Mercury contamination in a former mining plant of NW Tuscany (central Italy)

Mine dumps are man-made geological formations characterized by unique chemical, particle-size, bacterial, and physical and mechanical features, representing deposits or cumuli of crushed, grounded and roasted material. Processing plants of metallic poli-sulfide ore deposits have contributed to pollute soils and surface and ground waters in many areas worldwide. The southern sector of the Apuan Alps (northern Tuscany, Italy) hosts a number of small pyrite ± barite ± iron-oxide orebodies that have been exploited since at least the Middle Age, whose activity ceased at the end of the 20^th century. The most important mining areas were those distributed along a 10 km-long NE-SW strip in high Hg contents the southern portion of the Apuan Alps. In this framework, the Rezzaio treatment plant (Valdicastello Carducci, Pietrasanta), dismissed since 1991, was the site where ore bodies hosted within the metamorphic rocks of the Apuane Unit, mainly from Monte Arsiccio, Pollone and Buca della Vena mines were treated. The materials extracted from these mines were barite and iron oxides (both hematite and magnetite) used as weighting agents as drilling mud during petroleum drilling wells. Abandoned mining tunnels and dumps and plants for mineral treatment are still present in the area, posing a series of environmental threats. Our study focused on assessing the impact due to mercury contamination released by the treatment plant of Rezzaio by the past-mining activity. The aims of this study were to (1) determine the concentration of Gaseous Elemental Mercury (GEM or Hg0) in air and interstitial soil inside and outside the plant, including the working areas and the edifices where the workers were operating (e.g., offices, laboratory, rock storage); (2) assess the total amount of Hg in the top- and sub-soils, mostly developed on a small mining dump and (3) quantify the release of Hg by soil leaching tests by Milli-Q water. According to WHO and the Italian Legislative Decrees, the GEM values in air and outside the plant and in the plant edifices and mining structure are below the Recommended Exposure Limit. The spatial distribution of Hg indicates that up to 88 mg/kg were recorded in the top- and sub-soils, the highest contents being found on the small mining dump that is partly the bank of a creek. However, in most cases the concentration of Hg leachate were < 1µg/L, suggesting that mercury is likely trapped within crystalline silicate structures recalcitrant to chemical weathering. These results suggest that despite the high contents, mercury is not apparently playing a critical role as a contaminant in the Rezzaio area, being hosted in relatively insoluble minerals.