Assessment of heavy metal pollution in the Lupa gold field, SW Tanzania

Abstract

The concentrations of AS, Cd, Cr, Cu, Hg and Pb were determined in water, stream sediments, soil, and mine tailings from Lupa Gold Field (LGF), SW Tanzania, in order to assess their pollution levels. As, Cd and Cr were determined by automated Graphite Furnace Absorption Spectrophotometry. Cu and Pb were analysed by Air-Acetylene Flame, while Hg was analysed by Cold Vapour Atomic Absorption Spectrophotometry. The mean concentrations obtained were: As: 0.44 ppb (water), 1.2 ppm (sediments), 0.44 ppm (soil) and 5.2 ppm (tailings); Cd: 0.03 ppb (water), 0.03 ppm (sediments), 0.03 ppm (soil) and 0.50 ppm (tailings); Cr: 1.4 ppb (water), 70 ppm (stream sediments), 250 ppm (soil) and 270 ppm (tailings); Cu: 30 ppb (water), 68 ppm (sediments), 66 ppm (soil) and 455 ppm (tailings); Hg: 0.25 ppb (water), 1.1 ppm (sediments), 0.10 ppm (soil) and 8.70 ppm (tailings) and Pb: 0.50 ppb (water), 85 ppm (sediments), 22 ppm (soil) and 275 ppm (tailings). Leaching of As-based mineral flotation reagents as well as the decomposition of accessory As minerals such as arsenopyrite (FeAsS), enargite (Cu3AsS4) and tennantite (Cu12As4S13) most likely contributed to some high As concentrations in the mine tailings. The low Cd content in all samples reflects the low content of sphalerite (in which Cd replaces Zn) typical of the LGF ores. Cromium was low in water, most likely because of its low mobility; it was high in soils and tailings derived from mafic-ultramafic rocks. High Cu levels in tailings came from dissolution of chalcopyrite and bornite. Neutral to alkaline pH conditions (6.5-8.5) of water was most likely responsible for its low Cu levels. Amalgamation of Au concentrate likely introduced Hg into the environment. Dissolution of galena and anglesite was likely the source of Pb into the soils, sediments and tailings. The range of concentrations of the six heavy metals recorded in the Lupa gold field is of great concern to human health.