The Occurrence And Distribution Of Mercury And Co-Occuring Metals In Selected Namibian Areas Affected By Industrial And Mining Activities

Abstract

The occurrence and distribution of mercury (Hg) and other co-occurring heavy metals in the mining towns of Berg Aukas and Tsumeb in northern Namibia were studied Different forms of Hg and other heavy metals were characterized in soils, water and plants collected in these areas. Total metal concentration was determined using ICP–OES and total Hg was determined using the DMA 80 mercury analyser. Fractionation of Hg and other heavy metals were determined by performing a four step sequential extraction procedure that separates metals into four fractions, namely: exchangeable (F1), reducible (F2), oxidisable (F3) and residual ( F4). Total metal results showed that there are high levels of heavy metals in tailings from Berg Aukas in comparison to international guidelines with, for example, Hg and Pb having concentrations as high as 2.24 mg/kg and 18 195 mg/kg, respectively. Although Berg Aukas soils generally showed Hg values beyond guidelines threshold levels ( which is 1.0 mg/kg), the highest Hg concentration in soil (3.49 mg/kg) was found at Tsumeb. Total metal concentration in water samples from Berg Aukas showed Hg values higher than the maximum allowed for drinking water (up to 6 µg/L). Some plants collected at Berg Aukas also had high Hg levels reaching a maximum of 0.70 mg/kg in a sample collected from a pond. Distribution of Hg in tailings found at Berg Aukas revealed that about 90% of Hg was residual whereas the exchangeable (i.e. bioavailable) fraction accounts for about 7%. In contrast, the exchangeable Hg fraction in Berg Aukas soils was as high as 35% of the total Hg. This fraction may explain the presence of Hg measured in the surrounding plants and is a reason for concern due to the risks of Hg methylation as well as further uptake of the soluble forms of Hg by other living organisms. Other metals were also present in bioavailable fractions and correlation analysis enabled the identification of several metal compounds that were likely present in the study soils and tailings. Tailings are known to be highly contaminated with heavy metals and this study demonstrates that these metals are being dispersed into the surrounding soils that are used for agricultural purposes.