Geochemical characterization of drainage waters after closure of sulphides extraction activity (Salafossa, Northeastern Italian Alps)

After decommissioning, the potential impact of mining areas on the quality of water resources is a major concern for local communities. Acid mine drainage resulting from oxidation of metal sulphides associated with mineral veins or mining wastes is often responsible for leaching large amounts of metals in solution, which can be dispersed into the surrounding environment and affect the quality of receiving water bodies. The body ore in Salafossa (NE Italy) was one of the largest lead/zinc deposits in Europe. Mining activity started around 1550 but it was only around 1960 that the richest veins of sulphides were discovered. When the activity definitely stopped in 1985, a little more than 11 million tons of tout-venant (raw material), with an average content of 0.9% for Pb and 4.7% for Zn, were obtained from the mineral deposits. The aims of the present study were to investigate the geochemical properties of the mine drainage waters and to monitor the concentration of trace metals outflowing from the mine galleries into the Piave River, the major tributary downstream the mine. In spite of the sulphides body ore, there is no evidence of acid drainage waters in Salafossa, probably because of the buffering effect produced by carbonate host rocks. However, Zn and Tl, due to their high mobility, are present in solution mostly in ionic form. Conversely, the less mobile Pb (highest concentration: 4 μg L-1), is preferably partitioned in the solid phase. Additionally, the oxidizing conditions found in the drainage waters also allow the precipitation of some trace metals (As, Cd, Pb, Tl, Zn) in the form of Fe-Mn oxy/hydroxides and carbonates, which accumulate in the mine galleries sediments. 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), but their concentrations are diluted in the mine before being discharged into the Piave River. Although drainage waters 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.