Distribution of natural and anthropogenic lead in Mediterranean soils

The concentrations and isotopic composition of Pb have been combined with a selective sequential dissolution procedure to track the distribution of anthropogenic and natural Pb in the soil of semiarid climate and the penetration pathway of petrol-Pb within the soil profile. Soil samples were collected from soil profiles adjacent to a major highway (8-23 m) and from soil profiles 500 m away from the same highway. The selective sequential dissolution procedure was used to determine the distribution of Pb between the different soil components: soil carbonate, organic matter, Fe-oxides and hydrous oxides, and aluminosilicates. Natural Pb is associated mainly with aluminosilicates (~60%) and Fe-oxides (~30%), and only a small fraction with the soil carbonate and organic matter (~10%). In contrast to the distribution of natural Pb, the distribution of anthropogenic Pb, which accumulates mainly in the upper part of the soil profile, is ~40% with the soil carbonate, ~10% with organic matter, ~35% with Fe-oxides, and only ~15% with aluminosilicates. On the basis of concentration and isotopic composition of total Pb, the deeper horizon of the roadside soil (10-30 cm) and the soil sampled 500 m from the highway seem uncontaminated. However, the isotopic composition of the labile components in the soil (Pb_carb, Pb_org, and Pb_Fe-ox) indicates that these soils are actually contaminated. This contamination implies that over the period of vehicle pollution (~40 yr), petrol-Pb has penetrated through the entire roadside soil profile (25-30 cm). Furthermore, Pb isotopes in aluminosilicates of the upper part of the roadside soil indicate that this Pb too is anthropogenic. Measurements of Pb concentration and isotopic composition in roadside soil profiles sampled in 1997 and archived samples collected at the same location in 1982 provided a unique opportunity to trace petrol-Pb penetration into the soil.