Potential toxic elements contamination of soils in O’Kiep, an arid region of Namaqualand, South Africa

Abstract

The aim of this study was to evaluate the geochemical background and spatial distribution patterns of selected potential toxic elements (PTEs) in ten sub-surface soil samples (n=10) collected in the former copper mining area, O’Kiep. The major oxides and PTEs in the samples were identified and measured using X-ray fluorescence and X-ray diffractometer. The contaminated soils were compared to prestige soil and Soil Screening Values for All Land-Uses Protective of the Water Resource – references which can elucidate PTE contamination. The examined soils showed a high variability in the PTEs concentrations. O’Kiep soils show some degree of contamination with an average low pH of 3.5 and an Eh of 520 mV. The most abundant PTEs in the soils were S > Cu > F > Mn > Cl > Zn > Cr > V > Pb > Ni > Co. The soil type in downstream was classified as silt with a clay content of 1.1%. The mineralogical study of the soils revealed minerals such as Alumino-silicates, Ca-silicates and Mg-silicates aggregates, with a strong similarity between the downstream and upstream samples from the mining area, while the midstream only differred by the presence of hematite. The median values of oxides contents in soils follow a decreasing order as: SiO2 > Al2O3> Fe2O3 > K2O. Furthermore, factors that influence the distribution and migration of PTEs in soils were largely identified as mine waste heaps and slime dams further north from the sampling sites. Excessive concentration of PTEs in O’Kiep’s soils may lead to elevated potential risks to human health and the local environment. The material of mine waste heaps is very fine and can easily migrate with wind and water erosion. Therefore, further risk assessment analyses of mine waste heaps and the mine are required in O’Kiep.