Mobility of trace elements in drainage waters from a zinc- lead decommissioned mine (Northeastern Italian Alps)

The Salafossa mine is located in the eastern Dolomites belonging to the Alpine chain, within the territory of the Veneto region. The mineral body excavated in Salafossa was one of the largest lead-zinc-containing mineral deposits in Europe. The two metals are in the form of sulphides (sphalerite, ZnS and galena, PbS). Mining activity started around 1550 but it was only around 1960 when the richest veins of the minerals were discovered. Mining activity definitely stopped in 1985. One of the problems that characterizes the mining area consists in the generation of acid drainage waters, resulting from oxidation of metal sulfides located in correspondence of the ore body and of tailings, with consequent leaching of large amounts of metal species in solution, such as Fe, Mn, Pb, Cu, Zn and Tl. The products of oxidation, carried by drainage waters, can be dispersed into the surrounding environment resulting negative impact on the quality of the watershed. The aim of this study was the geochemical characterization of drainage waters (DWs) from both the mine and the Piave River, the major tributary downstream the mine, in order to assess the mobility of heavy metals in the area. DWs were found to be enriched in Zn (up to 16 mg L-1), Fe (up to 5 mg L-1) and Tl (up to 260 μg L-1), and they represent the primary means by which the metal species are conveyed by the galleries of the mine to the Piave River. The Zn and Tl, due to the high mobility that characterizes them, are mostly present in solution in ionic form; conversely, the Pb, less mobile, is preferably partitioned in the solid phase. Additionally, the oxidizing conditions found in the waters also allow precipitation of some metals in the form of oxides, hydroxides and carbonates, such as Fe and Mn, which, consequently, have determined their enrichment in sediments transported by drainage waters and accumulated along the mine galleries floor. Although DWs are still characterised by high concentrations of Tl (up to 30 μg L-1) at their outflow, dilution in the Piave River seems to be a natural process mitigating the impact of trace metals within the drainage basin.