Heavy metal pollution of road dust and roadside soil near a major rural highway.

The concentrations of lead, copper, cadmium, zinc, nickel and chromium were measured in road dust and roadside soils from a French major highway. The profiles of total levels in the soil as a function of distance from the road edge and as a function of depth were investigated. Lead, zinc, copper and, to a lesser extent, cadmium contamination were found in samples taken on the surface and in the immediate vicinity of the road. The observed concentrations decreased rapidly with distance and depth. Soil samples appeared to exhibit little nickel and chromium pollution. The geochemical phases on which heavy metals (lead, copper, zinc, cadmium and chromium) were fixed preferentially, as well as the potential mobility of the different metals under certain physical and chemical conditions were evaluated. Sequential extractions and single extractions with different reagents were performed. Chromium, which was present mostly in residual form, was mainly of natural origin in the studied samples and was not highly mobilisable. Cadmium was the most easily exchangeable element in case of variations in the physical and chemical conditions but the quantities involved remained small. Lead and copper did not appear to be highly mobile. Only drastic conditions, such as those that may be produced by accidental spillage of a chemical product (acid or complexing agent), mobilised them quantitatively. A significant risk of mobilisation was to be feared only in the case of zinc, which is very sensitive to acid pH.