Mercury contamination associated with small-scale gold mining in Tanzania and Zimbabwe

Mercury contamination associated with small-scale gold mining and processing represents a major environmental and human health concern in Eastern and Southern Africa. Approximately 200,000-300,000 persons are involved in small-scale gold mining activities in Tanzania and > 200,000 persons in Zimbabwe. Mercury (Hg) is used mainly for the processing of primary gold quartz veins and supergene gold mineralizations. Gravimetric material flow analyses show that 70-80% of the Hg is lost to the atmosphere during processing, 20-30% are lost to tailings, soils, stream sediments and water. For every 1 g Au produced, 1.2-1.5 g Hg are lost to the environment. Cumulatively, the anthropogenic Hg released annually into the atmosphere is approximately 3-4 t in the whole Lake Victoria Goldfields of Tanzania and > 3 t in Zimbabwe. Tailings are local 'hot spots' with high concentrations of As, Pb, Cu and Hg. Lateral and vertical dispersion of Hg lost to soils and stream sediments is very limited (laterally < 260 m, vertically < 20 cm). Dispersion of mercury from tailings is low because Hg is transported largely in the elemental, metallic form. In addition, Fe-oxide rich laterites and swamps appear to be natural barriers for the dispersion of metals in soils and streams. Ground and surface water quality data indicate very low dispersion rates during the dry season.