Mobility and fate of Thallium and other potentially harmful elements in drainage waters from a decommissioned Zn-Pb mine (North-Eastern Italian Alps)

The potential impact of decommissioned mining areas on the quality of water resources is an issue of major concern for local communities. Acid mine drainage resulting from hydrolysis and oxidation of metal sulphides associated with mineral veins or mining wastes is often responsible for leaching large amounts of potentially harmful elements (PHEs) in solution, which can be dispersed into the surrounding environment and affect the quality of the recipient water bodies. The aim of the present study was to investigate the geochemical properties of the mine drainage waters at the decommissioned Salafossa mine (North-Eastern Italian Alps), to highlight anomalous concentrations of PHEs outflowing from the currently flooded galleries and to elucidate their speciation. In spite of the Zn-Pb sulphides still present in the body ore, there is no evidence of acid drainage waters from the mine galleries as a result of the buffering effect produced by carbonate host rocks. Due to their high mobility, however, Zn and Tl are present in solution mostly in ionic form. Conversely, the less mobile Pb, is preferably partitioned in the solid phase. Additionally, the oxidising conditions of the drainage waters also allow the precipitation of some PHEs (As, Cd, Pb, Tl, Zn) in the form of Fe-Mn oxy-hydroxides and carbonates, which accumulate at the bottom of the mine galleries as fine “sediments” or concretions. Drainage waters inside the mine were found to be highly enriched in Zn (up to 16 mg L‐1), Fe (up to 5 mg L‐1) and Tl (up to 260 μg L‐1). Their concentrations, however, are partially diluted in the mine due to a mixing with less mineralised waters before being discharged into the Piave River, the major tributary downstream from the mining area. Although drainage waters are still characterised by high concentrations of Tl (about 30 μg L‐1) at their outflow, dilution in the Piave River seems to be the only natural process mitigating the impact of PHEs within the drainage basin.