Heavy metal content in soils of Réunion (Indian Ocean)

This study was designed to inventory heavy metal concentrations (Cd, Cr, Cu, Hg, Ni, Pb and Zn) in the soils of Réunion (volcanic island in the Indian Ocean). Thirty-nine sampling sites were selected to account for the entire range of soils present on the island. From one to three layers of cultivated and uncultivated soils were sampled, with 84 soil samples analyzed overall. Réunion soils were found to be acidic (median pHwater = 5.7) and heavy metal concentrations (median) were 0.15 mg/kg for Cd, 0.16 mg/kg for Hg, 52.9 mg/kg for Cu, 92.1 mg/kg for Ni, 146.1 mg/kg for Zn and 165.9 mg/kg for Cr. 83% of the soil samples studied had a Pb concentration below the detection limit (i.e. 0.75 mg/kg). We attempted to identify the heavy metal sources by assessing: (i) heavy metal distributions in soil profiles, (ii) heavy metal contents in cultivated soils relative to uncultivated soils, and (iii) relationships between soil heavy metal contents and the origin of the volcanic parent material. Cd and Pb distributions in the soil profile highlighted an impact of human activities. Volcanic eruptions could account for the high soil Hg concentrations, which ranged from 0.03 to 0.81 mg/kg. We demonstrated that the high soil concentrations of Cr, Cu, Ni and Zn were mainly due to the natural pedo-geochemical background.