Assessment of heavy metal enrichment and degree of contamination around the copper-nickel mine in the Selebi Phikwe Region, Eastern Botswana

Abstract

This paper focuses on heavy metal (Co, Cu, Fe, Pb, Mn, Ni and Zn) enrichment and degree of contamination in the soils windward of the Selebi Phikwe copper and nickel mine (eastern Botswana). Pollution by these metals was examined using several calculation methods: enrichment factors, modified degree of contamination, pollution load index and geoaccumulation index. Enrichment factors were further assessed by comparing present day soil metal contents obtained using actual Fe content in standard earth material obtained from literature with that measured in soils sampled from areas adjacent to the site of concern to represent “uncontaminated” background concentrations. The latter method was found to be the most relevant and the normalised enrichment values demonstrate moderate enrichment of Cu and Ni. The proposed modified calculation method of degree of contamination, which involves determination of arithmetic mean of contamination factors by individual studied metals, revealed moderate contamination by metals. The PLI, which basically is a measure of site quality, indicated deterioration due to metal contamination. These results were further supported by values obtained from indices of geoaccumulation method of pollution assessment. Principal component analysis at windward site of the mine extracted two major components: 1. high positive loading of Pb, Ni, Cu and Zn associated with anthropogenic influence by smelter emissions and 2.high positive loadings of Co, Mn and Fe are associated with contributions from natural processes. This evaluation would serve to create consistent calculation methods as a contribution towards building a consensus on methodologies applicable to the study of polluted soils.