Ten years of ground and surface water monitoring in the abandoned Hg-mine of Abbadia San Salvatore (central Italy): geochemical investigations before reclamation

Since February 2013, 21 periodical samples (mostly carried out seasonally) of ground and surface waters within and outside the former Hg-mine of Abbadia San Salvatore (Tuscany, central Italy) have been analyzed for the main composition, Hg, As, Sb, and, in most cases,  selected trace elements. The groundwater samples refer to a phreatic aquifer at depths between 5 and 10 m, characterized by low transmissivity and whose waters are interacting with terrains containing tailings derived from the production of liquid mercury by roasting cinnabar. The temperatures are thus strongly affected by seasonal variation, while the pH values are mostly circumneutral. The results evidenced the presence of relatively high concentrations of Hg and, to a minor extent, As and Sb, forcing the local authorities to intervene to test specific strategies to remove mercury. Strikingly high seasonal variations of the geochemical facies were observed and were apparently not related to meteoric precipitations. The variability of the main composition, i.e. Ca(Mg)-SO₄, Ca(Mg)-HCO₃ and, subordinately, Na-HCO₃, is intimately associated with the large differences recorded in terms of Hg, and, to a lesser extent, As and Sb. This is likely related to the water-rock interaction processes governed by the dissolution of carbonates and gypsum/anhydride (typical minerals recognized in the waste materials used to fill a paleo-valley in the SW margin of the mining area). The highest recorded Hg concentration was 407 µg/L during the wet period, decreasing down to 81.4 µg/L in the dry period, when the groundwater level decreases in most boreholes by up to 2 m. This also results in an increasing electrical conductivity. The low transmissibility of this shallow aquifer is clearly evidenced when the piezometers are purged before sampling, as they tend to be rapidly emptied. The groundwaters upstream and downstream of the mine are found to have a concentration of Hg < 1 ppb. This suggests that there is no interference between the mining area aquifer and the volcanic one. Apparently, no significant correlations were found between Hg and other metals, probably suggesting that the presence of liquid Hg often recovered in the piezometer cores is perhaps the main source of mercury. The speciation of Hg, As, and Sb of selected ground and superficial waters was computed by PHREEQC modelling. In addition, to simulate how Hg is vehiculated through the aquifer, a chemical transport model was developed. Presently, the installation of a hydraulic barrier is the most suitable solution to minimize the water-rock interaction processes, responsible for the recorded Hg variability, possibly coupled with additional operations, e.g. extractant agents or filters, to remove mercury before discharging the pumped waters into a surface stream.