Zn isotopes as tracers of anthropogenic pollution from Zn-ore smelters The Riou Mort–Lot River system
Introduction
The first measurements of Zn isotope variations in geological materials were performed fifty years ago (Blix, 1957) and limited analytical precision of ∼ 0.1% lead to the conclusion that there was no isotopic fractionation of Zn in terrestrial samples (Rosman, 1972). Recent development of analytical instrumentation such as multiple collector ICP-MS (MC-ICP-MS) has induced a great expansion of the knowledge of the transition metal isotope geochemistry (Halliday et al., 1995, Rehkämper et al., 2001, Johnson et al., 2004). Analytical protocols for Zn isotope analysis by MC-ICP-MS have been developed in the late 1990s (Marechal et al., 1999) and early 2000s (Marechal and Albarede, 2002, Archer and Vance, 2004, Chapman, 2006) and document substantial natural isotopic variations.
Due to active participation of Zn in multiple biological and low-temperature inorganic-chemical reactions, isotopic variations of Zn may be used as a tracer of biogeochemical and chemical processes. Marechal et al. (2000) have first applied Zn isotopes as a marine biogeochemical tracer of biological activity. Pichat et al. (2003) reported δ66Zn values in deep-sea carbonates varying from + 0.31‰ to + 1.34‰ and Bermin et al. (2006) recently determined a δ66Zn profile in a seawater column showing values of + 0.35 ± 0.08‰. This suggests that Zn isotopes may be used as tracer of marine sediments provenance (Bentahila et al., 2008) or to identify the role of Zn as a micronutrient in the oceans (Vance et al., 2006). Experiments by Pokrovsky et al. (2005) showed minor Zn isotopic fractionation (± 0.2‰) during Zn adsorption onto oxy(hydr)oxides and anhydrous oxides surfaces under abiotic conditions. Within a continental environment analyses were performed on rocks, such as loess, mantle-derived materials (Ben Othman et al., 2001, Ben Othman et al., 2003, Ben Othman et al., 2006), ore deposit samples (Wilkinson et al., 2005, Sonke et al., 2008) and volcanic-hosted massive sulphides (VHMS; Mason et al., 2005), giving a wide range of δ66Zn values: from − 0.43‰ to + 1.33‰ (Cloquet et al., 2006a, and references therein). Biological processes may also induce fractionation between Zn isotopes. For instance, Zhu et al. (2002) have reported Zn fractionation of + 1.1‰ for Zn proteins synthesized by yeast, relatively to δ66Zn in the nutrient solution. Gelabert et al. (2006) demonstrated that irreversible incorporation of Zn in cultured diatom cells (Thalassiosira weisflogii) induces an enrichment in heavy isotopes (Δ66Znsolid-solution = + 0.3 to + 0.4‰, compared to the growth media) attributed to the change of chemical status of the metal inside the cells. Weiss et al. (2007) reported large variations of δ66Zn in peat cores (up to + 1.05‰), partially related to biological Zn cycling in peat. In plant roots, heavy Zn isotope enrichments of up to + 0.27‰ and + 0.76‰ were observed relative to the litter layer (Weiss et al., 2005, Viers et al., 2007), whereas negative δ66Zn values were obtained for tree leaves.
While many studies focus on Zn isotopes during natural biogeochemical cycling, fewer deal with the use of Zn isotopes to trace anthropogenic contamination, which is a timely topic: significant variability of δ66Zn was reported in epiphytic lichen and aerosols samples near an urban area affected by aerosols or flue gases from a waste combustor (Cloquet et al., 2006a, and references therein), but the lack of systematic variation did not allow distinguishing between pollution sources. Dolgopolova et al. (2006) also measured δ66Zn in lichens around a mining and mineral facility and reported Zn isotopic signatures up to + 1.40‰ heavier than the natural dust derived from the local host rocks. Weiss et al. (2007) used the Zn isotopic signature of peat surface layers affected by atmospheric Zn contamination to estimate those of the probable sources, a mining site (δ66ZnJMC = + 0.32 ± 0.18‰, 2SD) and a smelting site (δ66ZnJMC = + 0.66 ± 0.16‰, 2SD). These values are ‘heavy’ compared to the negative δ66ZnJMC values measured by Mattielli et al. (2006) in smelter airborne particles (δ66ZnJMC = − 0.02 to − 0.52‰). Riverine suspended particulate matter (SPM) and sediments are both pivotal compartments for pollution transport and accumulation in hydrosystems, but only one study of anthropogenic Zn isotopic variations in SPM and sediments has been published so far (Petit et al., 2008). Cloquet et al. (2008) recently proposed a review of the known variations in the isotopic composition of zinc in the environment.
The aim of the present study was to understand and evaluate the impact of metallurgic activity on Zn isotopic fractionation and to assess the potential of Zn isotopes as a tracer of anthropogenic Zn at a river system scale. For this, Zn isotopic compositions of surface soils, smelting/mining wastes, river and reservoir sediments from a river system (Lot River, SW France) impacted by Zn-ore metallurgy were determined using MC-ICP-MS. Zinc isotopic signatures of the anthropogenic sources and polluted compartments (soils and river/reservoir sediments) were compared. Links between Zn-extraction processes, Zn-extraction yields and Zn isotopic fractionation are discussed.
Section snippets
Field settings
The Riou Mort and the Riou Viou Rivers drain the Decazeville basin (Aveyron, SW France) and are affected by heavy metal pollution while running through a former (1842–1987) industrial Zn-ore treatment plant (Fig. 1). The contaminated Riou Mort River exports important amounts of various metals into the downstream Lot–Garonne–Gironde fluvial-estuarine system (e.g. Audry et al., 2004a, Audry et al., 2004b, Schafer et al., 2006), contributing 23% of the total Zn flux into the downstream Gironde
Sampling sites
Samples of polluted soils, smelting and mining wastes (tailings), waters percolated through the tailings, river and reservoir sediments were collected (Fig. 1). Four river sediments were sampled (grab samples; 0–3 cm depth) at various points of the Riou Mort (F, E, V1), Riou Viou (V0). Six soils from flood terraces were sampled: in the Riou Mort River watershed upstream from the industrial site (V0), in the Lot River watershed upstream from the Lot/Riou Mort confluence (L0, C), downstream from
Soils and river sediments
Zinc concentrations in present-day soils and river sediments from the Riou Viou upstream the plant reach 877 mg/kg and 180 mg/kg, respectively (Table 1), which is similar to previous data at these locations (Audry et al., 2004a, Audry et al., 2004b). The river sediments from the Riou Mort and Riou Viou Rivers upstream the industrial site are characterized by low and similar δ66Zn signatures, + 0.36‰ and + 0.32‰, respectively. Near the Zn-ore facility, Zn concentrations increase to values up to
Discussion
In the following sections, we will first determine the most pertinent δ66Zn value of the local geochemical background and discuss the origin of the anthropogenic signatures, especially regarding the evolution of the Zn extraction processes. Anthropogenic signatures will be compared with those of the compartments that are the most affected by the polymetallic pollution: the sedimentary record at Cajarc, and to a lesser extent the floodplain soils of the Lot River system.
Conclusion
The δ66Zn signatures of the main anthropogenic Zn sources in the Lot watershed (i.e., tailings, tailing–percolating waters and coal fly ash) were compared to those of the geochemical background and of the most polluted compartments of the fluvial system (i.e., river sediments and soils) in order to determine the anthropogenic sources of Zn in these compartments and the associated metallurgic processes. The geochemical background δ66Zn signature, determined from sediments located upstream the
Acknowledgements
We would like to acknowledge F. Candaudap, M. Carayon and C. Causserand for technical assistance. M. Munoz and E. Pont are gratefully acknowledged for providing soil mineralization. Financial support was provided by grants from ‘l'Agence de l'Eau’, ‘le Ministère Français de la Recherche’ and the INSU-«ECODYN» programme. Finally, we would like to particularly aknowledge A. Zanet from the UMICORE company for his helpful collaboration. Two anonymous reviewers and the associate editor are thanked
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