Heavy metal (Cu, Zn, Cd and Pb) partitioning and bioaccessibility in uncontaminated and long-term contaminated soils

We investigated the pore-water content and speciation of copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb) in a range of uncontaminated and long-term contaminated soils in order to establish their potential bioaccessibility to soil biota, plants and humans. Among the samples, soil pH (0.01 M CaCl2) ranged from 4.9 to 8.2. The total metal content of the uncontaminated soils ranged from 3.8 to 93.8 mg Cu kg-1, 10.3 to 95 mg kg-1 Zn, 0.1 to 1.8 mg Cd kg-1 and 5.2 to 183 mg kg-1 Pb, while metal content in the contaminated soils ranged from 104 to 6841 mg Cu kg-1, 312 to 39,000 mg kg-1 Zn, 6 to 302 mg Cd kg-1 and 609 to 12,000 mg kg-1 Pb. Our analysis of pore-water found the Cu concentrations to be much higher in contaminated soils than in uncontaminated soils, with the distribution coefficients (Kd) correlating significantly with the log of dissolved organic carbon concentrations. Despite the high total metal content of the contaminated soil, Zn, Cd and Pb were not generally found at elevated levels in the pore-water with the exception of a single contaminated soil. A long period of ageing and soil weathering may have led to a substantial reduction in heavy metal concentrations in the pore-water of contaminated soils. On the other hand, Pb bioaccessibility was found to be comparatively high in Pb contaminated soils, where it tended to exceed the total Pb values by more than 80%. We conclude that, despite the extensive ageing of some contaminated soils, the bioaccessibility of Pb remains relatively high.