Intra- and inter-site variability of soil contamination in road shoulders – Implications for maintenance operations

This study addresses soil contamination in vegetated road shoulders with diffuse inflow of runoff. It aims (i) to characterize the spatial distribution of three metals (copper, lead, zinc) and PAHs, and (ii) to identify influencing factors for the inter-site differences. An extensive sampling campaign was carried out on forty road segments in the Paris region, targeting various distances and soil depths. Copper and zinc contamination was found to be mainly restricted to the first 30 cm from the road, where their concentrations ranged respectively between 40 and 270 mg.kg⁻¹, and between 150 and 950 mg.kg⁻¹ (with a few additional extreme values related to the presence of a galvanized steel guardrail). Lead contamination was moderate (< 100 mg.kg⁻¹ in more than 75% of the sites) and relatively uniform across the width of the shoulders. Conversely, highest PAH concentrations were found further from the road. These differences between contaminants were likely due to the varying importance of wet weather-related processes and atmospheric transport in their dispersion mechanisms. Copper and zinc concentrations correlated well with the traffic density, which alone explained ≥ 69% of the inter-site variability, whereas lead and PAHs did not exhibit such dependence. Soil organic matter was found to control the “ceiling” concentration of metals, thus limiting the maximum amounts that can be intercepted from road runoff. These results illustrate the feasibility of estimating contamination levels on road shoulders, and thus offer interesting perspectives for better consideration of the infrastructures' maintenance needs and improved routine operations.