Hg-rich ground waters from the former Hg-mining area of Abbadia San Salvatore (Mt. Amiata, central Italy): possible implications for a remediation process

Geochemical investigations of ground waters are an impelling request to quantify both water quality and anthropogenic impact particularly in abandoned mining sites. This study was focused on the geochemical and isotopic characterization of the ground waters from the former Hg-mining area of Abbadia San Salvatore (Mt. Amiata, central Italy), which was one the most important districts of Hg production worldwide and whose activity was closed at the end of the seventies. The exploited ore had a Hg content of 0.6 to 2.0 wt.% and was extracted down to the depth of 400 m. It has been estimated that 103,000 tons of liquid mercury were produced and that 10,000 tons were dispersed in the environment. From February 2013 to September 2014 a geochemical monitoring, consisting of 6 sampling surveys, of the ground waters from a piezometer network located inside and outside the mining area was carried out. This study has evidenced a relatively high geochemical heterogeneity since three distinct geochemical facies were recognized: Ca(Mg)-SO4, which is the prevailing chemical composition, Ca(Mg)-HCO3 and Na-HCO3. Dissolution of gypsum/anhydrite and carbonates and hydrolysis of sulfide minerals are likely the main geochemical process that produced the observed geochemical compositions. It was remarkable the fact that the studied waters showed significantly high concentrations of Hg (up to 853 μg L-1). Independently by the monitoring period, Hg concentrations for most waters had close to significantly higher values than the maximum allowable concentrations (1 μg L-1). In particular, the high Hg contents are likely related to interaction processes between the circulating waters and the heterogeneous material (e.g. cinnabar, slag, tailings) with which the southeastern part of the mining area had been filled. Remediation actions are to be planned while a periodical geochemical monitoring is going on in order to evaluate the dispersion halo of Hg along the groundwater flow path. The removal of mercury from groundwater has several criticalities due to: i) the high contents of Hg; ii) the large geochemical variations observed in the monitoring surveys and iii) the difficulties in operating with ground waters. In this respect, the construction of permeable reactive barriers can be as a possible solution for the removal of Hg, although, according to the literature, several materials, tested with laboratory experiments, can be used and, as a consequence, it is necessary to individuate specific piezometers with different Hg concentrations where pilot investigations are to be carried out before undertaking any remediation actions.