Soil contamination near the Kabwe Pb-Zn smelter in Zambia: Environmental impacts and remediation measures proposal

Kabwe Town and its surroundings (central Zambia) belong to the most contaminated districts in Africa due to mining and smelting of local Pb-Zn ores. To assess the extent and intensity of this anthropogenic contamination, samples of topsoil (from a depth of 0 to 3 cm), together with reference subsurface soil from a depth of 70-90 cm, were collected in the area. In the Kitwe Town and downwind, the Pb and Zn contents in topsoils were found to be significantly higher compared to the permissible ecological limits used in Canada with regard to various land uses. Other elements (Cu, Fe, Mn, Cr, Ni and Ba) in topsoil demarcate only a small area of the former ore processing and smelter grounds. The gastric bioaccessibility of metals in topsoils was tested by a US EPA-adopted in vitro method using a simulated gastric fluid. The results revealed that the intake of Cd, Cu, Zn, Co, and As does not exceed the tolerable daily intake (TDI) values for children but, for Pb, almost 50% of the samples exceeded TDI. Therefore, the concentrations of gastric bioaccessible Pb in highly contaminated topsoils or in resuspended soil-derived dust particles may be considered to be an important health risk in the Kabwe area. The amounts of plant-available metals in topsoils were established by extraction with a diethylen-triaminopentanacetic acid (DTPA) and triethanolamine (TEA) solution. These tests showed that the amounts of plant-available metals increase (median values, in mg kg(-1)) in the sequence: Cd (0.7) -> Cu (3.9) -> Mn (76) -> Fe (79) -> Zn (110) -> Pb (126). Chemical stabilization of the pollutants in soils using a phosphate amendment caused a significant reduction of the plant-available fraction of Pb and Cd, but did not suppress their gastric bioaccessibility. Based on our results, various measures were suggested to reduce the impact of the pollution on the Kabwe Town population.