Effects of groundwater and tailings interaction on the mobility of Thallium and other potentially toxic elements in a decommissioned Pb-Zn mining site (Raibl mine, north-eastern Italian Alps)

Mining activity at the Raibl district (northeastern Italian Alps) date back to 1320, reaching the peak production in the 20th century. Nearly 350,000 tons of lead-zinc (Pb- Zn) ore, primarily sphalerite (ZnS) and galena (PbS), were annually extracted during the last years of operation until the mine closure in 1991. The ore deposit also includes iron (Fe) sulfides (pyrite and marcasite), baryte, and secondary minerals such as Fe oxy-hydroxides, smithsonite, hydrozincite, and cerussite, with dolomite and calcite as gangue minerals. Between 1976 and 1991, nearly 4 million tons of mine tailings, byproducts of the milling and flotation processes employed to extract the elements of economic interest, were accumulated in tailings ponds located nearby the Rio del Lago stream, the main watercourse in the area. These tailings are highly enriched in Zn, Pb, and Fe, as well as PTEs, including Thallium (Tl), and represent a secondary source of Tl and other PTEs in surface and groundwater, especially under conditions of heavy rainfall and high river flow (Barago et al., 2023). Moreover, the carbonate host rocks provide a buffering effect, leading to neutral mine drainage (NMD), despite ongoing sulphide oxidation. Currently, the site is undergoing remediation, as concentrations of PTEs in surface and groundwater often exceed national regulatory limits. Specifically, Tl concentrations persist at levels one order of magnitude above the threshold limit (>2 μg L⁻¹) downstream the mining district. This study aims at assessing the interactions between groundwater and tailings accumulated in the impoundments which promotes the release of Tl and other PTEs in the dissolved phase. Boreholes were recently drilled in the tailings ponds, followed by on-site analyses using portable X-ray fluorescence spectroscopy (pXRF), a key geochemical technique for multi-elemental screening in contaminated sites (Barago et al., 2022). Subsequent sub-sampling of representative tailings levels allowed for a detailed characterisation of the material in the laboratory through inductively coupled plasma atomic emission and mass spectrometry (ICP-AES, ICP-MS). As expected, results con-firmed elevated concentrations of Zn (1.21 – 7.99 wt%), Pb (0.29 – 1.76 wt%), Fe (2.15 –14.2 wt%), As (505 – 2964 μg g-1), and Tl (65.8 – 677 μg g-1). Based on this characterisation, tailings samples with high Tl concentrations (rang-ing from 185 and 677 μg g-1) were selected for further extraction and leaching tests to evaluate the geochemical behaviour of Tl and other PTEs as well as the processes con-trolling their release in solution. Gaining insights into these mechanisms is crucial for assessing the environmental impact of Tl contamination and developing effective strategies to mitigate its mobility, contributing to the protection of surrounding eco-systems and water resources. ACKNOWLEDGEMENTS The authors want to thank Claudio Ellero for his help during field and laboratory activities. REFERENCES Barago, N., Pavoni, E., Floreani, F., Crosera, M., Adami, G., Lenaz, D., Larese Filon, F., and Covelli, S., 2022. Portable X-ray Fluorescence (pXRF) as a Tool for Environmental Characterisation and Man-agement of Mining Wastes: Benefits and Limits. Applied Sciences, v.12, 12189. Barago, N., Pavoni, E., Floreani, F., Crosera, M., Adami, G., Lenaz, D., and Covelli, S., 2023. Hydrogeo-chemistry of thallium and other potentially toxic elements in neutral mine drainage at the decom-missioned Pb-Zn Raibl mine (Eastern Alps, Italy). Journal of Geochemical Exploration, v.245, 107129.