Tracing the spatial distribution and mobility of metal/metalloid contaminants in Oxisols in the vicinity of the Nkana copper smelter, Copperbelt province, Zambia.

摘要

The copper mining and smelting activities in the Copperbelt province (Zambia) left enormous pollution related to the disposal sites for mining/smelting waste (tailings, slags) and to the continuous deposition of smelter stack particulates in the soil systems. Topsoils in the vicinity of the Nkana copper smelter were studied in order to assess the spatial distribution of metal/metalloid contaminants (As, Co, Cu, Pb, Zn) in a 32-km2 zone around the smelter (n=196 soil samples). The bulk concentrations of metal/metalloid contaminants were determined by ICP techniques, supplemented by the determination of the Pb isotopic composition of selected samples measured by quadrupole-based ICP-MS. The spatial distribution of the major contaminants indicated the highest contamination NW of the smelter stack, corresponding to the prevailing wind direction in the area. The highest metal/metalloid concentrations in the topsoils were: 255 mg As kg-1, 27,410 mg Cu kg-1, 606 mg Co kg-1, 480 mg Pb kg-1 and 450 mg Zn kg-1. Lead isotopes helped to differentiate the extent of metallic pollution and indicated the mixing of three major pollution sources (given as 206Pb/207Pb ratios): smelting activities (1.19-1.28), petrol combustion (1.07-1.09) and regional background (deep soil horizons, ca. 1.35) To assess the vertical mobility of these inorganic contaminants, a 110-cm-deep profile in Oxisol in a highly polluted zone in the near vicinity of the Nkana smelter was sampled and studied by combination of bulk chemical analysis, sequential extraction procedure (SEP) and Pb isotopic tracing. Investigation of the soil profile indicated that the contamination is mostly located in the uppermost soil horizons enriched in organic matter (<10 cm). Based on SEP, the isotopic composition and knowledge of smelter activities in the area, it was predicted that anthropogenic Pb (corresponding to a concentration of 1 mg kg-1, i.e., approx. 1.5% of total Pb) exhibited downward migration in the soil profile with estimated penetration rate of 1.36 cm year-1. Copper, being substantially bound in the exchangeable fraction, also showed significant mobility in the profile. As a result, the sources of anthropogenic emissions in the Kitwe smelting area represent an important source of mobile (and potentially bioavailable) metals.