–9.

Olyphant G A [43] evaluated the suitability of tire chips (average size of 2 inches) as structural fill in residential onsite septic distribution systems. Conventional rock aggregate was used as a control in the absorption field. While DOB₅ and NO₃-N were lower at the tire chip site than at the control site, total suspended solids, total coliform, aluminum, iron, and magnesium were higher at the tire chip site. However, the zinc, chromium, and copper concentrations were consistently below the drinking water standards.

Per- and poly-fluoroalkyl substances (PFAS) are synthetic organofluorine chemical compounds that are environmentally pervasive globally. These highly mobile, persistent chemicals do not naturally break down in the environment and bioaccumulate in people and wildlife. The U.S. Center for Disease Control stated that high levels of PFAS might cause health consequences, including certain types of cancers and decreased vaccine response in children.

There have been various approaches to removing PFAS from water. Johnson R [62] concluded that 74 to 99% of PFAS could be adsorbed onto iron-rich sand solid surfaces with part per million (ppm) levels of PFAS. Blotevogel J [63] estimated that the half-life reduction step of PFOA (with zero-valent zinc) is eight years. Accordingly, PFAS can be defluorinated by iron and zinc in steel wires and TDA rubber as reducing agents. The mechanism would be the same as zero-valent iron, which is proven to remediate halogenated hydrocarbons through reductive dehalogenation [64].

Overall, through combined mechanisms of zero-valent metal reductive dehalogenation, biological degradation using autotrophic and other PFAS-consuming anaerobic bacteria attached to the TDA surface and the sorption of PFAS-related chemicals onto TDA rubber, it is anticipated that TDA-baled blocks can remove more than 50% of PFOA and PFAS chemicals from landfill leachate, composting facility runoff, and other stormwater collection and infiltration systems.

6PPD belongs to the p-phenylenediamine chemical family and is categorized as an antiozonant, a protective agent against ozone for rubber compounds in tires. This addition helps enhance the longevity of the tires. According to the U.S. Tire Manufacturers Association (n.d.), antioxidants, antiozonants, and curing systems constitute 14% of passenger-tires and 10% of truck tires. Recently, it has been determined that the 6PPD-quinone chemical is present in harmful concentrations for coho salmon in runoff from roadways and in the surface water that receives the runoff after stormwater discharge [65].

6PPD’s half-lives due to primary transformation are 2.9 h in biologically active river water, 3.9 h in sterile river water, and 6.8 h in sterile deionized water under aerobic conditions by biotic and abiotic degradation [66]. Thus, 6PPD transforms into other transformation products relatively fast. The octanol–water partition coefficient (K_ow) is the concentration ratio of a chemical in octanol and water at equilibrium at a specified temperature, normally 25 °C. The log K_ow of 6PPD is 4.68 [66]. The log K_ow of 6PPD-quinone was estimated as between 5 and 5.5 [65]. The measured log K_ow was 4.30 ± 0.02 [67]. If the log K_ow of an organic compound is greater than 2, it is considered to be suitable for removal by adsorption by activated carbon [68]. Thus, 6PPD and 6PPD-quinone will be removed by organic carbon in the soil, tire rubber, and activated carbon readily.

Together with a layer of sand-TDA mixture surrounded by organic-matter-rich soil, TDA can address the 6PPD-related chemical pollution from road runoff. Empirical estimation has shown that 15 cm of TDA or TDA-baled block material could adsorb 99% of 6PPD from roadway runoff for 50 years. Park et al. [20,27] developed empirical equations for estimating tire rubber thickness to remove a fraction of organic chemicals, such as TDA, based on tire rubber’s physical and chemical properties. TDA comprises 75% to 80% weight for weight (w/w) of organic carbon material and 10 to 15% w/w of ferric material—steel wires. TDA attracts long-chain, non-polar organic chemicals, adsorbs heavy metals and organic compounds, degrades halogenated compounds, and precipitates nitrates.

In summary, the stormwater management system incorporated with TDA can adsorb heavy metals, SOCs, and nutrients such as nitrogen and phosphorus, with lower installation costs and ease of construction.

4. Contaminants Leached from End-of-Life Tires

End-of-life tires contain a wide range of additives, such as filler systems (calcium carbonate, carbon black, clays, silicas), stabilizers (antioxidants, antiozonants, and waxes), cross-linking agents (sulfur, accelerators, and activators) and secondary components (pigments, oils, resins, and short fibers) [69]. These tires may also contain other contaminants due to their exposure to other chemicals as a sorbent. Car tires have a variety of chemical classes, including dithiocarbamates, guanidines, phenolics, phenylenediamines phthalates, polyaromatic hydrocarbons (PAHs), sulfur donors, sulfonamides, thiazoles, thiurams, and heavy metals [70]. Nonetheless, as elucidated in the subsequent sections, the quantities of these chemicals leaching from tires are negligible. Consequently, these leached chemicals, when assessed against regulatory limits and water-quality-based tests, do not present any threat to either the ecosystem or human health [69,70,71,72,73].

In contrast to the worries surrounding tire chemicals, the hydrophobic properties of tires can transform end-of-life tires into effective adsorbents. This quality limits the migration of contaminants into the surrounding environment, thereby diminishing the likelihood of environmental damage.

4.1. Leaching Test Methods

The Toxicity Characteristic Leaching Procedure (TCLP) was used to determine concentrations of leached tire components. Magni S [74] conducted an in-depth analysis of tire components and secondary contamination chemicals and reported 31 PAHs, 31 metals, 8 benzothiazoles, 6 phenols, 15 phthalates, 9 hydrocarbon oils, 10 amine-nitrosamines, 9 halogenated organic aromatic compounds, 35 others, polymer contents, and ashes. However, under environmental conditions, the composition of the leachates will be quite different due to variable solubility in water and other chemical properties. Downs L [75] and Ealding W [76] used TLCP to assess the relative toxicity of the extracted leachate from TDA [29]. The TCLP is a method used to assess the mobility of organic and inorganic pollutants in liquid, solid, and multi-phase wastes. This process complies with the Resource Conservation and Recovery Act (RCRA). This regulation empowers the US Environmental Protection Agency (EPA) to regulate hazardous waste throughout its entire lifecycle, from production to disposal. However, it is important to note that TCLP test results should not be used to evaluate the environmental impact of TDA in civil engineering applications, as the test was designed to mimic the conditions of a municipal solid-waste landfill and to identify whether samples are hazardous waste for disposal purposes.

TCLP uses a solution with an acidic pH level of less than 5 to simulate the leaching potential for waste in a landfill setting. When evaluating the environmental impact of TDA in civil engineering applications, leaching tests should be within a pH range of 5.6 (rain) to 8. The World Health Organization [77] and the United States Environmental Protection Agency (US EPA) [78] have established that drinking water’s pH level should range from 6.5 to 8.5. This is to prevent the excessive concentration of dissolved pollutants that could be sourced from acidic waters and the accumulation of scale deposits that can result from alkaline water. Thus, groundwater at a pH < 3.5 does not meet the National Secondary Drinking Water Regulations (NSDWRs) regardless of zinc exceeding the Secondary Maximum Contaminants Levels (SMCLs) [79]. Therefore, analytical results, regulations, and site conditions must be carefully evaluated when TDA is approved for civil engineering applications. Based on the TCLP toxicity standards, regulatory limits for any target elements were not exceeded, and TDA was not found to be a hazardous waste [29].

The NSDWRs establish non-compulsory water quality standards. Hence, the US Environmental Protection Agency (EPA) does not enforce SMCLs. Instead, these standards, including taste, color, and odor, are set primarily for aesthetic reasons and are not considered a risk to human health.

4.2. Pathway of Compounds in Tires Leaching the Environment

Compounds used in tire manufacturing can leach into the atmosphere as gases, solids, and solutions. Organic compounds are volatilized into the air, but their health effects are minimal unless tires are stored in a confined space. Tire wear particles are a significant source of environmental microplastic pollution [80]. These particles increase the surface area of tires, which accelerates chemical leaching. Stormwater management systems are essential for minimizing microplastic and water contamination. TDA can be used to remove heavy metals and organic chemicals from stormwater runoff.

The concentrations of tire components leaching from end-of-life tires after exposure to groundwater or runoff are affected by pH, temperature, tire particle size [81], and contact time [23]. TDA used in civil engineering applications imposes the most negligible impact on the environment compared with ground and granular tires due to less surface area for leaching tire components. Figure 3 was developed to illustrate the pathway of tire components leaching from end-of-life tires. TWPs and crumb rubber are prone to exposure to ultraviolet (UV) lights and rainwater; thus, their leaching potentials are much greater than TDA and whole tires. Unlike tire wear particles, crumb rubber used in athletic fields, tire chips used in playgrounds, and open-dumped whole tires, TDA is used underground, covered by geotextile, gravel, and cover (asphalt). Open-dumped whole tires or tire-wear particles exposed to rainwater may leach tire components, and leached 6PPD may convert to 6PPD-quinone through accelerated autooxidation, e.g., sunlight (ultraviolet) and ozone. However, TDAs installed in civil engineering applications are anticipated to have less environmental impact because TDA is covered underground.

Once tire components leach, they interact with various media, such as soil, water, and organics in soil and water, and undergo biodegradation, abiotic degradation, volatilization, and adsorption. The residual tire components will flow to the waterbody, contacting aquatic species. Unlike tire wear particles, crumb rubber used in athletic fields, and tire chips used in playgrounds, ASTM Type B TDA is used underground, covered by geotextile, gravel, and cover (asphalt), as shown in Figure 4.

Local and state stormwater management regulations require the treatment of runoff generated across all redeveloped and newly constructed impervious surfaces. Due to site constraints, surface treatment options for the Woodbury site were limited, so TDA layers were used to collect, manage, and infiltrate stormwater into the ground beneath a parking lot area. This is considered one of the green best management practices (BMPs) that can be integrated around the site with features consisting of a green roof over a building, a rain garden within curb islands, and cisterns to capture roof drainage for water reuse, among other environmentally conscious measures. All TDA-implemented projects are used underground; thus, there is no ultraviolet exposure and human contact, implying concerns raised by other forms of end-of-life tires are irrelevant to TDA. The pathway of tire components to the environment is discussed in the following sections.

4.2.1. Leaching

TDA applications in civil engineering occur in uncontaminated areas, unless TDA is used to remove inorganic and organic contaminants through adsorption and chemical reaction as landfill leachate collection layer media or wastewater treatment media. Thus, results of tests performed under the condition violating the National Drinking Water Regulations (NDWRs) MCLs and SMCLs should not be used to determine the suitability of TDA for civil engineering applications since the water is already contaminated, such as in landfills and wastewater. The test results obtained under conditions that satisfy NDWRs and NSWRs are used to discuss the potential environmental impacts of TDA on the surrounding environment.

When tire components leach out, we encounter two conditions (Figure 2):

(1)

Water percolating through tires

Under unsaturated conditions, tires subjected to repeated dry–wet cycles were found to leach more heavy metals than under saturated conditions [5]. However, as the soil porosity increases, the leaching will be less due to a shorter contact time, while the mobility of leachate will increase.

(2)

Tires in the waterbody

Due to the longer contact time than unsaturated conditions, leaching tire components tend to have higher concentrations. Heavy metals exchange with, precipitate, or adsorb to other inorganics in soils. Most leached organic components biodegrade to CO₂ or other byproducts, adsorb to soil matrices, or are persistent in the environment [27].

Several factors, including particle dimensions, pH levels, and the ratio of solid to liquid (S/L), can influence the extent to which tire components leach out from the tires [82,83,84,85,86,87]. According to Selbes M [81], the size of processed scrap tires significantly affected the chemical leaching from end-of-life tires. However, for TDA, the leaching of inorganic constituents such as Al, As, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, and Se was found to be negligible, indicating that TDA is a safe and reliable resource for civil engineering applications as it does not release significant amounts of potentially hazardous compounds into the environment.

4.2.2. Transport in Soil and Water

Leached tire components adsorb onto organics in soils, biodegrade, precipitate, and are transported downstream. Heavy metals leached from tires react with the soil matrix; thus, their mobilities are low. Synthetic organic tire components are adsorbed to organics existing in soils since they are primarily hydrophobic. Polar compounds are readily biodegradable, so they do not significantly impact the environment even if leached. Some previous leaching studies have been conducted under worst-case scenarios, e.g., acidic or alkaline conditions. For civil engineering applications, this is highly unlikely, and tests should be designed to simulate conditions from rainwater or groundwater [61].

4.3. Metals Leached from Tires

Table 4. The range of metals in soils and tires [88]—modified.

Metal	Soils (mg/kg)	Tires (mg/kg)
Al	10,000~30,000	81~420
Fe	7000~55,000	2.12~533
Mn	20~3000	2
Cu	2~100	1.8–29.3
Zn	10~300	8378–13,494
Pb	2~200	1~160

compares metal concentrations commonly found in leached water samples with those in soils. Most metals in background soils have higher concentrations than tires, except for zinc. Thus, the leaching of heavy metals from tires is generally not a significant environmental concern.

Many tests have been performed to evaluate metal leaching from various tire sizes. The concentrations ranged from ‘not detected’ to ~10,000 mg/L. Jeong H [82] stated that Zn had the highest concentration among metals in all tire samples tested. The sequence of average metal concentrations was as follows: Zn, Cu, Pb, Sn, Sb, Ni, Cr, As, and Cd.

The concentration of heavy metals leached from tires can vary depending on factors such as the tire’s age, type, environmental conditions, and testing method. The concentration ranges of heavy metals leached from tires can be relatively low; some approximate ranges of heavy metal concentrations reported in various studies on leaching from tires were as follows:

• Lead (Pb): 0.01 to 3.3 mg/L
• Cadmium (Cd): 0.0007 to 0.03 mg/L
• Zinc (Zn): 0.9 to 38 mg/L
• Copper (Cu): 0.03 to 1.1 mg/L
• Nickel (Ni): 0.002 to 0.2 mg/L

While these concentrations may not pose significant risks to human health or the environment, continued exposure can increase the risk of health effects. Moreover, environmental guidelines have been established to reduce heavy metal leaching from tire components into the environment as part of scrap tire management.

From various published studies, the following metals would not affect groundwater quality: lead [26,75,89,90], copper [90,91,92], chromium [90,91], mercury [58,75], and cadmium [26,75].

TCTC [93], Downs L [75], and Ealding W [76] found that the metal concentrations in the leachate did not exceed the National Drinking Water Regulations (NPDWRs) and, thus, metals would not present a threat to the adjacent environment [91,94]. In addition, Humphrey D [57] and Maeda L [90] stated that TDA was not likely to lead to an increase in metals that meet the NPDWRs above the naturally occurring background levels in the nearby region [90].

In a leaching test, Han C [79] found that zinc concentrations exceeded NSDWRs SMCLs only when the pH was below 3.5. However, other contaminants did not exceed the SMCLs established to safeguard public drinking water consumption. Based on this research, the Minnesota Pollution Control Agency (MPCA) prohibited the placement of shredded tires below the water table. It is worth noting that rain and snow typically possess pH values near 5.6, well above the pH threshold 3.5. The World HealthOrganization [77] and the United States Environmental Protection Agency [90] have advised maintaining drinking water pH between 6.5 and 8.5 to minimize dissolved contaminants from acidic waters and prevent scale deposits from alkaline water. Consequently, results obtained outside the pH range of 6.5 to 8.5 should not be considered when assessing the suitability of TDA for civil engineering applications.

Zinc leached from tires at a higher rate, at lower pH [83,95] and with a larger surface area per mass [87], but the concentrations varied widely [87]. Hartwell L [85] and Nelson S [96] observed that zinc leached at 2.5~2.7 mg/L and reached a steady state in ~3.5 h near the EPA freshwater limit of 0.12 mg/L with crumb rubber.

Edil T [89] observed zinc, manganese, and iron leaching in the field lysimeters beneath the areas containing tire chips. The metals surpassed the Preventive Action Levels (PALs) set by Wisconsin for several metals on several occasions. Based on further testing, Bosscher P [97] concluded that the likelihood of tire chips impacting groundwater quality was improbable.

The Wisconsin Groundwater Preventive Action Levels (PALs) were exceeded in the leachate for several metals. However, it was concluded that the possibility of tire chips affecting groundwater quality was improbable [97].

Selbes M [81,98] found minimal leaching of all the monitored parameters under neutral pH conditions from tire chips, sized from 2.5 cm × 2.5 cm to 15 cm × 5 cm. In addition, low levels of aluminum were also detected [85,91,93,99].

Manganese was found to be released from TDA [75,82,89,92,99,100,101,102,103], and in a few cases, the concentrations exceeded its NSDW Regulations [76]. In a four-year field study [92], the manganese concentration decreased over time and approached background concentrations in four years. Selbes M [81] found that iron and manganese leaching rates were comparable across all sizes of tire chips and for all leaching solutions. However, smaller tire chips had higher dissolved organic carbon and nitrogen leaching.

Cadmium levels were typically at least two orders of magnitude lower than the NPDWRs MCL of 0.005 mg/L, implying that released cadmium from TDA is not a significant concern. Zinc levels were initially an order of magnitude less than its NPSWRs SMCL of 5 mg/L and reduced by two orders of magnitude by the end of the study. Iron frequently exceeded its NSDWRs SMCL of 0.3 mg/L in most samples, and manganese concentrations often surpassed the NSDWRs SMCL of 0.005 mg/L. Previous research has shown that leachate from TDA typically elevated cadmium, iron, manganese, and zinc concentrations [26,75,92,95,103,104].

Barium sulfate is a common additive in rubber products because it enhances its aging resistance and weatherability [105]. Therefore, it is possible that TDA could contain elevated levels of barium. However, studies have shown that the concentrations of barium in TDA leachate are below the NPDWRs MCL of 2 mg/L, even in the sampling well located in the TDA trench [92]. As other studies have found [89,91], TDA would not increase barium concentrations to a concerning level.

As, Pb, Cd, and Sb have been shown to infiltrate from the waste zone by about 0.5 to 1 m in soil and about 0.5 to 3 m in the aqueous phase, indicating a rapid decrease in heavy metal concentrations [105]. Furthermore, heavy metals have been shown to migrate at a total distance of only 10–20 cm, compared to 130 cm for dissolved constituents [36,106].

In the case of the TDA-incorporated stormwater management systems, the impacts on the surrounding environment will be better or equal to typical stormwater management systems using earthen materials. However, if there is a concern with leached metals from TDA, 2~10 ft (0.6~3 m) of a soil buffer layer is recommended based on the field observations and a safety factor.

4.4. SOCs Leached from Tires

Halsband C [66] identified PAHs (pyrene and phenanthrene), benzothiazoles (benzothiazole and 2-mercaptobenzothiazole), phenols (4-tert-octylphenol and 3-tert-butylphenol), methyl stearate, quinolines and amines (N-(1,3-dimethylbutyl)-N′-phenyl-1,4-benzenediamine and diphenylamine) from crumb rubber granulate.

VOCs are incorporated during the tire production process to enhance the mixing of rubbers and promote elasticity [107]. Miller W [107] cited that approximately 8% of these compounds were sorbed into the rubber. As discussed in the previous section, TDA can adsorb SOCs if present at higher levels than potentially leached levels.

In a study by Humphrey D [100], VOCs and semi-volatile organic compounds (SVOCs) were examined at two sites where TDA was situated above the water table and three locations where TDA was positioned below the water table. The data collected at the three locations where the TDA was placed beneath the groundwater table indicated that even a minor flow through about 0.6 m (2 feet) of soil typically decreases the concentrations of the examined compounds below the detection limits of the test method.

Most studies investigating organic-compound leaching from TDA in typical civil engineering applications implied that TDA showed negligible effects on the surrounding groundwater quality [26,60,89,92,94,102,108]. Compared to the available literature on inorganic compounds, studies on organic compounds cover a more comprehensive array of constituents.

Humphrey D [92] conducted a four-year field study and detected the cis concentration of cis-1,2-dichloroethene in most samples in a four-year field study at a site where samples were taken below the groundwater table at the 0.6 m downstream of the TDA trench. The highest concentration of cis-1,2-dichloroethene was 9.8 µg/L, slightly above the California Maximum Contaminant Level (MCL) of 6 µg/L.

Several studies have detected benzene in TDA or tire-derived-product (TDPs) leachate [92,94,107]. Benzene was suspected of accumulating from gasoline contamination on tire chips. As expected, benzene leached out when first exposed to a water-based environment [96]. Rinsed tire chips had lower benzene concentrations than unwashed tire chips [75].

Miller W [107] observed that toluene was either not detected or detected at low concentrations initially but increased exponentially during a three-month laboratory experiment. However, the field research did not detect toluene at low concentrations. Gunter M [94] reported either non-detect or low concentrations (0.0012 mg/kg of the tire) at the beginning of the laboratory experiment. Seven months later, toluene was still not detected. Humphrey D [92] conducted a four-year field study in which TDA was placed below a ground table. They reported that toluene was well below the NPDWRs MCL of 100 µg/L. Finney B [26] conducted a 15-month leach field study and did not find toluene in the TDA effluent.

Aniline, a compound used as an anti-degradant in rubber manufacturing [109], has been detected in studies where tire shreds were placed below a groundwater table [92,102,110]. In a four-year study, concentrations ranged from <10 µg/L to 380 µg/L [102]. Aniline was detected during the first three years of the study but was never detected downstream of the tire shred trenches. Furthermore, the compound was not detected at any of the sampling locations in the last samples of the four-year study [92].

Benzoic acid was detected but at low concentrations [92]. It was not detected when measured downstream of the TDA trench below the groundwater table, indicating strong immobility. Thus, it was concluded that it would not impact groundwater quality. Phenol was found during the first three years of a study where tire shred trenches were placed beneath the groundwater table, but the compound was not detected downstream of the trenches or in the final samples of the four-year study [92]. Burnell B [109] reported that phenol concentrations were below the detection limit of 9 µg/L in all samples in a sewage disposal drain field study.

Research investigating the migration of TDA leachate under basic conditions detected Total Petroleum Hydrocarbons (TPH) and PAHs [93,95]. However, Edstrom R [102] did not detect any carcinogenic PAHs downstream in any of the samples.

It can be said that SOCs leached from TDA do not cause any noticeable impacts on aquatic species and human health due to minimal leaching and its immobility in groundwater.

5. Fate of Leached TDA Components

5.1. Fate of Leached Metals from TDA

Leached heavy metals from end-of-life tires are detected in surface water and groundwater samples. Hoppe E [4] conducted a 10-year experimental project to evaluate the potential of using TDA in highway embankment construction in Virginia. The project used approximately 1.7 million discarded tires. Over the decade-long monitoring period, groundwater samples were taken from the embankment’s upstream (control well) and downstream (test well). Based on environmental and engineering evaluations, the TDA-filled embankment proved satisfactory. Consequently, the TDA-filled embankment was recommended as a viable and environmentally friendly way to manage end-of-life tires. This study’s results were used to discuss the fate of the most detected heavy metals, Al, Cu, Fe, Pb, Mn, and Zn, leached from TDA.

5.1.1. Aluminum (Al)

There is no NPDWRs MCL for aluminum. When aluminum metal contacts with water, it immediately reacts by forming a thin layer of aluminum oxide. Aluminum in leached TDA tends to be filtered out by soil matrix and remain in solution rather than precipitating or adsorbing onto soils or other materials. This means that aluminum can be transported in surface water or groundwater, and its fate will depend on various factors such as the flow rate and chemical characteristics of the receiving water bodies.

5.1.2. Copper (Cu)

The NPDWRs MCL for iron is 1.3 mg/L. Copper (Cu) is held in soils via exchange and specific adsorption processes. In typical native soil concentrations, copper precipitates are not stable. However, this might not apply to waste-soil systems where precipitation could be a key retention mechanism. Cavallaro N [111] proposed that an exchange phase with clay minerals could be a sink for Cu in noncalcareous soils. In contrast, in calcareous soils, the specific adsorption of Cu onto CaCO₃ surfaces might regulate the Cu concentration in the solution [111,112,113,114]. Cu has a high affinity for soluble organic ligands, which can significantly increase its mobility in soils. As indicated in the adsorption step, Cu is more extensively adsorbed by soils and soil components than other studied metals, except for Pb. The retention and mobility of Cu in soils are complex and driven by various factors, such as the soil type, pH, and organic ligands.

Copper is not likely to pose a threat to groundwater quality once leached from TDA [91,92].

5.1.3. Iron (Fe)

The NSDWRs SMCL for iron is 0.3 mg/L. Iron is removed from soils by chemical precipitation, adsorption, complexation, redox reactions, microbial activity, and plant uptake. The effectiveness varies depending on the specific soil conditions in the soil, including organic matter content, pH, redox potential, and other present ions and compounds. Average iron concentrations were 0.47 mg/L for the control well and 0.45 mg/L for the downstream test well. However, the peak level of 1.20 mg/L was recorded in the upstream control well. The t-test showed no statistically significant difference between the iron levels for the two wells. Unlike other studies that reported higher iron levels in the leachate from tire shreds, this study found no statistically significant rise in iron levels in the downstream test well [4].

5.1.4. Lead (Pb)

The NPDWRs MCL for Pb is 0.015 mg/L, or 15.0 µg/L (parts per billion). Soluble Pb added to the soil is immobilized through various chemical reactions with soil components such as clays, carbonates, hydroxides, organic matter, phosphates, and sulfates, reducing Pb solubility. When the pH exceeds 6, Pb either gets adsorbed onto clay surfaces or forms lead carbonate. Research indicates that Pb was retained the most by soils and soil constituents among all the trace metals tested, although these studies were conducted in well-defined, simple matrices such as 0.01 M CaCl₂. Nonetheless, Pb sorption could decrease when complex ligands and competing cations are present [115,116]. Lead has a strong affinity for organic ligands, which can significantly increase the mobility of Pb in soil.

Hoppe E [4] found that the average Pb concentration was 7.7 g/L for the upstream control well and 9.0 g/L for the downstream test well. Out of the six Pb analyses performed for each well, three resulted in “non-detects” for the upstream control well, and two resulted in “non-detects” for the downstream test well. One reading for the downstream test well was above the MCL, but the t-test showed no significant difference between the Pb levels in both wells. The average Pb levels for both wells remained below the MCL. The two wells had no statistical differences [4]. Therefore, lead is not likely to threaten groundwater quality [26,75,91,94].

5.1.5. Magnesium (Mn)

Mn does not have an established MCL or SMCL. Hoppe E [4] found that the average Mn concentration was 4.90 mg/L for the upstream control well and 5.05 mg/L for the downstream test well. The downstream test well initially had a slightly higher concentration than the upstream control well. However, the upstream control well had a higher concentration than the downstream test well for the last two readings. The t-test showed no statistically significant difference between the Mn levels in the two wells. Hence, it can be inferred that Mn would not leach from TDA into the groundwater.

5.1.6. Zinc (Zn)

The NSDWRs SMCL of Zn is 5 mg/L. Zn is a naturally occurring element in soils, typically found in concentrations of 10~100 mg/kg [117]. The presence of Zn in TDA can pose an environmental concern, as it accounts for 1~2% of the tire’s total weight. Thus, Zn has the potential to leach into water sources, such as stormwater runoff from roadways [118]. Factors including pH, TDA size, and leaching time were all important considerations affecting the amount of leachate [86].

Clay minerals, carbonates, or hydrous oxides readily adsorb Zn in soils. This adsorption to soil surfaces increases with pH. When the pH exceeds 7.7, it generates hydrolyzed species that are adsorbed to soil surfaces and adhere easily. This means that Zn is less likely to move around in the soil and end up in nearby water sources. Zn can also attach to inorganic and organic molecules, changing the soil’s interaction. This can affect how much zinc the soil absorbs and how much might end up in water sources. Zn’s relatively high solubility means that precipitation is not an effective way to keep it from leaching out of the soil.

Hoppe E [4] detected Zn only in two readings, one each in the control and downstream test wells. The average concentration was 0.13 µg/L in the upstream control well and 0.12 µg/L in the downstream test well. These findings were somewhat unexpected, considering that other studies had reported elevated Zn concentrations. The Zn concentrations were very low and statistically identical between the upstream and downstream wells.

Recent reports have addressed the issue of Zn in urban receiving waters. When evaluating the impact of leachate from rubberized hot-mix asphalt on Zn accumulation in stormwater runoff from roadways, Finney B [119] found that rubberized hot-mix asphalt (RHMA) was responsible for only a minor portion of the Zn concentration in the runoff. The primary sources along the roadway were tire wear particles and galvanized materials. The report proposes developing methods to capture these contaminants on the road before they enter water bodies. While tire wear may contribute some Zn to surface water, it is likely to make up only a small percentage (5~10%) compared to other Zn pollution sources in the environment, such as industrial and commercial activities. High zinc concentrations can be toxic to aquatic organisms, although it is generally not considered a threat to human health in runoff [120].

5.2. Fate of Leached SOCs from TDA

The fate of leached SOCs (SOCs) from TDA is affected by several factors, including the specific characteristics of the TDA, the types of SOC present, the environmental conditions, and the time passed since being introduced into the environment.

One of the most well-known SOCs is Polychlorinated Biphenyls (PCBs). After an experiment performed by Kellough R [121], placing shredded and whole tires in a tank for 60 days, no PCBs or organochlorides (OCs) were detected. Methyl Isobutyl Ketone (MIBK) is another common volatile organic compound (VOC) used in tire manufacturing. After passing through a TDA fill system, the concentrations were lower than before and, in most cases, lower than regulatory standards. The system also effectively prevented the leaching of benzene [90].

In the case of benzene, a naturally occurring VOC found in TDA, studies conducted by Gunter M [94] and Miller W [107] suggested that benzene concentrations in the leachate decrease rapidly and exponentially over time. Benzene can accumulate on TDA from gasoline pollution, so the initial high benzene concentration is likely from the first exposure to water. Over time, benzene concentrations in the leachate are expected to decrease to insignificant concentrations due to several processes, such as adsorption to soil particles, biodegradation, and volatilization. Acetone has also been detected in tire shred studies [60,92,94], but due to the low concentrations and readily biodegradable nature, acetone would not affect the groundwater quality [92].

Another important SOC to consider is pesticides. In a study by Park J [27] involving 51 pesticides, it was predicted that 37 of them would be removed by a tire rubber layer thickness of 20 cm. Moreover, when pesticides pass through a layer of tire rubber, these substances are sorbed onto the tire rubber and slowly desorb or degrade, suggesting recovery of tire rubber sorption capacity.

Park J [20] performed a study to evaluate the retardation of the movement of VOCs through a sand–bentonite mixture by adding shredded tire chips. The results indicated that tires could supplement the modern landfill clay liner system to mitigate VOC transport more than conventional engineering containment systems.

Previous studies [101,122,123] that suggested leachate from TDA fill may have toxic effects when used under the groundwater table did not consider a prior dilution before mixing with surface water or the possibility that the leachate could pass through the soil. These studies reported reduced survival rates following leachate exposure under the groundwater table, but these would be different conditions for most TDA applications in civil engineering [92].

TDA leachate is influenced by variables such as the distance from fill placement, target species, TDA size, and location regarding the groundwater table. Sheehan P [101] used a groundwater model to estimate that a 3 m buffer zone between the tire shred fill and the closest surface water body is adequate to ensure the safety of aquatic organisms. A 3.0 m buffer between a tire shred fill and the nearest surface water would provide sufficient distance for aquatic life protection [92]. Long-term research is needed to better understand the fate of different types of SOCs and their potential environmental impacts.

5.3. Impact of Leached Tire Components on Groundwater Quality

TDA has been found to exert a minimal influence on groundwater quality. This section reviews the findings of published reports discussing TDA’s effect on groundwater quality using mainly the 10-year study results by Hoppe E [4]. Assessing characteristics such as hardness, total organic carbon (TOC), total organic halides (TOXs), and specific conductance allows a better understanding of water quality and potential risks associated with TDA use. NSDWRs SMCLs are non-enforceable guidelines for contaminants that may affect the aesthetic quality of water, such as taste, odor, and color. MCLs are enforceable standards for contaminants such as bacteria, viruses, and certain chemicals that pose a significant risk to public health. Both are mentioned to compare tested levels with the regulatory standards set by the US EPA.

5.3.1. Hardness

Hardness is not regulated in the NPDWRs and NSDWRs, but total dissolved solids (TDS) are regulated at 500 mg/L. High levels of hardness can affect the taste and appearance of water. There was no statistically significant change in hardness attributable to tire shreds’ pH. The hardness averaged 24.60 mg/L as CaCO₃ in the upstream control well and 23.98 mg/L as CaCO₃ in the downstream test well. The pH of the upstream control well averaged 4.82, and that of the downstream test well averaged 4.30. The confidence limit of no change in pH between wells was closer to 95% instead of being routinely greater than 99% with the other variables. Still, there was no statistically significant change in pH between the wells. Iron and zinc concentrations are expected to increase in an acidic (pH < 7) environment, but the concentrations were much lower than expected [4]. Edil T [61] observed the increase in hardness over time while none of the elements (As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Se, Na, and Zn) exceeded the regulations in simulated laboratory leaching studies.

5.3.2. Total and Dissolved Organic Carbon (TOC and DOC)

There is no MCL for TOC in the NPDWRs and NSDWRs. TOC is a measure of organic matter present in the water. Elevated levels can indicate the presence of microorganisms or bacteria, which can affect the potability and safety of the water. The average TOCs in the upstream control and downstream wells were 1.62 and 1.0 mg/L, respectively. A total of 32 samples were analyzed for each well. The upstream control well had 16 non-detects, while the downstream test well had 28 non-detects. There was no statistical indication that tire shreds were increasing the level of total organic carbon in the groundwater [4]. Selbes M [81] found that crumb rubber and tire chips leached DOC of 12 and 2.7 mg/kg tire, respectively. Edil T [61] observed a TOC increase of 150~220 mg/kg tire in a laboratory batch test using 266-L stainless steel tanks in one year. However, the increase may be due to microbial growth.

5.3.3. Total Organic Halides (TOXs)

There is no regulatory limit in the NPDWRs and NPSDWRs for total organic halides. TOXs are a group of organic compounds that may be present in the water due to pollution. High levels may indicate pollution and can pose a health risk. The average TOX in the upstream control well was 0.033 mg/L; in the downstream test well, it was 0.30 mg/L. A total of 32 samples were analyzed for each well. The upstream control well resulted in 24 non-detects and the downstream test well accounted for 28 non-detects. There was no statistical indication that tire shreds were increasing in level in the groundwater [4].

5.3.4. Specific Conductance

Electrical conductivity serves as a gauge for the ionic activity and content in water. A greater concentration of dissolved ions results in higher conductivity. The presence of inorganic dissolved solids such as chloride, nitrate, sulfate, and phosphate anions (negatively charged ions) or aluminum, calcium, magnesium, and sodium cations (positively charged ions) influence the conductivity in water. The basic unit for measuring conductivity is the mho (inverse of an ohm) or siemens. Specific conductance, the resistance’s reciprocal, is typically reported in mhos/cm². The upstream control well had an average value of 143.9 mhos/cm², while the downstream test well recorded an average of 135.6 mhos/cm². No statistical evidence indicated that tire shreds influenced the specific conductance level in the groundwater [4]. It is unlikely that TDA would impact specific conductance or the concentration of nitrate, oil and grease, and sulfate in the leachate. Electrical conductivity serves as a measure of water’s ionic activity and content. The higher the ionic (dissolved) constituent concentration, the higher the conductivity. Conductivity in water is affected by the presence of inorganic dissolved solids such as chloride, nitrate, sulfate, and phosphate anions (ions that carry a negative charge) or sodium, magnesium, calcium, iron, and aluminum cations (ions that have a positive charge). The basic unit of conductivity measurement is the mho (inverse of an ohm) or siemens. Specific conductance is the reciprocal of the resistance. Specific conductance, reported in mhos/cm², has an MCL of 1600. The average value for the upstream control well was 143.9 mhos/cm², whereas the data from the downstream test well averaged 135.6 mhos/cm². There was no statistical indication that TDA affects the level of specific conductance in the groundwater [4]. TDA is not likely to affect specific conductance or the concentration of nitrate, oil and grease, and sulfate in the leachate [92].

5.4. Impact of Leached Tire Components on Aquatic Species and Human Health

Assessing the impact of leached tire components on aquatic species and human health is an essential area of concern. A straightforward interpretation of their toxicity is needed to take appropriate measures against the possible risks. According to Gualtieri M [83], tire-wear leachates were less toxic than those from other rubber samples in their tests. Specifically, it was reported that the 48 h EC50 (the concentration of a substance that results in a 50% reduction in the growth or reproductive capabilities of a test organism) of the tire wear leachates was approximately 100 times higher than the EC50 of the most toxic rubber samples they tested. This suggests that the leachates from tire wear may be less harmful to the environment and have a lower toxicity rate than other types of rubber [65].

Wanielisa M [42] conducted aquatic toxicity tests for tire crumbs with fathead minnows and concluded that tire crumbs are not toxic when tested with tap and distilled water. Tire crumb filtrate increased the survival of fathead minnows to greater rates than that experienced in the control chambers.

In field studies, Maeda R [45] found that using TDA as a fill material saturated with water did not negatively affect water quality in the surrounding area in field tests. The study found that a TDA-soil system can remove iron, manganese, zinc, cadmium, methyl isobutyl ketone, benzene, and phosphate from stormwater runoff. The concentration of metals in the leachate depended on the mass fraction of exposed and free metal in the TDA. TDA with more metal available for oxidation may result in higher concentrations of metals in the leachate. Nevertheless, whether seasonally or constantly saturated, TDA fills are unlikely to impact the neighboring water bodies’ quality negatively. This implies that TDA could be a sustainable option instead of traditional fill materials like gravel or sand for stormwater management purposes while avoiding harm to groundwater quality. Using TDA might reduce the possibility of groundwater pollution while providing a cost-effective approach to managing stormwater runoff. The study also suggested that using the TDA–soil system can effectively remove several pollutants from urban stormwater runoff, including cadmium, iron, lead, zinc, and organic compounds like acetone, oil and grease. The research indicated that TDA could be responsibly utilized as a fill material, without detrimentally affecting the quality of the water bodies it drains into over short and long timeframes. More research is needed to evaluate the environmental impact of TDA on groundwater quality, particularly in the long term; however, the study’s findings gave valuable insights into the potential environmental advantages of using TDA for stormwater management [45].

Halsband C [66] investigated tire crumb rubber leaching in marine environments. They showed that toxicology studies were conducted on tire wear particles (TWPs) and crumb rubber granulates (CRGs) in various aquatic environments with different species. Nonetheless, results significantly varied due to disparities in tire composition, the methods employed for leachate generation, and species sensitivity [87,124]. The precise elements in TWP and CRG leachates that trigger toxic responses in aquatic ecosystems are yet to be completely identified. Moreover, more uniform procedures are necessary for generating leachates, characterizing their chemical composition, and assessing their potential hazard, which currently makes comparing toxicity data for CRGs/TWPs challenging. It was proposed that existing regulations for soluble contaminants could be modified to establish a leachate guideline [125]. Furthermore, developing methods to distinguish between particle effects and those derived from additive chemicals is essential [87,124,126].

The US EPA’s Federal Research Action Plan (FRAP) on tire crumb rubber characterization [127] suggested limited human exposure to chemicals in tire crumb rubber when released into the air or simulated biological fluids. Only trace quantities of metals were discharged into simulated biological fluids, and the release of numerous organic chemicals into the atmosphere was either below detection thresholds or within the test chamber’s background levels. Contrary to the common assumption of complete (100%) bioaccessibility, the discharge of metals into simulated biological fluids was low. This indicates that only a particular portion of the present metals is potentially absorbable by the body. This is a positive finding, as it means there may not be as much of a health risk from exposure to metals found in crumb rubber as previously thought. Typical values for various metals and extractable SVOCs varied from 1 mg/kg to 15,000 mg/kg for Zn. The crumb rubber samples were all noted to contain bacteria, which could potentially constitute a health hazard. It is important to note that releasing metals into simulated biological fluids is only one aspect of the overall risk assessment, and other potential health risks associated with exposure to tire crumb rubber may still exist.

p-Phenylenediamines (PPDs), widely utilized in the rubber industry, have been consistently detected in numerous environmental segments for years. PPDs are highly reactive with ozone but less with oxygen [117]. The dominant byproduct is 6PPD-quinone. Cao G [128] found that five quinones originated from PPDs by ubiquitous in urban runoff, roadside soils, and air particles. It was found that 6PPD-quinone was toxic to coho salmon but less so to other fish (chum salmon) [129,130]. PPDs leached from TDA were much lower than those from tire wear particles, and PPDs and 6PPD-quinone were not toxic to other fish. Furthermore, TDA is mainly used underground. Thus, the chance of forming 6PPD-quinone will be low when TDA is used in civil engineering projects. Therefore, it can be postulated that TDA and filter layers, including soil matrix, remove 6PPDs and 6PPD-quinone in tire-wear particles and dissolved in the runoff.

It can be concluded that tire components leached from end-of-life TDA would not cause serious ecological and human health issues since the tire component concentrations leached from TDA are low enough and adsorb onto the soil. TDA can also adsorb contaminants, provide favorable surfaces for microbial growth, and promote anaerobic dehalogenation as a reducing agent.

5.5. Factors Affecting TDA-Component Leaching

The considerable differences in results from both laboratory and field studies suggest that the leaching behavior of TDA hinges on its aquatic chemistry, the dimensions of the tire product, exposure duration, and downstream dilution volume. A more detailed exploration of these factors follows.

5.5.1. pH

Many researchers evaluated the impact of pH on potential contaminants that may be leached from tire products [76,81,91,125]. Under acidic conditions, metal detection reached its highest levels. Even though some proposed that pH had minimal or no effect on the leaching of organic compounds [96], many studies examining pH demonstrated that alkaline conditions lead to higher organic concentrations [90]. Ealding W [77] observed swelling of plastic containers containing tire shreds immersed in a low-pH solution (pH 4). However, this study generated conditions favorable to biological activity. This issue was mitigated by replacing acetic acid with benzoic acid, thus affirming that the TDA was not the source of gas production. Neutral pH conditions were optimal for TDA applications [77,82].

5.5.2. Size of End-of-Life Tires

The impact of TDA or tire chip size on the leaching behavior of their constituents was assessed [82,96]. Smaller fragments (e.g., 2.54 cm × 2.54 cm) tended to leach organics more efficiently. Miller W [107] discovered that the size of the tire shred was the primary determinant of the leaching of organics, though the specific impact differed depending on the compound in question. The findings indicated that tire chip size influenced the removal of benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds. Medium (between 5.08 cm × 5.08 cm and 10.08 cm × 5.08 cm) and large (15.24 cm × 5.08 cm) pieces leached more benzene than small pieces (between 1.27 cm × 1.27 cm and 2.54 cm × 2.54 cm), while small pieces leached more toluene and 1,2,3-trimethylbenzene [96]. The benzene behavior does not follow the usual trend where smaller pieces tend to leach more, and no explanation was provided. Ethylbenzene, xylenes, 2-ethyl toluene, and 1,2,4-trimethylbenzene appeared to behave similarly regardless of tire shred size [96].

Selbes M [82] investigated the behavior of dissolved organic carbon and inorganic compounds that may leach from various tire-derived products (TDPs) in a 28-day study. The results showed that crumb rubber leached significantly more than tire chips for all sizes tested, except for Fe, as the iron’s primary source in the tire chip came from the wires removed before preparing the crumb rubber. The smallest tire chip size (2.54 cm × 2.54 cm) leached the compounds of interest the most, while leaching behavior was similar for the other dimensions (5.08 cm × 5.08 cm, 10.16 cm × 5.08 cm, and 15.24 cm × 5.08 cm). Further analysis suggested that the increased side surfaces of the smaller tire pieces likely contributed to the increased levels of dissolved organic carbon [82]. When normalized for the side-surface area, data suggested that organic leaching is more dependent on the side-surface area than the total size of the TDA. Selbes M [82] theorized that this is likely due to the freshly cut and exposed side material rather than the more-weathered top and bottom surfaces. When comparing tire chips and crumb rubber, all detected constituents were significantly higher except for iron due to the steel wire in tire chips [82]. Behavior like iron is expected from other steel components. The behavior of the inorganics found in steel is expected to be less dependent on the actual size of the TDA and more dependent on the amount of protruding steel wire, which is governed by the specifications and the material manufacturing. It is interesting to note that although the leaching rate and desorption rate of tire components are affected by end-of-life tire sizes, the adsorption of SOCs by end-of-life tires is not affected by the size, since the rate-limiting parameter in transfer through the tire matrix is diffusion [23].

Khan A [125] conducted TCLP tests with various sizes of crumb rubber and tire chips. Crumb rubber demonstrated higher leaching for all constituents identified except for iron, which is likely because the iron in tire chips mainly originates from wires removed before the crumb rubber was prepared. Among tire chips within a practical application’s particle size range, the 2.5 cm × 2.5 cm tire chips typically showed more leaching than other sizes. The discrepancy in leaching from 5 cm × 5 cm, 10 cm × 5 cm, and 15 cm × 5 cm tire chips was relatively minor or insignificant.

5.5.3. Contact Time

The leaching of tire components from end-of-life tires is a function of contact time, while adsorption to tires was not, as found by Kim J [23] and Selbes M [82]. Khan A [125] found that the leaching rate of four metals (Al, Fe, Mn, and Zn) under acidic conditions revealed a quick initial leaching rate for Zn, which then slowed but remained constant. In contrast, the rates for Al, Fe, and Mn were consistent from the onset of the experiments and showed no signs of decreasing. This observation was likely due to the release of Zn from the rubber component of the tires, as suggested by its relatively similar leaching patterns to those of DOC, where 40~50% of DOC leached during the first week and 20~25% during the four weeks of experiments. Conversely, the steady and unchanging dissolution rates of iron, aluminum, and manganese likely result from their origin in the tire chip wires.

Conversely, the adsorption rate is fast. Typically, the contact time, also known as the Empty Bed Contact Time, in a granular activated carbon bed for the adsorption of organic compounds varies from 5 to 20 min in drinking water treatment applications. Thus, it is safe to assume that a similar contact time is good for removing metals and SOCs using TDA. Since the concentrations of tire components leached from TDA are all below the NPDWR and NSDWR SMCLs except for the cases with Fe and Zn, a longer contact time will not cause serious ecological and human health issues when applied to civil engineering projects.

5.5.4. Downstream Dilution Volume and Soil or Reaction Zone

When leached tire components are diluted with surface water, the concentrations will be too low to cause any ecological and human health issues. Leached tire components dilute with groundwater. Metals are quickly adsorbed and precipitated in soil and filtered. SOCs are adsorbed onto organic carbon in the soil matrix. This dilution and reaction zone will make the water quality impact minimal, although the dilution rate may be lower than surface water.

6. Conclusions and Recommendations

It is imperative to find a systematic way to recycle approximately 300 million tires discarded annually. The tire-derived aggregate (TDA) for civil engineering applications represents one possible value-added sustainable reuse option. TDA serves as a lightweight fill material possessing commendable engineering qualities, thus offering a substitute for standard fill materials. It has proven advantages such as lightness, shear strength, permeability, vibration dampening, thermal insulation, filtration, interlocking properties, and the ability to break capillary action. In numerous civil engineering applications, TDA encounters water, which may lead to the leaching of organic and inorganic compounds, potentially compromising the quality of surrounding ground and surface waters.

This study examined TDA’s physical, chemical, and microbiological properties, along with new and additional data on the leaching of tire components and their ecological and human health issues. In addition, recent findings on TDA’s benefits and ecological and human health issues were addressed. After evaluating published studies on end-of-life tires on leached tire components and their ecological and human health issues, the following conclusions and recommendations were drawn.

6.1. TDA as a Means of Removing Contaminants

It has been shown that although end-of-life tires leach tire components, tires can also adsorb potentially toxic SOCs from the water. End-of-life tires can remove heavy metals such as mercury (Hg), lead (Pb), copper (Cu), and zinc (Zn). Zinc and iron in tires can be used as reducing agents to dehalogenate PFAS and tri- or more halogenated hydrocarbons such as trichloroethylene (TCE) and tetrachloroethylene (PCE) to fewer toxic byproducts. TDA can adsorb 6PPD-quinone like many other nonpolar and halogenated hydrocarbons. In addition, TDA can act as a catalyst for hydrogen sulfide removal. The adsorption capacity increased as the tire surface area increased for metals but remained the same for SOCs.

6.2. Leached Metals from TDA

The elements—As, Ba, Ca, Cd, Cr, Co, Cu, Fe, Mg, Mn, Mo, Na, Ni, Pb, Se, and Zn—leached from TDA or tire chips were below the detection limit or did not exceed federal and state Maximum Contaminant Levels (MCLs) under field conditions at a pH between 6.5 and 8.5. Iron and zinc were the dominant metals leached from TDA. Once metals are leached from tires, they are trapped within 0.3~3 m by the soil matrix for over 10~20 years, indicating extremely low mobility. In addition, leached iron and zinc may react with contaminants in the water and precipitate or adsorb onto the soil matrix, acting as a contaminant-cleaning agent. In addition, leached iron and zinc may react with minerals in the ground to form insoluble precipitates or adsorbed complexes. These complexes are not easily transported through the soil, making them less likely to move into groundwater.

6.3. Leached SOCs from TDA

The amount of SOCs that leach from TDA into water samples may vary depending on the test conditions, such as the exposure time, presence of headspace, organic carbon content in water, sample collection method, etc. Although the risk of exposure to chemicals leaching from TDA in aquatic environments is typically low, regulations have been established to control the quantity of these compounds discharged into the surroundings. TDA-leached SOCs did not exceed the federal and state regulatory limits under simulated field conditions. TDA can remove SOCs as a reducing agent by adsorption or microbial degradation on the surface of TDA. The microbial degradation of SOCs adsorbed on the TDA will extend the exhaustion time of the adsorption capacity. Therefore, TDA should be considered a favorable medium rather than a material concerning civil engineering projects due to its cleaning potential for organic and inorganic contaminants.

6.4. Ecological and Human Health Effects of TDA

Although leached compounds from tire-wear particles and particles themselves have been found to be toxic to a few specific aquatic species, no conclusive study has shown the harmful effect of TDA on aquatic species. The low concentration of leached substances and the dilution effect suggest that using TDA in civil engineering projects is unlikely to cause significant toxicity to aquatic species, including coho salmon and even humans. The potential pathways for human exposure to leached chemicals from Tire-Derived Aggregate (TDA) in civil engineering projects primarily include (1) dermal contact, (2) the inhalation of volatilized compounds, and (3) the ingestion of contaminated drinking water. However, (1) and (2) are improbable due to low likelihood, and previous research indicated that leached chemical levels remained below regulatory thresholds. Consequently, it can be asserted that the potential health risks associated with exposure to leached chemicals from TDA in civil engineering projects are minimal.

The risk associated with using TDA in civil engineering projects is relatively minor compared to other daily risks. For example, rubber gaskets similar to tires in chemical composition have been used in drinking water distribution pipes for over a century despite the potential leaching of “toxic” rubber components. Does the discovery of such leaching necessitate the removal of all such gaskets from our water distribution infrastructure? The decision of whether or not to use TDA in civil engineering projects should be made on a case-by-case basis, considering the specific application and the potential risks and benefits.

6.5. Gaps in Knowledge

The following gaps in knowledge were identified from the extensive literature review.

• Many studies were performed on the effects of end-of-life tires on environmental and human health issues. They showed concerns about a few specific species or exposures to tire wear particles and crumb rubber. However, TDA that has less exposure to the environment than other forms of end-of-life tires is viewed the same, sometimes inhibiting the TDA application to various civil and environmental engineering projects. Thus, the guidelines for determining the suitability of TDA for a specific project need to be developed, specifically regulation compliances, limitations and constraints, design considerations, TDA installation procedures, and post-care/maintenance.
• Long-term evaluations of TDA-implemented civil engineering projects are needed to improve future projects and avoid potential mistakes, specifically environmental impacts, technical data for design, and performance.
• Further research in geotechnical and environmental fields should focus on identifying potential projects where TDA can be effectively implemented. Particular emphasis should be placed on expanding its use as an adsorbent, a filtering medium, and a support medium for microbial growth. The potential for TDA to serve as a suitable medium for removing emerging contaminants should also be further explored.