Mobility of Thallium and other trace elements in mine drainage waters from two carbonate-hosted Lead-Zinc ore deposits in the northeastern Italian Alps

The ore deposits mined at Salafossa and Raibl (northeastern Italian Alps) were two of the largest Pb-Zn mineral deposits in Europe where the two metals were mainly present as sulphides (sphalerite and galena). Currently, the two mining districts are decomissioned and their galleries were partially flooded after the mine closure. In this context, water-rock interaction and the oxidation of sulphides are often responsible for increasing the mobility and dispersion of potentially toxic trace elements in the surrounding environment. Among minor constituents of sulphides, Tl is one of the most toxic elements and there is great concern regarding its occurrence, behaviour and fate in areas affected by both active and decommissioned mining activity. The geochemistry of the mine drainage waters was investigated at the two mining districts with the final aim of providing further information regarding their chemical composition, highlighting anomalous Tl and other trace element concentrations. In spite of the oxidation of sulphides which appears to occur, the buffering effect related to carbonate host rocks prevents the formation of acid mine drainage at both the investigated sites. Drainage waters were, however, found to be enriched in Zn, Pb and Tl (maximum of 16204, 80.1 and 261 µg L-1 at Salafossa and 5243, 70.1 and 80.1 µg L-1 at Raibl, respectively). Although drainage waters are diluted after being discharged into their respective riverine systems, Tl still remained at high levels in the freshwater downstream from the mines, especially at Raibl. There, the Tl-enriched groundwater flowing into the tailing ponds appears to affect the chemical composition of the freshwater of the Rio del Lago stream.