Mercury in soil profiles from metal mining and smelting areas in Namibia and Zambia: distribution and potential sources

Whereas mercury (Hg) has been extensively studied in gold mining areas (including artisanal mining), it is often overlooked as a minor contaminant in these districts, within which industrial base-metal mining and smelting are in operation. The aims of this study were to investigate Hg in tropical soils from mining and smelting areas in Namibia and Zambia and to apply statistical methods to generate models for the prediction of Hg concentrations in the soils studied. Twenty-one soil profiles (n = 159 soil samples) were collected in metal mining districts in the northern parts of Namibia (Tsumeb, Berg Aukas, Kombat) and in the Zambian Copperbelt (Kitwe, Mufulira). Total Hg was analysed by atomic absorption spectrometry and compared statistically with other physico-chemical and chemical soil parameters. Mercury concentrations in potential sources (feed and wastes from smelters as well as mine tailings, n = 35) were also determined. Mercury concentrations in soils from mining/smelting areas were significantly higher in northern Namibia (range 0.0038-4.39, mean 0.39, median 0.02 mg kg(-1)) than in the Zambian Copperbelt (range 0.0055-0.39, mean 0.02, median 0.01 mg kg(-1)). This phenomenon is related to the higher levels of Hg in the mine tailing materials as well as the feed/wastes from the smelters in Namibia (specifically in Berg Aukas and Tsumeb). Only 27 % (Namibia) and 26 % (Zambia) of the soil samples exceeded geochemical anomaly thresholds (0.033 and 0.016 mg kg(-1), respectively), generally indicating a low Hg pollution level. The highest Hg concentrations were observed in the uppermost soil layers. Total Hg correlated significantly with other contaminants and, in the Zambian dataset, also with C-org and S-tot. Based on measurements of total Hg in soils from the mining/smelting areas in Namibia and Zambia, only one fourth of the samples exceeded geochemical anomaly thresholds, and they indicated a relatively low level of Hg pollution. Elevated Hg concentrations were only observed in topsoils in the immediate vicinities of mine tailings and active smelters. Constructed regression models were found to be useful for prediction of Hg concentrations at both of the regions studied.