Occurrence and risk assessment of PAHs from athletic fields under typical rainfall events

With increasing living standards, people's demand for sports fields is increasing. An artificial athletic field has many advantages over an open grass field. As they do not require a growing season, fertilizers, pesticides, or both, they use less water and require less time, labor, and effort to produce (Claudio 2008). As such, the number of artificial athletic fields is increasing, and their application scope is expanding, from schools and sports fields to parks, communities, and public entertainment venues. More than 13,000 artificial turf courts and 47,000 small-scale courts were used for football training in the EU in 2016, and this number continues to increase (ECHA 2017). The United States has thousands of synthetic turf fields (especially for children and adolescents) that many people frequently use. As an essential part of artificial athletic fields, surface materials are directly in contact with the users. Depending on the sports functions such as running, playing basketball, playing football, etc., surface characteristics such as composed materials and roughness vary from different athletic fields. For example, the surface materials of artificial turf are composed of plastic grass and rubber particles, while the surface materials of badminton courts are mainly composed of rubber. For economic reasons, rubber materials are mainly produced from discarded car tires (Cheng et al. 2014). According to the European Chemicals Agency (ECHA), 43% of the waste rubber is used for artificial turf (including fillers), and 45% of the waste rubber is used for sports fields such as track and field fields, tennis courts, and basketball courts. About 21% of the waste tire-derived rubber particles in Europe are used as fillers for artificial turf (Verschoor et al. 2021). Because different chemical compounds (benzene, phthalates, alkylphenols, etc.) and additives (activators, plasticizers, vulcanizers, etc.) are employed in the manufacture of tires, it is a sophisticated procedure (Bocca et al. 2009). In addition, in the production and laying process, some organic additives such as antioxidants and pigments are also added to the surface materials (Li et al. 2010). With the increasing use of sports grounds, the degradation of materials caused by exogenous processes (e.g., rainfall, weathering, and mechanical wear) inevitably leads to the release of chemical substances such as polycyclic aromatic hydrocarbons (PAHs), metals, and volatile organic compounds (VOCs) (Zhang et al. 2021). Although several nations and areas have established rules and regulations to limit the use of dangerous chemicals in the rubber for making artificial turf, the rubber itself may still include a number of dangerous elements (Canepari et al. 2018; Gomes et al. 2021), which could potentially leach out of the materials during use, pollute the environment, and endanger human health.

Environmental contaminants known as PAHs are pervasive, bioaccumulative, and hazardous. Early in the 1980s, the United States Environmental Protection Agency (U.S. EPA) identified 16 PAHs as priority pollutants that the International Agency for Research on Cancer (IARC) had also identified as potential human carcinogens. Outdoor conditions such as oxidants, sunlight, high temperatures, and rain can weaken the mechanical resistance of the surface materials of sporting grounds (Wachtendorf et al. 2017). Consequently, toxic substances such as PAHs are likely released from athletic field materials into air and runoff, which impose potential health risks via inhalation or dermal absorption, in addition to ecological risks by ending up in sewage waters, groundwater, and natural surface waters (Krüger et al. 2013). Celeiro et al. (2018) found 14 kinds of PAHs in the air above artificial turf. In addition, eight PAHs including naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, and chrysene were detected in the runoff samples, with a total concentration of 0.68–1.55 μg/L. The partial distribution of PAHs in the air and runoff was then demonstrated. Schneider et al. detected high concentrations of heavy metals, PAHs, and VOCs in rubber crumbs, including aluminum (5,383 mg/kg), cobalt (168 mg/kg), benzothiazole (48 mg/kg), 2-(2-hydroxyphenyl) benzothiazole, and 4-tert-octylphenol (14 mg/kg) (Schneider et al. 2020). Kalbe et al. (2013) used particles grained from artificial turf surface materials to study the leaching behavior of PAHs in a column experiment. They found that the artificially aged turf materials could release PAHs and the leaching concentration increased over aging time.

Numerous safety evaluations have found no danger of chemical exposure from recycled rubber used in artificial fields. However, it may still cause a series of health concerns. For example, some toxic compounds (specify these compounds here) were frequently detected in human urine after playing over the athletic fields (van Rooij & Jongeneelen 2009). As a result, there are growing concerns about the safe use of artificial fields, particularly those using surface materials made of recycled rubbers. Unfortunately, previous studies on the health risks of recycled rubber's health risks have some limitations, including small sample numbers and little analysis of pertinent exposure routes and situations (Peterson et al. 2018). In addition, to our best knowledge, previous studies mainly focused on investigating pollutants from specific fields such as artificial turf. Different athletic fields may result in various pollutant distribution partners due to the varying materials composition, surface structure, catchment area, rainfall characteristics, etc. However, the occurrence of PAHs in different types of athletic fields remains understudied. Especially, the risk assessment of PAHs released from athletic fields other than artificial turf remains inadequate.

In our previous study, we investigated heavy metal pollution in the runoff from artificial athletic fields and found that the health and environmental risks could not be neglected (Zhang et al. 2021). To further assess the risks of pollutants in stormwater runoff from artificial athletic fields, this study aimed to investigate the occurrence and health risks of PAHs in runoff (under three typical rainfall events) from various types of athletic fields, including artificial turf, badminton, basketball, plastic runway, and tennis court (Zhang et al. 2021). The specific objectives were (i) to distinguish the distribution of PAHs in runoff from various kinds of artificial sports fields; (ii) to study the effect of rainfall characteristics on the distribution and first flush behavior of PAHs in runoff from different types of athletic fields, and (iii) to assess the ecological and health risk of PAHs in runoff from various athletic fields.

The liquid–liquid extraction method was used to extract PAHs from the runoff samples (Kruger et al. 2011). Before the extraction, 200 mL of the sample was filtered through 0.45 μm glass fiber filters (GF/F, Whatman, pre-combusted at 450 °C for 4 h) and added with 5 ng of 2-fluorobiphenyl and 4–4′-terphenyl-d₁₄ (o2si, USA). Then, 15 mL of dichloromethane (CNW, Germany) was combined with each sample in a separating funnel. The organic phase was separated 15 min later after moderate shaking. The extraction was repeated once with 15 mL fresh dichloromethane. The organic phases were combined into a single mixture, dried with anhydrous sodium sulfate, and then concentrated with a nitrogen blower until almost dry. The concentration was transferred to a C18 cartridge for solid-phase extraction (SPE) (CNW LC-C18 SPE). Each C18 cartridge was prepared with 5 mL of dichloromethane and 5 mL of n-hexane (CNW, Germany) before sample loading and was then dried in a vacuum. The analytes on the cartridge were eluted three times with 5 mL of n-hexane. The extract was then evaporated to almost dryness with a moderate nitrogen flow, and it was then reconstituted with n-hexane to a final volume of 1 mL. Sixteen priority PAHs’ mixed standards were bought from o2si, USA. After that, a gas chromatograph–mass (GC–Mass) spectrometer (Agilent 7890B-5977C) was used to quantify the PAHs in the extracts.

The calibration curves were established with six data points (0.05, 0.1, 0.3, 0.5, 1.0, and 5.0 g/mL) that were used to create the calibration curves. For GC separation, a DB-5 ms silica-fused capillary column was used. The temperature of the GC oven was initially set at 40 °C for 2 min, raised to 300 °C at a rate of 10 °C/min, and then kept at 300 °C for 10 min. The recovery of the standards at each calibration point was monitored and served as quality control. In the dichloromethane:N-hexane solvent (1:3, v/v), the recovery was achieved from standards of known concentrations with acceptable linearity (r² = 20.997). The mean recovery percentages ranged from 95.2 to 99.5%. The relative standard deviation (i.e., coefficient of variation) varied from 0.2 to 4.5%. Recovery rates between 50 and 120% and a coefficient of variation value under 20% are considered to be within acceptable limits (Yuan et al. 2019). Thus, the quality assurance in this study was adequately confirmed. The limit of detection (LOD) and the limit of quantification (LOQ) were calculated based on 3 and 10 times the standard deviations. The LOD values ranged from 0.001 to 0.020 μg/L, while the LOQ values ranged from 0.002 to 0.091 μg/L. Statistical analysis was conducted with SPSS and Origin 2018. The analytical parameters for each PAH are shown in Table S1.

The lowest risk concentration of BaP was converted to a TEQ of 0.0005 μg/L (TEQ_QV). The ecological risk level was calculated by TEQ_CARC/TEQ_QV, which was divided into four grades: no risk (<0.1), low risk (0.1–1), low-to-moderate risk (1–10), moderate-to-high risk (10–100) and high risk (≥100) (Ravindra & Mor 2019).

Compared with the average concentration, the EMC also considers the impact of runoff flow on the concentrations of pollutants. Therefore, the EMC is a crucial event-specific parameter representing the actual time variations in concentrations (Perera et al. 2021). The EMC of PAHs in runoff from each athletic field is presented in Table 1. A total of 6 out of the 16 EPA PAHs were detected in all the samples, namely naphthalene (Nap), fluorene (Flu), phenanthrene (Phe), fluoranthene (Fla), pyrene (Pyr), and chrysene (Chr). This indicates that PAHs were ubiquitous in athletic field runoffs. The sources of PAHs in the runoff could be mainly from the surface materials of athletic fields. The EMC of ∑₆PAHs in the five athletic fields ranged from 3.96 to 23.23 μg/L, which was several times higher than the EMC (a few micrograms) of runoff in urban traffic areas (Zheng et al. 2014).

In addition, the surface of the athletic field is often made from rubber polymer, reinforcing elements (such as carbon black), aromatic extender oils, antioxidants, pigments, etc. (Perkins et al. 2019). Ethylene propylene diene monomer (EPDM) is frequently used as rubber fillers in sporting grounds. In use, PAHs may be released during the pyrolysis of the surface materials. PAHs are semi-volatile and highly hydrophobic. Thus, there are more PAHs distributed in the gas phase than in the water phase. The thermal stability of the athletic field surface material is much weaker than the road surface. Especially in summer, high temperatures will accelerate the release of organic compounds from the surface material (Jim 2017). Besides, the athletic field could undergo UV, ozone, and mechanical force during use. These processes can accelerate the cracking and peeling of the polymer matrix, so the internal additives and other pollutants containing PAHs could be released (Wachtendorf et al. 2017). Celeiro et al. (2018) have demonstrated that watering the surfaces of athletic fields contributed to the PAHs leaching from the rubber material.

Except for Nap, the concentrations of five PAHs, including Flu, Phe, Fla, Pyr, and Chr, in the athletic field runoff followed in the order of artificial turf > badminton court > basketball court > plastic runway > tennis court. To promote the reuse of waste resources, most of the rubber used in surface materials of athletic fields is recycled tires. Magnusson & Mácsik (2017) show that recycled tires are more susceptible to cracking than new tires. PAHs are one of the byproducts of tire cracking (Quek & Balasubramanian 2012). Thus, PAHs are inevitably released from the surface with rubber, causing a higher amount detected in the runoff from the artificial turf and badminton court. In addition, the smaller the size of the filling particle, the easier it releases contaminants (Menichini et al. 2011; Kim et al. 2012). In the artificial turf, the surface is composed of small rubber particles, while the badminton court is composed of compacted, smooth surfaces. Thus, the release of PAHs from the artificial turf was higher than that from the badminton court. The PAHs concentration in the plastic runway and tennis court was lower than that in other fields. The reason might be due to the lower release from surface materials. Unlike rubber as the main composition in artificial turf, the surface materials of the plastic runway and tennis court mainly comprised EPDM and siloxane-modified polyurethanes. Even though EPDM and siloxane-modified polyurethanes could release PAHs during pyrolysis, their release could be much less than from rubber. Therefore, the concentration of PAHs in the plastic runway runoff was lower. The PAHs concentration in the tennis court was the lowest, which might be due to its compact surface. Unlike other fields, the surface of the tennis court is smoother, where PAHs might not be released from that compact surface as quick as from the rough surface of the other fields.

The PAHs distribution in the athletic fields is compared with reported data in the literature. Celeiro et al. (2021) analyzed PAH concentrations in the runoff from eight football fields. They found that the average concentrations of Flu, Chr, Fla, and Nap were 0.03, 0.04, 0.07, and 0.04 μg/L, respectively, which were much lower than the concentrations (specify the values here) detected in this study. For Pyr, the concentration detected in the present study was 0.09–0.94 μg/L, which was similar to the concentrations (0.05 –0.9 μg/L) detected in eight football fields by Celeiro et al. (2021). The comparison shows that the distribution of PAH in athletic fields is highly dependent on athletic fields' locations, types, and sampling points.

2-ring PAHs have poor stability in an aqueous solution (Zhang et al. 2019). Therefore, the proportion of 2-ring PAHs in the runoff was relatively low. The relatively high distribution of 3- to 4-ring PAHs in all the samples could be originated from rubber polymer, aromatic extender oil, antioxidants, processing aids, plasticizers, etc., from the surface materials of the athletic fields (Lopes et al. 2000). The previous analysis showed that the samples with higher total concentrations were located in the artificial turf area, and the proportion of 4-ring PAHs was more significant. Celeiro et al. (2021) detected seven kinds of PAHs leaching from the material of artificial turf, and the total concentration was 6.0–54.0 μg/g and the largest portion was Pyr (4-ring PAHs), which reached a concentration of 30.0 μg/g. The dominance of 4-ring PAHs in athletic runoff in this study is consistent with the previous survey.

There was no clear rule between rainfall duration and PAHs concentration under a particular rainfall event, except for the 2nd rainfall event. During the 2nd rainfall event, except for the tennis court, the PAHs concentration increased first and then decreased over time in the early stages of the rainfall (0–30 min). This suggests that scouring had a more significant effect on the PAHs in runoff in the early rainfall stage. At this time, the primary source of PAHs may be the release of surface particulate matter. During the later period of rainfall, the concentration of pollutants increased again. The concentration was positively related to the rainfall intensity during the 2nd rainfall event. This suggests that surface materials released PAHs into runoff environments (Wachtendorf et al. 2017). However, during the 1st and 3rd rainfall events, the PAHs concentration seemed random during the sampling process for the five types of athletic fields. Generally, except for the tennis court, the concentration of PAHs from all the athletic fields under the 2nd rainfall event was the highest. It seems that rainfall duration only exerted a significant influence on high concentrations but not on low concentrations.

The carcinogenic risk indexes of runoff from the five athletic fields are presented in Figure 6. Among them, the cancer indexes of the badminton and basketball court runoff during the 2nd rainfall event and the indexes of the artificial turf runoff under the 2nd and 3rd rainfall events were higher than 1.0 × 10⁻⁶. This indicates that the artificial turf, badminton, and basketball court runoff could pose a health concern to the users. The EMC of the PAHs in those fields was also the highest (Section 3.1), suggesting a positive relationship between health risk and the concentration of PAHs. Therefore, the use of recycled materials in badminton courts, basketball courts, and artificial turf should be strictly controlled to reduce the potential human health risk. The carcinogenic risk was not found under the remaining rainfall events. Nonetheless, the migration of PAHs into the surrounding environment via runoff and their cumulative effect may have a significant impact on the surrounding environment.

This study investigated and compared the distribution and risks of PAHs in runoff from five representative athletic fields (artificial turf, badminton court, basketball court, plastic runway, and tennis court) under three typical rainfall events. Six out of 16 PAHs in the U.S. EPA's priority pollutant list were detected in all the samples, namely Nap, Flu, Phe, Fla, Pyr, and Chr. The sources of PAHs in the runoff could be mainly from the surface materials of athletic fields. The EMC of Σ₆PAHs in the five athletic fields ranged from 3.96 to 23.23 μg/L, which was dozens of times higher than the EMC of runoff in urban traffic areas. These results indicate that PAH pollution was ubiquitous in the athletic field runoffs. Four-ring PAHs (Pyr and Chr, >30%) were more significant than 2-ring and 3-ring PAHs. Except for Nap, the PAHs concentrations followed in the order of artificial turf > badminton court > basketball court > plastic runway > tennis court. The surface character of the athletic fields, such as the composition of materials and roughness, played a crucial role in the release of PAHs. Besides, strong rainfall intensities facilitated the release of PAHs, and the first flush effect was weak if a vigorous rainfall intensity appeared later during a rainfall event. Ecological risk assessment showed that PAHs from the artificial turf, badminton court, and plastic runway runoff corresponded to moderate-to-high risk. Health risk assessment showed that PAHs from the artificial turf, badminton court, and basketball court runoff could pose a potential carcinogenic risk to humans via dermal contact. Therefore, using recycled materials to construct badminton courts, basketball courts, and artificial turf should be adequately controlled to reduce potential hazards to human health.

This research was supported by the National Key R&D Program of China (Grant No. 2020YFC1808804); the Construction of High-Level Teaching Teams in Universities of Beijing – the Youth Top-Notch Talent Cultivation Program (CIT&TCD201804051); the National Natural Science Foundation of China (51508017 and 51978032); and the Youth Beijing Scholars program (No. 024).

X.Z. performed the design of experiment, conducted date analysis and funding acquisition, wrote the original draft, reviewed and edited the article. Y.G. and Y.W. performed sampling, sample analysis, and data analysis, wrote the original draft, reviewed and edited the article. J.L. reviewed and edited the article. Y.J. did sampling. Y.T. performed graphical abstract. Z.Z. reviewed the article. C.T. and Y.W. investigated sample analysis. H.L. conducted funding acquisition. Y.H. conceptualized and revised the article.

All authors contributed to the preparation of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.