Distribution of trace elements in soils from the Sudbury smelting area (Ontario, Canada)

The sequential extraction procedure proposed by the European Commission Measurement and Testing Programme, combined with Scanning Electron Microscopy and Energy Dispersive X-ray Analysis (SEM/EDS), was applied to identify and quantify the chemical and mineralogical forms of Cu, Ni, Fe, Mn, Zn, Pb, Cr and Cd present in the topsoil from a mining and smelting area near Sudbury (Ontario, Canada). The possible mobility of the chemical forms was also assessed. The metal fractions: (1) soluble and exchangeable, (2) occluded in manganese oxides and in easily reducible iron oxides, (3) organically bound and in form of sulphides, (4) residual mainly present in the mineral lattice structures were separated. Cu and Ni were the major metallic contaminants, occurring in soils in broad ranges of concentrations: Cu 11-1890 and Ni 23-2150 mg kg(-)1. Cu was uniformly distributed among all the extracted fractions. Ni was found associated mainly with the residual forms, accounting for 17-92%, with an average of 64%, of the total Ni present in the soils. Fe, Mn, Zn, Pb, Cr and Cd, while occurring in most analysed samples in normal soil concentrations, were primarily held in the residual mineral fraction (on average >50%). The soluble and exchangeable forms made a small contribution (less than or equal to8.1%) to the total content of metals extracted. At least 14% of the total Cd, Mn and Pb was mobilised from the reducible forms. The oxidizable fraction assumed mean values higher than 10% only for Pb and Zn. Statistical treatment of the experimental data showed significant correlations between total metal content of the soils, some soil properties such as pH value, clay and organic matter content, and metal concentrations in the various fractions. SEM/EDS analysis showed Fe in form of oxides and sulphides in soils and Cu, Ni, Mn, Zn and Cr in association with iron oxides. Numerous black carbonaceous particles and precipitates of aluminium fluoride salts, observed in the solid residue left after `total' digestion, were found to contain Fe, Ni and Cr.