Tracing the metal dynamics in semi-arid soils near mine tailings using stable Cu and Pb isotopes

Abstract

The dynamics of Cu and Pb were studied in eight soil profiles (Luvisols, Chernozems), located at various distances from the tailing dams near a closed Cu-Pb-(Zn)-Ag mine at Kombat (Namibia) by a combination of isotope and elemental studies, leaching techniques and speciation modelling. Tailings, soils and bedrocks were analyzed for the bulk Cu and Pb concentrations, for their chemical forms using the sequential extraction procedure and for the isotope compositions of Cu (δ65Cu) and Pb (206Pb/207Pb, 208Pb/206Pb).In spite of the differences in the contamination of the studied profiles by tailing dust, both soil types exhibit characteristic trends in the behaviour of metals and distribution of their isotopes. While most Luvisols exhibit an increase in the metal content towards depth in the profile, Chernozems exhibit the opposite trend (decrease in metal content with depth) or similar contents in the whole profile. Luvisols were mostly characterized by changing δ65Cu values as a function of depth. The value of δ65Cu decreases with depth to the Bt horizon and then the δ65Cu value increases with increasing depth. Chernozems mostly yielded uniform Cu concentrations and isotope compositions in all the soil horizons, clearly contrasting with the isotopically lighter compositions of the carbonate bedrocks. Positive δ65Cu values in the tailing material affect the surface of the soil, especially soil sampled in its vicinity. In most Luvisols and Chernozems, the isotope composition of Pb exhibits similar patterns with a low 206Pb/207Pb ratio on the surface of the profile and with a slight increase to depth. The low 206Pb/207Pb values at the surface of the profiles are caused by dust from the tailings.The elemental and isotopic patterns of both contaminated and uncontaminated soil types reflect not only the distance from the tailing dam but also pedogenetic processes in the profiles and geochemical behaviour of both contaminants. The copper elemental and isotope patterns reflect the formation of more mobile species in the surface environment and movement of solutions and colloids and precipitation onto secondary minerals in Luvisols. Plant uptake of lighter isotopes in Chernozems can influence the Cu isotope patterns in these soil types. The limited movement of colloids and higher sorption capacity of Chernozems is manifested in the isotope compositions of Cu and Pb in their profiles.