Zn isotopes as tracers of anthropogenic pollution from Zn-ore smelters The Riou Mort–Lot River system

Zn isotopes were used to trace the anthropogenic sources in the Lot watershed (Aveyron, SW France) where a small river, the Riou Mort drains an industrial exploitation of zinc ore. The zinc isotopes in industrial tailings are highly fractionated relatively to Zn ore (δ66Zn ∼ 0.16‰), due to metallurgical processes, reaching δ66Zn values up to + 1.49‰. Zn extraction yields for these samples were calculated using presumable conservative elements (Fe, Ca, Mg, K, Na and Mn) and vary from 95.4% to 99.4%. Extraction yields are related to the intensity of Zn isotopic fractionation. Different Rayleigh isotope fractionation scenarios for Zn refining yielded αore-tailings ranging from 1.00012 to 1.00062. The δ66Zn of the Riou Mort sediments downstream from the former Zn-ore facility is presently + 0.91 ± 0.04‰, i.e. significantly different from the signature of upstream sediments and of the regional geochemical background, + 0.31 ± 0.06‰, (2SD, n = 3). Sediment core δ66Zn values in the 40 km downstream Cajarc hydroelectric reservoir are also consistently ‘heavier’ (+ 0.75 to + 1.35‰) than the regional geochemical background. Both top-core Zn concentrations and isotopic signatures can be explained by the contribution of 9% of present-day Riou Mort sediments. δ66Zn variations in the 137Cs-dated downcore sediments are suggested to reflect historical changes in metallurgical processes of the Viviez facility, such as improved Zn extraction efficiency and the related evolution of tailings. In sediments deposited during 1952–1972, the mean δ66Zn is + 0.95 ± 0.08‰, representative of the combined electrolysis and thermal process used at that time for the Zn extraction. In sediments deposited during the late seventies, δ66Zn increases, reaching a maximum of 1.35‰ during the eighties. These heavier δ66Zn are likely related to further increased Zn extraction after replacement of thermal waste processing by electrolysis in 1976/77. The nature of the extraction processes, especially electrolysis, may therefore play a major role in the Zn isotopic fractionation produced. The soils located on the Lot riverbanks have chemical and Zn isotopic compositions (+ 0.74‰ to + 0.96‰) close to the Cajarc sediments, indicating that they were also impacted by tailings erosion and leaching. The soils located in the vicinity of the plant display lower δ66Zn (+ 0.40‰ to 0.48‰). That corresponds to mixtures of several atmospheric sources like coal fly ashes, ore and tailing dusts. The relation between Zn isotopic fractionation, extraction yields and/or extraction processes clearly demonstrates the efficiency of δ66Zn as tracer of environmental contamination.