Stock and vertical distribution of microplastics and tire and road wear particles into the soils of a high-traffic roadside biofiltration swale

Urban highways are a central infrastructure in megacities and represent diverse sources of microplastic pollution. Understanding the relative contribution of these microplastic sources, including the abrasion of macrolitter and the generation of tire and road wear particles (TRWP) is necessary to better assess the sources and fate of microplastics in terrestrial environments. This study focuses on microplastic (MP) and TRWP infiltration in the soil of a biofiltration swale alongside a high-traffic highway in the north of Paris, France. First, using manual core sampling, soil samples were collected and analysed by micro-Fourier Transformed Infrared (micro-FTIR) imaging and Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC/MS) to determine the stock and vertical distribution of microplastics and TRWP. Secondly, the relative contribution of major microplastics and TRWP sources into the soils were estimated. Finally, these field results were then used to provide an estimate of the mass balance of microplastics and TRWP on that site. Results revealed a median concentration of 5.4 μg MP g⁻¹, three orders of magnitude lower than the median TRWP concentration of, 1.16 mg TRWP g⁻¹. In both types of contaminations, surface samples presented a significantly higher microplastic abundance than deeper layers, with 90 % of the overall stock located in the surface layers. The major contributors to microplastics accumulation in the soils were found to be fragmentation of macrolitter found on-site, followed by road runoff. The TRWP accumulation on-site was significantly lower than the TRWP production from the local traffic, suggesting TRWP may either remain trapped in the road or are deposited away from the road. To better understand the distance around a road that is affected by TRWP and microplastic emissions, further studies should be conducted at varying distances around highways. The present study focusing on a site highly exposed to microplastic and TRWP pollution helps prioritize the sources and improves the understanding of the short scale transfer dynamics of the plastic pollution stemming from an urban road.