Assessment of agricultural soil contamination by potentially toxic metals dispersed from improperly disposed tailings, Kombat mine, Namibia

The Kombat tailings dam, surrounded by agricultural lands, has been exposed to water and wind erosion over a long period of time. The objectives of this research were: (1) to characterize the tailings and the surrounding agricultural soils with respect to the mineral and trace element composition; (2) to determine the degree of soil pollution using soil contamination indicators; (3) to assess the environmental risk of polluted agricultural soil; and (4) to identify dominant type (mechanical and/or chemical) and dominant agent (water and/or wind) of metal dispersion from the tailings. A sequential extraction procedure was used to determine binding mechanisms involved in the retention of metals in tailings and soils under the influence of tailings, which indicate the trace metals bioavailability, the threat to groundwater pollution, as well as the dominant type of dispersion. Among seven analysed elements, copper and lead showed significantly high concentrations in tailings, especially in dry season (up to 9086 mg/kg and 5589 mg/kg, respectively). As a consequence, adjacent arable soils have high concentrations of Cu and Pb (up to 150 mg/kg and 164 mg/kg, respectively). Enrichment factors for lead and copper reveal severe contamination, while geoaccumulation indices disclose moderate to strong contamination by both elements. The combined pollution index points out high contamination. The main binding phase for Cu and Pb is the reducible fraction (oxides, hydroxides, oxyhydroxides). Similar metal distributions in the sequential extraction fractions of tailings and soils support the assumption that wind and water disperse tailings predominantly by mechanical transport to the surrounding agricultural soil. Although agricultural soils are contaminated with Pb and Cu, these metals are relatively strongly bound to the soils and are of medium risk for their mobilisation after The Risk Assessment Code (up to 20% for Cu and up to 36% for Pb). Though rehabilitation of tailings dam, as well as limitation of certain crop use on polluted agricultural land, is recommended. (c) 2014 Elsevier B.V. All rights reserved.