Tyre wear particles and metals in highway roadside ditches: Occurrence and potential transport pathways.

Tyre wear particles (TWP) pose significant environmental concerns, necessitating a comprehensive understanding of their environmental distribution for accurate risk assessment. Roadside soil has not been extensively studied for TWP occurrence and distribution. This study aims to characterise the occurrence and distribution of TWP and associated metals in roadside soils and to investigate the correlations between these contaminants. Soil samples were collected from two road ditches along a Swedish national motorway at varying depths and distances from the contamination source. TWP in fractions <500 μm were analysed using PYR-GC/MS. Results indicated that TWP concentrations in soil samples ranged from 0.74 ± 0.20 to 12.40 ± 1.88 mg/kg d.w., consistent with other studies, and decreased with distance from the road, similar to Zn. In one ditch, TWP concentrations remained constant with depth, unlike concentrations of Co and Cr, which increased, while in the other ditch, TWP and most metals did not decrease with depth or distance from the outlet. Strong correlations were found between concentrations of TWP and Zn in one, but not the other, where Zn might have followed different transport due to leaching. Metal correlations in both ditches suggest traffic-related but not necessarily tyre wear origins. These findings are crucial for risk assessments of traffic-related pollutants, particularly TWP, and their spread into soils.