Hydrogeochemistry of thallium and other potentially toxic elements in neutral mine drainage at the decommissioned Pb-Zn Raibl mine (Eastern Alps, Italy)

Decommissioned mines represent a worldwide concern due to the potential long-term effects related to the dispersion of potentially toxic elements (PTEs) in the environment. In this study, 176 water samples were collected in the period 2018-2021 at the carbonate-hosted Pb-Zn Raibl mine which is affected by neutral mine drainage (NMD). The post-flotation tailings are the main source of PTEs (Zn, Pb and especially Tl) in the river drainage system. Compared to other dissolved PTEs (Zn, Pb, Cd), Tl was found to be more mobile, reaching concentrations up to 120 mu g L-1 in waters flowing in the tailings impoundments. Modelling results suggest that Tl is mainly present in the Tl(I) ionic and mobile form thus suggesting that relevant natural attenuation processes for this element are not expected in the investigated area. In contrast, Zn and Pb attenuation pathways appeared to be governed by pH-dependent speciation and sorption processes, whereas elevated Zn concentrations were likely also limited by hydrozincite precipitation. Metalloids such as As and Sb were almost entirely released into the slightly alkaline waters of the mine drainage system, which are generally characterised by longer residence time or standing waters whereas As and Sb concentrations were negligible in the tailings-seepage waters. However, intense rainy events may increase PTE concentrations of one order of magnitude, especially in the tailings impoundment groundwater as a result of a rise in the water table, and PTE total dissolved loads of three orders of magnitude, in the main stream during high flow events, thus representing the most critical factor in regulating the remobilisation and downward dispersion of PTEs in the river drainage system.