Road dust-associated microplastics as a carrier of plastic additives in urban small-scale river sediment

ABSTRACT

Microplastics (MPs) and plastic additives have attracted global attention as emerging environmental pollutants. Herein, we analysed MPs and 56 organic plastic-derived chemicals in road dust (n=3), stormwater (n=4) and urban small-scale river sediment (n=8) samples collected from a downtown area of Kumamoto, Japan. The MP levels were 57,500–160,000 items/kg dw, 2.2–42 items/L and 1,600–15,800 items/kg dw in road dusts, stormwater and river sediments, respectively. The compositions of the polymers (tyre road wear particles [TRWPs], polymethyl methacrylate [PMMA] and polyethylene terephthalate [PET]) were consistent among the analysed samples; this result indicates that road dust–associated MPs were transferred to the river sediment via stormwater. A large amount of glass beads, an indicator of traffic-related particulate contaminants, was found in the road dust and sediments, suggesting that the urban small-scale river was strongly impacted by traffic activities. In total, 32 plastic-derived chemicals were identified in road dust and sediments with 6,900–32,000 and 620–13,000 ng/g dw concentrations, respectively. Notably, these concentrations were positively correlated with the abundance of MPs, indicating that MPs act as carriers of plastic-derived chemicals. We determined the concentrations of plastic additives in 28 traffic-related plastic products to identify the potential source of additive-derived chemicals in the road surface environment. Several road marking sheets and road paints contained large quantities (up to 1.7% w/w) of high-concerned substances, including phthalate esters, benzotriazole ultra-violet (UV) stabilisers and benzophenones. The additive profiles of the MPs and traffic-related products were similar to those of some PMMA-based MPs detected in the samples and PMMA-based road paints. This indicates that PMMA-based road paint was identified as an original source of MPs, contributing to 24%–54% of the total number of MPs in the road dust, stormwater and river sediment. In conclusion, an urban small-scale river was highly contaminated with road dust–associated MPs and plastic additives, and high-priority plastic products were identified as an important source of those contaminants in urban road areas.

INTRODUCTION

Microplastics (MPs; plastic particles under 5 mm in size; GESAMP, 2015) have attracted global attention as one of the most concerned contaminants. MPs released into the environment potentially have adverse effects on biota. Sussarellu et al. (2016) reported feeding modification and reproductive disruption for adult oysters exposed to polystyrene (PS) beads (2 and 6 μm in diameter). The chemical toxicity derived from the plastic additives has been a great concern. Wagner et al. (2024) documented >16,000 chemicals potentially used or retained in plastic materials and >4,200 plastic additives are concerned about their persistency, bioaccumulative properties and toxicity. In general, plastic additives are not covalently bonded with polymers; thus, such chemicals can be easily released from plastics. Leaching of organic/inorganic plastic additives is promoted under a digestive fluid condition (Masset et al., 2021, 2022) and the chemicals released are accumulated in the tissues of organisms (Tanaka et al., 2020). As an example of the ecological impact of additive chemicals, a transformation product of a rubber additive chemical, 6PPD-quinone (6PPD-Q; CAS#: 2754428-18-5), have a lethal effect on a portion of salmons at an environmentally relevant concentration (Tian et al., 2021, 2022). These results indicate that understanding the environmental behaviour of MP particles and plastic additives is important; however, few studies have investigated the environmental behaviour of both contaminants simultaneously.

Attention towards the terrestrial regions as a primary source of MPs into the environment is increasing. Globally, scientists have investigated the pollution status of MPs in various terrestrial compartments including soil (Qiu et al., 2022), road dust (Yang et al., 2023), freshwater sediment (Yin et al., 2023), surface water (Wang et al., 2021) and groundwater (Lee et al., 2024) samples. To reduce the MPs emission into the environment, the sources must be adequately controlled and categorised as point and non-point sources. The wastewater treatment plants (WWTPs) are considerably a major point source of MPs in terrestrial areas (Cesa et al., 2017). Open dumping sites and mechanical recycling facilities have recently been identified as important point sources, especially in developing countries (Suzuki et al., 2022; Yamahara et al., 2024a). The importance of controlling non-point sources of MPs, especially stormwater runoff, has been highlighted in recent studies. Cho et al. (2023) reported that stormwater runoff contributed to 99% of the total MP load into the Han Cheon River in Korea. Similarly, Imbulana et al. (2024a) reported a higher emission of MPs by surface runoff than untreated wastewater, contributing 99% of the annual MP emission into rivers; this indicates that stormwater runoff is an important contributor to MPs in the freshwater ecosystem.

Urban small-scale rivers can be one of the most highly contaminated waters with MPs because of their low dilution effect and proximity to heavy human activities (Kabir et al., 2021, 2022). Urban small-scale rivers receiving stormwater may be highly polluted by MPs and their additive chemicals because of the large quantities of MPs input into small water bodies, but evidence of the pollution in these rivers is lacking. Thus, this study aims to examine the pollution status of MPs and plastic additives in an urban small-scale river sediment, elucidate the transport pathway of MPs and plastic additives into the sediment and identify the potential and original sources of MPs and plastic additives.

MATERIALS AND METHODS

CHEMICALS

Table S1 demonstrates the standard chemicals used for the additive analysis.

SAMPLE COLLECTION

SEDIMENT, ROAD DUST AND STORMWATER

Eight sediments were sampled in an urban small-scale river (width: 10–12 m), flowing in a downtown area of Kumamoto City, Japan (population density: 7,368 people/km²) (Fig. 1). There were several rainwater pipes, but no outlets for domestic and industrial wastewater existed along the river. The main outlets of the stormwater were located near the SED-1 and -5 stations (Fig. S1). Two weirs were constructed near the SED-5 and -8 stations (Fig. S1). Surface sediments were collected using a stainless-steel shovel (for SED-1 and -5) and Van Veen grab (for other sites). All samples were wrapped with aluminium foil and stored at −20°C until analysis. The road dust and stormwater samples were collected to elucidate the source and inflow pathway of MPs and plastic additives in the river sediments. Three road dusts were collected from a main road along the river in October 2021. The road dust was swept from four corners at each sampling point and combined as one sample. The stormwater samples were collected from two outlets during two rain events on 21^st and 26^th April, 2022. The first samples were collected after six antecedent dry days or 25 days, if the excepted rain events were of the intensity <10 mm/day. First sampling was performed during a rain event with a precipitation intensity of 9 mm/h and an accumulative precipitation of 10 mm, which was named “First flush samples.” In the second rain event, the precipitation intensity was similar to the first event (9 mm/h), but the accumulative precipitation reached 53 mm. A total of 116-mm of precipitation occurred between the first and second sampling. The second samples were “steady-state samples.” Approximately 10 L of stormwater was sampled using a stain-less steel bucket and filtered with 100 μm stainless steel sieves on site. The sieves were packed in plastic bags and transferred to the laboratory.

Fig. 1 Information of samples analyzed in this study: a) Picture of sampling road dust, stormwater, and river sediment, b) Sampling points of road dust, stormwater, and river sediment, c) Pictures of 28 traffic-related plastic products

TRAFFIC-RELATED PLASTIC PRODUCTS

In total, 28 traffic-related plastic products (including 1 automobile tyre, 9 road paints, 6 road marking sheets, 7 reflectors and 5 poles/road cones) were collected as potential sources of MPs and plastic additives in the road surface environment (Fig. 1). We previously identified most products as the dominant sources of MPs in a road surface environment (Yamahara et al., 2024b). Samples were obtained from do-it-yourself shops and specific commercial companies, and fragments were dropped at the roadside.

ANALYSIS OF MICROPLASTICS AND GLASS BEADS (GBs)

MPs (100–1,000 μm) in the samples were analysed following the procedure of Yamahara et al. (2024a) with a slight modification. The dried road dust and sediment samples were passed through a 1 mm mesh stainless steel sieve to remove large particles, such as leaves, metals and stones. The suspended matter in the stormwater deposited on the stainless steel sieves was recovered by washing with ultrapure water. All the samples including the suspended matter in 10 L of stormwater, 0.6–1 g dry weight (dw) of road dust and 5 g dw of sediment were treated with 30% hydrogen peroxide (H₂O₂; Fujifilm–Wako Pure Chemical Industries, Japan) for 3 days at 50°C to decompose the organic matter in the samples. The solid residues were recovered on a 100-μm mesh nylon filters, and the MP candidates were density-separated using a 60% (w/v) sodium iodide (NaI) solution (ρ=1.8 g/cm³, Fujifilm-Wako Pure Chemical Industries, Japan). The floating fractions were passed through the 100-μm mesh nylon filters, and this protocol was repeated three times. The residues on the nylon filters were dried at 50°C overnight, and each MP candidate was picked up with tweezers and packed into a small plastic bag under a stereomicroscope (S9, LEICA, Germany).

The MP polymers were determined using two types of Fourier-transform infrared spectrometer (FTIR; Cary 630 with a germanium crystal, Agilent, USA: for black rubber-like particles; IR Affinity 1S with a diamond crystal, Shimadzu, Japan: for the other particles). The spectrum of each sample was compiled from 20 scans recorded more than 600–4,000 cm⁻¹ at a resolution of 4 cm⁻¹. In addition, all the samples were analysed for GBs. GBs are used in road marking paints at concentrations of up to 33% (w/w) to improve road visuality for drivers (Zannonni et al., 2016; Fig. S2). GBs have been proposed as indicators of traffic-related particulate contaminants in several studies (Migaszewski et al., 2022; Turner and Keene, 2023; West-Clarke and Turner, 2024). Herein, glass beads were counted by analysing the sedimentation fraction obtained by density-separation because of the high density of GBs (2.5 g/cm³; Järlskog et al., 2022).

ORGANIC PLASTIC ADDITIVES

In total, 56 organic plastic additives, including plasticisers (n=17), antioxidants (n=11) and their degradation products (n=3), UV stabilisers (n=11), flame retardants (n=13) and a vulcanisation accelerator (n=1), were analysed in road dust and sediment samples. The analysis was performed following of Yamahara et al.’s (2024a) procedure with a slight modification. The solid samples were passed through a 500-μm mesh stainless-steel sieve and grounded with a clean mortar to improve the sample uniformity. Each solid sample (60-mg dw. for road dust and 300–400-mg dw. for sediment) was placed in a 10-mL glass tube. Before extraction, 50 ng of 17 surrogate standards (DMP-d₄, DEP-d_4, DiBP-d₄, DnBP-d₄, BBP-d₄, DEHP-d₄, DnDP-d₄, BHT-d₂₁, UV-P-d₃, UV-326-d₃, UV-327-d₂₀, UV-328-d₁₂, BP-3-d₃, TCEP-d₁₂, TDCIPP-d₁₅, TPHP-d₁₅ and TBOEP-d₂₇) were spiked into each sample. The plastic-derived chemicals were extracted using 1.5 mL of dichloromethane (DCM; Fujifilm–Wako Pure Chemical Industries, Japan) by vortexing for 30 s. and ultrasonication for 15 min. The suspended matter in the extract was settled by centrifugation at 1,640×g for 5 min, and the supernatant was recovered using a Pasteur pipette to a 5 mL micro-concentration tube. This protocol was repeated three times for each sample, and the extract was concentrated at <0.5 mL under a gentle nitrogen stream. Next, 100 ng of 2-MeNA-d₁₀ was spiked into the extract as an internal standard. The final extract volume was adjusted to 0.5 mL, and 1 μL of the extract was automatically injected into a gas chromatograph mass spectrometer (GC/MS; Agilent 7890-B-5977B, Agilent Technologies, USA) in the selected ion monitoring mode. Separation was achieved using an HP-5MS column (length: 30 m, i.d.: 0.25 mm; film thickness: 0.25 μm; Agilent Technologies). The GC oven temperature programme started at 80°C, held for 1 min, and increased at 20°C/min to 160°C and 3°C/min to 300°C, and finally held for 15 min. The flow rate of He carrier gas (purity: 99.999%) was 1.2 mL/min. Approximately 3–10 mg of the traffic-related plastic products were analysed following the above-mentioned method, except that 100 and 200 ng of surrogates and internal standard were used, and the final extract volume was adjusted to 1 mL.

The plastic additives in individual MPs (>400 μm) were qualitatively analysed via Yamahara et al.’s (2024b) method, with a slight modification. The additive-derived chemicals were extracted with 1 mL of DCM under ultrasonication for 20 min. The extract was completely evaporated under a gentle nitrogen stream, reconstituted with 70 μL of DCM and injected into a GC-MS system operating in the scan mode.

QUALITY ASSURANCE AND QUALITY CONTROL

This information has been mentioned in the supporting information.

RESULTS AND DISCUSSION

ABUNDANCE, POLYMER TYPES AND SIZE DISTRIBUTION OF MICROPLASTICS

MPs were detected in all the analysed samples (road dust, stormwater and sediment) (Fig. S3). A total of 191 MPs were identified in road dust samples with abundances of 54,400–160,000 items/kg dw (mean: 90,600±60,100; median: 57,000) (Table 1). The mean level of MPs was comparable with that of road dust in Iran (98,030 items/kg dw: Abbasi et al., 2017; 74,769 items/kg dw: Abbasi et al., 2019) and higher than those reported in multiple studies reviewed by Yang et al. (2023). In total, 696 MPs were identified in the stormwater samples with abundances of 2.2–42 items/L (mean: 18±17; median: 14). MPs were 2–5 times more abundant in the first flush (9.9–42 items/L) than in the steady-state samples (2.2–18 items/L). A similar trend was reported by Sugiura et al. (2021) and Cho et al. (2023). Shafi et al. (2024) reported that the first flush event considerably contributed to the transport of MPs. Imbulana et al. (2024b) observed an increase in the MP abundance in surface river water during the initial phase of rainfall, suggesting the importance of controlling first flush events for mitigating the loading of MPs into the aquatic environment. In total, 303 MPs were identified in the sediments with abundance of 1,550–15,800 items/kg dw (mean: 7,280±5,960; median: 4,040). The abundance of MPs was approximately 5 times higher in the sediment near the stormwater outlets and weirs (SED-1, -5 and -8; 14,400±1,280 items/kg dw) than the other stations (3,030±1,140 items/kg dw). Eibes and Gabel (2022) reported that weirs act as a sink of riverine MPs by decreasing the flow velocity, and similar trends were reported by Mani et al. (2015) and Wicaksono et al. (2021). MP levels at the three points (SED-1, -5 and -8) were comparable with those of the Myanmar rivers (max.: 11,946–13,855 items/kg dw), where large quantities of plastic garbage have been discarded (Tun et al., 2024). Yin and Zhao (2023) reviewed 81 papers on MP pollution in freshwater sediments at 1,685 sampling sites worldwide. It is difficult to compare the abundance of MPs with those reported in other studies because of the differences in the analytical protocols, target polymers, particle size and detection methods used. Nevertheless, the contamination levels determined herein were approximately 7 times higher than the global average (1,290.88 items/kg dw; Yin and Zhao, 2023), suggesting that the urban small-scale river was highly contaminated with MPs.

Table 1 Abundances and polymer types of microplastics in road dust, stormwater, and river sediment

ID	Glass beads			ALL Microplastics (MPs)			TRWPs			Non-rubber MPs
	Weight (g or L)	Count (items)	Abudance (items/kg or L)	Weight (g or L)	Count (items)	Abudance (items/kg or L)	Weight (g or L)	Count (items)	Abudance (items/kg or L)	Weight (g or L)	Count (items)	Abudance (items/kg or L)	Non-rubber polymer types (n=14)
													PMMA	PET	PP	PE	EVA	CA	PU	PAS	PVC	Epoxy	PS	PDAP	PA	ACN
Road dust (RD: n=3)
RD-1	0.15	61	409,180	0.65	104	160,032	0.65	33	50,575	0.65	71	109,458	59	9						1	2
RD-2	0.15	58	392,133	0.66	38	57,489	0.66	20	30,257	0.66	18	27,231	14	4
RD-3	0.25	112	452,089	0.90	49	54,384	0.90	41	45,505	0.90	8	8,879	2			2	1			1		1		1
TOTAL RD	0.54	231	—	2.2	191	—	2.2	94	—	2.2	97	—	75	13		2	1			2	2	1		1
Stormwater (SW: n=4)
SW-1 FF	2.0	0	0	8.0	335	42	8.0	76	10	8.0	259	32	180	45	9	8	7	6	2	2
SW-1 SS	2.0	23	12	14	260	18	14	67	4.7	14	193	13	172	13	1	4	1				2
SW-2 FF	2.0	0	0	8.0	79	9.9	8.0	10	1.3	8.0	69	8.6	19	32	2	3	4	7	1				1
SW-2 SS	2.0	2	1.0	10	22	2.2	10	6	0.59	10	16	1.6	9	2	4			1
TOTAL SW	8.0	25	—	41	696	—	41	159	—	41	537	—	380	92	16	15	12	14	3	2	2		1
Urban small-scale river sediment (SED: n=8)
SED-1	5.1	139	27,214	5.1	72	14,063	5.1	9	1,758	5.1	63	12,305	62	1
SED-2	5.3	105	19,910	5.3	18	3,416	5.3	9	1,708	5.3	9	1,708		6		1			1			1
SED-3	5.2	35	6,830	5.2	11	2,124	5.2	2	386	5.2	9	1,737	6				1					1	1
SED-4	5.2	50	9,661	5.2	8	1,553	5.2	3	583	5.2	5	971	3	1		1
SED-5	0.85	74	87,544	5.3	84	15,760	5.3	64	12,008	5.3	20	3,752	8	7		1		1	1		1	1
SED-6	5.1	109	21,659	5.1	20	3,960	5.1	13	2,574	5.1	7	1,386	2	4												1
SED-7	5.1	47	9,284	5.1	21	4,110	5.1	13	2,544	5.1	8	1,566	5			1									2
SED-8	5.2	0	0	5.2	69	13,244	5.2	28	5,374	5.2	41	7,869	18	17	1				2		1	1			1
TOTAL SED	37	560	—	41	303	—	41	141	—	41	162	—	104	36	1	4		1	4		2	4			3	1

Abbreviations; TRWPs: Tire road wear particles, PMMA: Polymethyl methacrylate, PET: Poly(ethylene terephthalate), PP: Polypropylene, PE: Polyethylene, EVA: Ethylene vinyl acetate, CA: Cellulose acetate, PU: Polyurethane, PAS: Acryl-styrene-based copolymer, PVC: Polyvinyl chloride, PS: Polystyrene, PDAP: Poly(diallyl phthalate), PA: Polyamide, ACN: Acrylonitrile; FF: First flush, SS: Steady state

In total, 15 polymers were identified (Table 1). The major components (>90%) of MPs in the samples were TRWPs, PMMA and PET (Fig. 2). TRWPs are well-known as a major component of MPs in road dust (Vogelsang et al., 2019). Considerably, TRWPs are generated by abrasion between a tyre tread part and the pavement at rates of 53–160 mg/vehicle/km (Liu et al., 2022). A similar dominance of PMMA and PET in road dust has been previously reported (Kitahara and Nakata, 2020; Su et al., 2020; Yukioka et al., 2020; Yamahara et al., 2024b). The consistency of the polymer profiles (TRWPs, PMMA and PET) for road dust, stormwater and the sediment (Fig. 2) indicates that MPs in road dusts were transported by the stormwater and settled on the sediment of the small-scale river. Imbulana et al. (2024b) observed a large increase in the PMMA-MPs level in Japanese urban river water during rain events, which would have resulted from the release of MPs in road dust (especially PMMA) via stormwater runoff.

Fig. 2 Characteristics of microplastics (MPs) and glass beads (GBs) among road dust, stormwater, and creek sediment (a: polymer types of MPs; b: size distribution of MPs and GBs) ^*each value in parentheses in (a) represent a percentage

As for the size distribution of MPs, 100–400 μm-sized MPs accounted for 62%–87% of the total number of particles detected in the samples (Fig. 2). In particular, approximately half of the MPs in the stormwater were distributed in a 100–200-μm fraction. The first flush contained a high proportion of 100–200 μm-sized MPs (60%) compared with the steady-state sample (33%) (Fig. S4). This result indicates that small-sized MPs (SMPs) can be easily discharged by stormwater runoff, especially during first flush events. A similar trend was reported by Sugiura et al. (2021) and Cho et al. (2023). Imbulana et al. (2024b) observed a rapid increase of SMPs in urban river water during the initial phase of rain events, suggesting a high diffusion potential for SMPs. Hence, the sediment had a lower proportion of 100–200 μm-sized MPs (15%) than the other samples, despite the large input of SMPs via stormwater runoff.

ABUNDANCE AND SIZE DISTRIBUTION OF GLASS BEADS

GBs were detected in all road dust samples (392,000–452,000 items/kg dw; mean: 418,000±30,900; median: 409,000) (Fig. S2). The mean GB levels in road dust was approximately five times higher than that of the MPs. However, in the stormwater samples, the abundance and frequency of GBs was extremely low (0–12 items/L) compared with that of MPs (2.2–42 items/L) (Table 1). This indicates that GBs cannot easily be transported by stormwater, which may result from GBs with a higher density than MPs. In the sediment, GBs were detected in 7 of 8 stations at 6,830–87,500 items/kg dw (mean: 26,000±28,200; median: 19,900). The mean GB levels in the sediments were approximately three times higher than that of MPs, even though it is difficult for stormwater to transport GBs. This suggests that GBs were continuously integrated in the sediment. High abundances of GBs were found in the sediment near the stormwater outlets (SED-1: 27,200 items/kg dw; SED-5: 87,500 items/kg dw) (Table 1; Fig. S1). However, in SED-8, which did not have a stormwater outlet, no GBs were found despite the high abundance of MPs, suggesting that GBs could be a suitable indicator for tracking the source of traffic-related particulate contaminants. The GB levels in the sediment were 35–550 items/kg dw (med.: 280; coastal/intertidal sediment, England; Turner and Keene, 2023), 100–28,000 items/kg dw (med.: 1,400; Thames Estuary sediment, England; West-Clarke and Turner, 2024) and 20–52,500 items/kg dw (med.: 820; urban river sediment, Poland; Migaszewski et al., 2022). Herein, the median GB levels (19,900 items/kg dw) were greater than those reported previously. The presence of abundant GBs suggests the strong impact of traffic-related particulate contaminants on the urban small-scale river.

The dominant size of GBs in road dust and stormwater was 100–300 μm (Fig. 2). The highest peak in the distribution pattern of GBs corresponded to the 200–300-μm fraction. A similar distribution pattern was reported by West-Clarke and Turner (2024). However, river sediment tended to retain larger particles of GBs (300–600 μm) than the road dust and stormwater (Fig. 2), indicating that small-sized GBs were relatively easy to flow through the river current, whereas large-sized GBs tended to settle in the river sediment. This size distribution pattern suggests that large GBs (>300 μm) are more suitable indicators of proximity to sources of traffic-related particulate contaminants.

CONCENTRATIONS AND PROFILES OF THE PLASTIC ADDITIVES

ROAD DUST

In total, 32 of 56 target chemicals were identified in the road dust samples (Table 2). The total additive concentrations were 6,900–32,000 ng/g dw, and plasticisers showed the highest concentration (med.: 6,800 ng/g dw), followed by antioxidants (med.: 1,600 ng/g dw), UV stabilisers (med.: 320 ng/g dw) and flame retardants (med.: 240 ng/g dw).

Table 2 Concentrations of plastic additives in the road dust and river sediment (ng/g dw)

Chemicals	Road dust (n=3)						Urban small-scale river sediment (n=8)
	DF%	Mean±SD	Median (Min.-Max.)	RD-1	RD-2	RD-3	DF%	Mean±SD	Median (Min.-Max.)	SED-1	SED-2	SED-3	SED-4	SED-5	SED-6	SED-7	SED-8
ΣPlastic additives	—	17,000±13,000	11,000 (6,900-32,000)	32,000	11,000	6,900	—	3,600±4,200	2,200 (620-13,000)	13,000	4,000	2,000	840	2,300	780	620	5,200
Vulcanization accelerator (n=1)
Benzothiazole	100	120±70	81 (77-200)	200	81	77	100	7.9±6.6	4.8 (2.9-21)	5.6	3.6	7.8	3.2	15	3.9	15	21
Plasticizer (n=14)
DMP	100	14±4.4	12 (11-19)	19	11	12	100	2.2±1.1	1.8 (1.2-4.5)	2.1	1.9	1.5	1.4	3.2	1.6	1.2	4.5
DEP	100	100±33	87 (81-140)	81	87	140	100	31±25	23 (4.6-68)	11	50	11	11	58	34	4.6	68
DiBP	100	190±70	180 (120-260)	260	180	120	100	7.0±6.4	4.5 (1.5-21)	5.5	3.5	21	2.0	8.8	3.5	1.5	10
DnBP	100	1,900±400	1,800 (1,500-2,300)	2,300	1,500	1,800	100	210±130	200 (76-400)	260	91	400	83	350	130	76	260
BBP	100	17±1.5	17 (16-19)	16	17	19	75	4.2±4.9	2.5 (<1.7-15)	1.8	<1.7	3.1	1.9	8.5	<1.7	3.8	15
DCHP	100	5,600±9,000	670 (120-16,000)	16,000	670	120	100	1,600±3,500	11 (3.8-9,800)	9,800	3.8	44	4.3	7.3	5.1	14	3,100
DEHP	100	3,300±2,100	2,200 (1,900-5,700)	5,700	1,900	2,200	100	870±1,200	510 (180-3,700)	310	3,700	750	220	710	180	220	840
DnOP	100	690±960	190 (91-1,800)	1,800	190	91	100	240±440	32 (3.3-1,300)	1,300	14	26	3.3	220	6.4	37	350
DnDP	100	780±710	390 (360-1,600)	390	1,600	360	100	24±23	21 (4.0-72)	34	6.6	30	5.2	72	4.0	14	27
ΣPAEs	—	13,000±12,000	6,200 (4,900-27,000)	27,000	6,200	4,900	—	3,000±3,900	1,400 (330-12,000)	12,000	3,900	1,300	340	1,400	370	370	4,700
TXIB	100	310±45	310 (260-350)	260	310	350	100	51±37	41 (3.3-110)	46	34	100	20	61	36	3.3	110
ATBC	100	35±26	40 (6.6-57)	40	57	6.6	63	2.6±2.7	1.7 (<0.28-7.1)	5.4	0.81	7.1	<0.28	2.5	<0.28	<0.28	4.4
DEHA	100	47±9.5	47 (37-56)	56	37	47	38	2.4±3.4	n.d. (n.d.-7.6)	7.1				7.6			4.4
DEHT	100	940±1,300	230 (180-2,400)	2,400	230	180	100	26±22	18 (9.0-73)	16	10	73	23	46	9.0	19	12
TOTM	100	75±56	47 (39-140)	140	39	47	100	17±18	11 (3.0-56)	13	14	56	5.6	26	3.0	5.9	9.3
ΣEPs	—	1,400±1,300	670 (630-2,900)	2,900	670	630	—	99±70	74 (28-240)	88	59	240	49	140	48	28	140
ΣPlasticizers	—	14,000±13,000	6,800 (5,500-29,000)	29,000	6,800	5,500	—	3,100±3,900	1,600 (380-12,000)	12,000	3,900	1,500	380	1,600	410	400	4,800
Antioxidant (n=6)
BHT	100	27±14	22 (15-43)	43	22	15	100	3.5±2.6	2.3 (1.1-7.4)	6.6	1.1	5.4	7.4	1.2	2.4	2.2	1.6
BHT-Q	100	200±35	220 (160-220)	160	220	220	100	210±180	100 (79-550)	550	84	310	370	84	110	90	79
BHT-CHO	100	72±67	39 (27-150)	150	27	39	100	24±22	16 (4.1-66)	4.1	4.9	10	42	9.3	66	21	33
2,4-DTBP	100	1,000±1,700	27 (19-3,000)	27	3,000	19	63	1.5±1.5	1.5 (n.d.-4.8)	1.9		<0.61	<0.61	1.4	2.4	1.6	4.8
6PPD	100	350±180	260 (240-560)	560	260	240	63	11±11	9.5 (n.d.-31)	12		8.0		31		11	22
TDtBPP	100	430±170	370 (300-630)	630	300	370	75	37±22	49 (<17-60)	54	<17	57	20	60	<17	44	59
ΣAntioxidants	—	2,100±1,500	1,600 (900-3,800)	1,600	3,800	900	—	290±180	200 (90-630)	630	90	390	440	190	180	170	200
UV stabilizer (n=8)
UV-P	100	82±93	29 (27-190)	190	29	27	100	80±150	8.8 (1.7-450)	120	3.3	14	1.7	450	3.5	3.3	44
UV-326	100	19±4.0	17 (17-24)	24	17	17	100	6.8±2.8	6.9 (3.4-11)	3.4	9.1	4.4	3.8	7.4	6.4	11	8.9
UV-327	33	9.2±10	<12 (<12-21)	21	<12	<12	50	21±52	1.9 (<1.8-150)	<1.8	7.6	<1.8	<1.8	2.6	150	<1.8	4.9
UV-328	67	5.2±2.0	6.0 (<5.7-6.7)	6.0	6.7	<5.7	100	3.2±2.1	3.3 (0.89-6.1)	2.9	1.0	4.3	0.92	5.6	0.89	3.7	6.1
ΣBUVSs	—	110±110	53 (44-240)	240	53	44	—	110±160	43 (6.4-470)	130	21	23	6.4	470	160	18	64
BP-3	100	5.4±1.0	5.6 (4.3-6.3)	6.3	4.3	5.6	75	1.9±1.4	2.1 (n.d.-3.4)	1.4	3.4	3.0	1.9	2.2	3.4
Tinuvin® 770	100	160±87	150 (77-250)	77	250	150	50	5.8±747	2.7 (n.d.-17)	17				7.1		5.4	17
Octinoxate	100	19±7.0	16 (14-27)	16	14	27	88	3.1±1.9	2.5 (2.1-7.9)	2.7	2.4	2.4	2.1	2.5	2.6		7.9
Octocrylene	33	7.0±12	n.d. (n.d.-21)			21	13	0.48±1.2	n.d. (n.d.-3.3)			<0.67		3.3
ΣUV stabilizers	—	300±49	320 (250-340)	340	320	250	—	120±160	59 (10-480)	150	27	28	10	480	170	23	89
Flame retardant (n=3)
TCIPP	100	180±58	200 (110-220)	220	200	110	88	18±27	6.1 (<3.5-75)	5.7	4.5	75	2.8	44	<3.5	6.4	18
TBOEP	67	37±37	36 (n.d.-74)	74	36		25	3.4±6.3	n.d. (n.d.-15)		12			15
TEHP	100	44±16	35 (34-62)	34	35	62	100	5.9±5.7	5.4 (0.82-18)	6.1	1.6	18	0.85	4.7	7.1	0.82	8.4
ΣFlame retardants	—	260±81	270 (170-330)	330	270	170	—	29±32	15 (3.7-93)	12	18	93	3.7	64	7.1	7.2	26

Phthalate esters (PAEs) were the most frequently detected chemicals in the road dust. Di(2-ethylhexyl) phthalate (DEHP; CAS#: 117-81-7; med.: 2,200 ng/g dw) was the most dominant PAE, followed by di-n-butyl phthalate (DnBP; CAS#: 84-74-2; med.: 1,800 ng/g dw), dicyclohexyl phthalate (DCHP; CAS#: 84-61-7; med.: 670 ng/g dw) and di-n-decyl phthalate (DnDP; CAS#: 84-77-5; med.: 390 ng/g dw). These three dominant PAEs in the road dust (DEHP, DnBP and DCHP) have been designated as substances of very high concern (SVHCs) because of their reproductive toxicities and endocrine disrupting properties (ECHA 2018, 2019). Table S4 shows that the total PAE concentration measured herein was relatively higher than that of previous studies. Five emerging plasticisers (EPs; Σ₅EPs=670 ng/g dw at a median level) were also identified in the road dust, accounting for 10% of the total plasticisers (Table 2). We previously reported a similar ratio of PAEs and EPs (med.: PAEs:EPs=6,300:670 ng/g dw) for soils in an open dumping site in Thailand (med.: PAEs:EPs=3,800:310 ng/g dw; Yamahara et al., 2024a). Although there has been an increase in the demand for EPs as alternatives to phthalate-based plasticisers (Bui et al., 2016), the potential toxicity of EPs has been reported in several studies (Qadeer et al., 2022).

Three major antioxidant-related chemicals were identified in the road dust: tris(2,4-di-tert-butylphenyl) phosphate (TDTBPP; CAS#: 95906-11-9; med.: 370 ng/g dw), N-1,3-dimethylbutyl-N’-phenyl-p-phenylenediamine (6PPD; CAS#: 793-24-8; med.: 260 ng/g dw) and 2,6-di-tert-butyl-p-benzoquinone (BHT-Q; CAS#: 719-22-2; med.: 220 ng/g dw). TDTBPP, an organophosphate ester (OPE), is an undesirable degradation product of the antioxidant Irgafos^® 168 (Yang et al., 2016) and has been found in various media (airborne fine particles, e-waste dust, house dust, sediment and plastic debris) at unexpectedly high concentrations (Liu and Mabury, 2018; Venier et al., 2018; Tanaka et al., 2019, 2023; Yu et al., 2020; Yamahara et al., 2024a). A recent report highlighted the high concerns regarding TDTBPP, including a rapid increase in concentration in Arctic air, high persistency and a higher risk relative to the traditional OPEs (Liu et al., 2023). The 6PPD is the most common antioxidant used in rubber products, including automobile tyres (Chen et al., 2023). The 6PPD level in road dust was comparable with those reported previously, reviewed by Liang et al. (2024). The quinone-derivative of 6PPD (6PPD-Q) has a lethal effect on some salmon species (Tian et al., 2021, 2022; Brinkmann et al., 2022; Greer et al., 2023; Bohara et al., 2024). Hiki and Yamamoto (2022) reported that ozone concentrations in the air were related to the presence of 6PPD-Q in road dust. Although the occurrence of 6PPD-Q was not measured herein, comparable levels of 6PPD-Q and 6PPD have previously been found in road dust (Huang et al., 2021; Deng et al., 2022; Hiki and Yamamoto, 2022). The concentrations of BHT-Q, a transformation product of synthetic phenolic antioxidants, including butylated hydroxytoluene (BHT; CAS#: 128-37-0), were 10 times higher than that of BHT (med.: 22 ng/g dw). The dominance of BHT-Q and TDTBPP (in addition to 6PPD-Q) in road dust shows the importance of understanding the environmental fate and ecological risk of original and their derivatives.

In total, eight UV stabilisers were identified in road dust. The concentrations of three stabilisers in the following order: bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate (Tinuvin^® 770; CAS#: 52829-07-9; med.: 150 ng/g dw), 2-(2H-benzotriazol-2-yl)-p-cresol (UV-P; CAS#: 2440-22-4; med.: 29 ng/g dw), 2-(5-Chloro-2-benzotriazolyl)-6-tert-butyl-p-cresol (UV-326; CAS#: 3896-11-5; med.: 17 ng/g dw) and octinoxate (CAS#: 5466-77-3; med. 16 ng/g dw). Tinuvin^® 770 is a hindered amine light stabiliser (HALS) that confers photo-stabilisation by scavenging the free radical intermediates in the photo-oxidation process (Paine et al., 2014; Kot et al., 2021). Deng et al. (2024) provided the first evidence of the abundant and ubiquitous occurrence of HALSs as emerging pollutants in indoor/outdoor dust. The Tinuvin^® 770 concentration determined herein is almost the same as that found for parking lot dust (med.: 185 ng/g dw, range: 72.9–977 ng/g dw; Deng et al., 2024). UV-P is a congener of benzotriazole UV stabilisers (BUVSs), which is regarded as a chemical group of highly-concern triggered by the designation of UV-328 in Annexe A of the Stockholm convention as a persistent organic pollutant (UNEP, 2023). In fact, UV-P reportedly exhibits agonistic activity on human oestrogen receptors (OR) α and β and an antagonistic activity on the human androgen receptor (Sakuragi et al., 2021). However, little is known about the occurrence in road dust. Nakata et al. (2013) analysed four BUVSs (UV-320, -326, -327 and -328) in 22 road dust samples from Kumamoto, Japan. The levels of the three major BUVSs other than UV-320 (Σ₃BUVSs; med.: 27 ng/g dw; range: 3.0–167 ng/g dw) were comparable with those measured herein (Σ₃BUVSs; med.: 24 ng/g dw; range: 17–51 ng/g dw) and in Toronto, Canada (Σ₃BUVSs; med.: 20 ng/g dw; range: 1.0–52 ng/g dw; Parajulee et al., 2022).

Three OPEs, tris(2-chloroisopropyl) phosphate (TCIPP; CAS#: 13674-84-5; med.: 200 ng/g dw), tris (2-ethylhexyl) phosphate (TEHP; CAS#: 78-42-2; med.: 35 ng/g dw) and tris(2-butoxyethyl) phosphate (TBOEP; CAS#: 78-51-3; med.: 36 ng/g dw), were identified in the road dust. The OPE levels were moderate compared with those reported previously (Table S5). The dominant OPE was TCIPP similar to Pang et al. (2023), Wu et al. (2023), Li et al. (2018) and Zhang et al. (2020). In general, OPEs in road dust considerably originated from the release of hydraulic fluids and car dust (Tokumura et al., 2017; Li et al., 2019). However, atmospheric deposition was recently identified as another major source of OPEs in street dust (Pang et al., 2023).

RIVER SEDIMENT

In total, 32 additive chemicals were identified in the river sediment samples. The concentrations total additive was 620–13,000 ng/g dw (med.: 2,200 ng/g dw), which was approximately 5 times lower than that of road dust. The concentrations of the individual additives decreased in the following order: plasticisers (med.: 1,600 ng/g dw), antioxidants (med.: 200 ng/g dw), UV stabilisers (med.: 59 ng/g dw) and flame retardants (med.: 15 ng/g dw). The concentration profiles of the additives in the sediment were mostly similar to those of the road dust. For instance, the concentrations of the additives decreased in the following order: DEHP (med.: 510 ng/g dw), DnBP (med.: 200 ng/g dw), BHT-Q (med.: 100 ng/g dw), TDTBPP (med.: 49 ng/g dw) and TXIB (med.: 41 ng/g dw); this result implies that road dust is a potential source of plastic additives in river sediment. The concentrations of some chemicals were locally high (e.g. DCHP in SED-1 and -8: 3,100–9,800 ng/g dw; UV-P in SED-1 and -5: 120–450 ng/g dw; UV-327 in SED-6: 150 ng/g dw).

Overall, the contamination levels of the additive-derived chemicals measured herein were moderate or slightly high compared with other studies. The PAE levels measured herein (Σ₁₁PAE; med.: 1,400 ng/g dw; range: 330–12,000 ng/g dw) were comparable with or greater than those measured for urban river sediments (Wang et al., 2006; Sun et al., 2013; Teil et al., 2014; Li et al., 2017; Liu et al., 2020; Yang et al., 2020), as reviewed by Xu et al. (2024). However, high PAE concentrations have been reported for the sediments of the Xi River, China (range: 22,400–369,000 ng/g dw; Li et al., 2016) and the Salt River, Taiwan (med: 11,743 ng/g dw; range: 2,448–63,457 ng/g dw; Wang et al., 2024), receiving multiple wastewaters, including domestic, industrial, sewage wastewater and stormwater. The urban small-scale river targeted herein has no contribution from domestic and industrial wastewater; therefore, the PAE concentrations in the sediment remained moderate or slightly high.

The concentrations of synthetic phenolic antioxidant (SPA) in the sediment decreased in the following order: BHT-Q, BHT-CHO (med.: 16 ng/g dw) and BHT (med.: 2.3 ng/g dw). The relatively high concentrations of derivatives (BHT-Q and BHT-CHO) in the sediments were similar to the results obtained for road dust (BHT-Q>BHT-CHO>BHT) and in other studies analysing SPAs in the sediment of St. Lawrence River, USA (med.: BHT-Q: 98 ng/g dw>BHT-CHO: 60 ng/g dw>BHT: 9 ng/g dw; Castilloux et al., 2022). Moreover, TDTBPP and 6PPD were identified in more than half of all sediment samples. The contamination level of TDTBPP in the sediment (med.: 49 ng/g dw; range: <17–60 ng/g dw) was comparable with or greater than that of Taihu Lake, China (med.: 25.5 ng/g dw; range: n.d.–135 ng/g dw; Ye et al., 2021). However, Ye and Su (2022) reported higher concentrations of TDTBPP in the river sediments in an industrial area (med.: 79 ng/g dw; range: 7.79–1,970 ng/g dw) and an e-waste recycling area (med.: 247 ng/g dw; range: n.d.–825 ng/g dw). Only a few studies have been conducted on the contamination status of 6PPD in river sediment. The median level of 6PPD in this study (n=8; 9.5 ng/g dw) was comparable with or lower than those reported in other studies (n=32: 14.4 ng/g dw; Zeng et al., 2023; n=30: 25 ng/g dw; Zhu et al., 2024).

Herein, BUVSs were dominant UV stabilisers in the sediment, accounting for 70% of total UV stabilisers at a median level. The pollution levels of BUVSs in this study (Σ₆BUVSs; med.: 43 ng/g dw) were comparable with or greater than those reported in most previous studies (Wick et al., 2016; Peng et al., 2017; Parajulee et al., 2018; Hu et al., 2021; Castilloux et al., 2022; Du et al., 2023; Khare et al., 2023; Zhao et al., 2024; Table S6). Parajulee et al. (2018) investigated the effect of rainfall and snowmelt events on the contamination level of BUVSs in rivers. Reportedly, the rivers that are not directly affected by domestic and industrial wastewater can be polluted at moderate or high levels by stormwater runoff. Nakata et al. (2013) analysed four BUVSs (UV-320, 326, 327 and 328) in road dust and estimated that 1,700 μg of BUVSs is stored on the road surface of Route 3 in Kumamoto, Japan, thereby indicating that stormwater runoff is an inflow pathway of BUVSs into an urban small-scale river without a wastewater outlet. However, higher BUVSs concentrations than those measured in this study have been reported for Japanese rivers receiving domestic and industrial wastewater (Nakata et al., 2009; Kameda et al., 2011; Table S6), suggesting a large contribution from BUVSs in personal care products such as sunscreen lotions and shampoos via WWTP effluent (Montesdeoca-Esponda et al., 2012, 2013).

Three OPEs (TCIPP, TEHP and TBOEP) were identified in the river sediment. The average level of total OPEs (29 ng/g dw) was moderate among the studies reviewed by Luo et al. (2020). The profile of OPEs in the river sediment (TCIPP>TEHP, TBOEP) was consistent with that of the road dust, implying a similar source of OPEs for the road dust and sediment.

RELATIONSHIP BETWEEN MICROPLASTICS AND PLASTIC ADDITIVES

The relationship between the concentrations of MPs and plastic additives (detection frequency: >70%) in the samples to assess the function of MPs as carriers of plastic-derived chemicals. Fig. 3 shows a significantly positive correlation (r=0.74, p<0.01) between the concentrations of the total plastic additives and MPs. The individual concentrations of 20 chemicals were positively correlated with MP abundances (Fig. S5), suggesting that MPs act as a vehicle of plastic-derived chemicals in the environment, and a similar trend was observed in our previous study (Yamahara et al., 2024a). Notably, non-rubber MPs showed strong correlations with the total additive concentration (r=0.93; p<0.0001), whereas TRWPs represented no significance (r=0.55; p>0.05) (Fig. 3). The concentrations of seven chemicals (DCHP, DEHP, ATBC, DEHT, TOTM, UV-328 and TEHP) were not significantly correlated with TRWP abundances but were significantly correlated with the abundances of non-rubber MPs (Fig. S5). This result confirms that the plastic additive chemicals in this study area were mainly derived from non-rubber MPs. However, TRWPs may play an important role as carriers of rubber-related chemicals and other traffic-related contaminants (e. g., vulcanisation accelerators, polycyclic aromatic hydrocarbons and heavy metals), which were not the main targets of this study. For instance, benzothiazole (BTH; CAS#: 95-16-9), a molecular marker of tyre wear particles (Rødland et al., 2023), was strongly correlated with TRWPs (r=0.84; p<0.01; Fig. S5) and 6PPD (r=0.96; p<0.00001; Fig. S6), implying that tyre wear particles contributed to the occurrence of rubber additives.

Fig. 3 Correlation between the concentrations of microplastics (MPs) and plastic-derived chemicals in road dust and river sediment: (a) all MPs vs. total additives, (b) TRWPs vs. total additives, (c) non-rubber MPs vs. total additives: the statistical values (r and p) are Spearman’s rank correlation coefficients

POTENTIAL SOURCES OF MICROPLASTICS AND PLASTIC ADDITIVES

In total, 43 plastic additives were detected while analysing 28 traffic-related products (Fig. 4; Table S7). The concentration of the total additives decreased in the following order: polyvinyl chloride (PVC) road marking sheets (RMSs) (n=4; med.: 15,000 μg/g), PMMA road marking paints (RPs) (n=6; med.: 12,000 μg/g), reflectors (n=7; med.: 470 μg/g) and EVA-RMSs (n=2; med.: 200 μg/g). The high concentration of plastic additives in the PVC-RMSs was derived from large quantities of DEHP (up to 1.7% w/w), UV-531 (CAS#: 1843-05-6; up to 0.7% w/w), octocrylene (CAS#: 6197-30-4; up to 0.3% w/w) and DEHA (up to 0.4% w/w). The additive profiles of the PMMA-RMPs comprised three main types of PAEs (DnBP, DCHP and DnOP) and UV-P, with the median concentrations reaching approximately 1.1% and 0.015% w/w, respectively. The median concentrations of the total additives in the EVA-based RMSs and RPs were 240 and 60 times lower than those of PVC-RMSs and PMMA-RMPs, respectively (Table S7), suggesting that EVA-based RMS and RMP are more eco-friendly products concerning the plastic additive concentrations. Various BUVSs were found in the reflectors at relatively high concentrations compared with those of other products (e.g. max.: UV-234: 0.42% w/w in Tape-2; UV-328: 0.23% w/w in Tape-1; UV-329: 0.46% w/w in REF-4). In the future, following the UV-328 restriction, there is a potential for the restricted use of other BUVSs because of the adverse effects on ecosystems. Plastic reflective products could suitably monitor the replacement of BUVSs. A tyre tread reportedly contain high concentrations of rubber-derived chemicals, BTH (13 μg/g) and 6PPD (35 μg/g), but not very high levels of the total plastic additives (120 μg/g); therefore, the concentrations of TRWPs and plastic additives in the road dust and sediment were not significantly correlated (Fig. 3). Some of these traffic-related products deteriorated and broke on the road surface (Fig. 1) and can be a source of MPs and plastic additives.

Fig. 4 Concentrations and compositions of organic plastic additives in traffic-related products (n=28) ^*abbreviation of the name of plastic additives are listed in Table S1-1

Finally, we identified the source of the PMMA-MPs from the plastic additive profiles by referring to Yamahara et al. (2024b). We individually analysed the additive profiles in 30 pieces of PMMA-based MPs (>400 μm) detected in road dust, stormwater and river sediment. Fig. 5 shows the GC chromatograms of MPs and traffic-related products. Several pieces of the MPs detected in the road dust, stormwater and river sediment had quite similar additive profiles to those of the road paints, including the braille block (Fig. 5); this suggests that these MPs originated from RMPs and were transported from road dust to river sediment via stormwater runoff. To estimate the contribution rate of RMPs to MPs, the source of the MPs in which additives were not analysed was indirectly determined by comparing the shape, colour and polymer types with MPs for which the source had been directly identified. Consequently, 37% MPs in road dust, 54% in stormwater runoff and 24% in river sediment were derived from RPs at a number basis. As mentioned above, the PMMA-RMPs contained high levels of plastic additives. Some of these additive chemicals (such as DnBP, DCHP and UV-P) are SVHCs because of their adverse effects on organisms, including endocrine disruption and reproductive toxicity. The relatively high concentrations of DCHP and UV-P observed in the road dust and river sediment can be explained by the presence of the road paint-based MPs containing these chemicals. In addition to organic plastic additives, Yamahara et al. (2024b) reported high concentrations of Pb (3.8% w/w) and Cr (0.65% w/w) derived from a yellow inorganic pigment of PbCrO₄ in a PMMA braille block (BB-2). These results highlight the concerns about the ecological risk of additive-rich MPs settled in the urban small-scale river sediment, in addition to the necessity to replace these products of high concern with more eco-friendly substances.

Fig. 5 Total ion chromatograms obtained by GC-MS analysis for microplastics (MPs) in road dust, stormwater, and river sediment. a) similar additive profiles between PMMA-based road paint (RP-3) and MPs, b) similar additive profiles between PMMA-based braille block (BB-1 and -2) and MPs, c) other PMMA-based MPs

CONCLUSION

The urban small-scale river was highly polluted by road dust–associated MPs via stormwater runoff. The concentrations of MPs and plastic additives were significantly positively correlated, suggesting that MPs function as a carrier of these chemicals. In addition, a number of GBs was detected in road dust and river sediment, indicating the strong impact of traffic activities on urban small-scale rivers. Traffic-related plastic products, such as RMSs, road paint and reflectors, contained large quantities (up to 1.5% w/w) of harmful plastic additives (PAEs, BUVSs and benzophenones, which are endocrine disruptors and reproductive toxicants) and are potential sources of these chemicals on the urban road surface. Notably, PMMA-based road paint was identified as the source of 24%–54% of MPs detected in the study area by source tracing using specific additive profiles as chemical indicators. However, the sample size was limited and further monitoring is required with a large sample size for a robust understanding.

ACKNOWLEDGEMENTS

We thank to Mr. Junpei Okita (Kumamoto University) for supporting sample collection under heavy rain condition. Special thanks are also due to Mr. Hiroki Ueda and the Ueda (Kumamoto University) Automobile Company (Kumamoto, Japan) for donating a used car for chemical analysis. This study is partly supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant #: 22H03762), and the Environment Research and Technology Development Fund (JPMEERF21S11900) of the Environmental Restoration and Conservation Agency provided by Ministry of the Environment of Japan.

SUPPLEMENTARY MATERIAL

Text S1, Quality assurance and quality control (QA/QC); Fig. S1, Pictures of stormwater outlets and weirs constructed along the urban small-scale river in Kumamoto City, Japan; Fig. S2, Stereomicroscopic photos of glass beads (GBs); Fig. S3, Photos of microplastics detected in road dust, stormwater, and river sediment; Fig. S4, Comparison of abundance and size distribution of microplastics between first flush and steady state stormwater samples; Fig. S5, Correlations between concentrations of microplastics and plastic additives in the road dust and sediment samples; Fig. S6,. Correlations between concentrations of benzothiazole and 6PPD in the road dust and sediment samples; Table S1-1, Detail information of target analytes in this study (n=56); Table S1-2, List of deuterium-labeled chemical standards (n=18); Table S2, Contamination levels of target analytes in procedural blanks (n=8); Table S3, Recovery rates (%) of surrogates; Table S4, Comparison of concentrations of phthalate esters in road dust; Table S5, Comparison of concentrations of organophosphate esters in road dust; Table S6, Comparison of concentrations of BUVSs in river sediment samples; Table S7-1, Concentrations of organic plastic additives in 28 traffic-related plastic products (unit: μg/g) ^*categories; Table S7-2, Concentrations of organic plastic additives in 28 traffic-related plastic products (unit: μg/g) ^*individuals

This material is available on the Website at https://doi.org/10.5985/emcr.20240033.

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