Environmental legacy of mercury mining: insights from soil speciation in the Mt. Amiata district (central Italy)

Mercury (Hg) contamination in soil represents a major environmental concern, particularly in former mining districts where large quantities of Hg-rich ore deposits were extracted and processed. This study investigates the Hg concentrations and speciation in 304 top- and sub-soil samples (10-150 cm) collected from the eastern sector of the Mt. Amiata district (central Italy), one of the world's most important Hg-producing regions. Total Hg contents exhibit an extremely wide range (six orders of magnitude), since they vary from 0.033 to 1,030 mg/kg and from 0.014 to 1,250 mg/kg in top- and sub-soils, respectively. Thermal desorption, integrated with leaching tests and mineralogical analyses, allowed to identify Hg-bearing phases and assess their environmental mobility. Cinnabar (α-HgS) accounts for a median of 58.1% (top-soil) and 60.8% (sub-soil), while metacinnabar (β-HgS) represents 13.2% and 19.8%, respectively. Additional contributions are related to Hg bound to organic matter, clay minerals and, subordinately, to more soluble species (HgCl₂ and HgSO₄). Only 3.94% of soils exceeds the Italian legal threshold for geoavailable Hg (1 µg/L), indicating limited mobility under current geochemical conditions. However, soils containing soluble Hg phases or labile Hg-organic complexes exhibit enhanced release for low pH and cation exchange capacity. These findings demonstrate that Hg is mostly hosted in minerals recalcitrant to alteration. However, site-specific environmental variations such as soil acidification, organic matter composition, cation exchange capacity or changes in ionic strength and/or redox potential could increase the solubility of the Hg-bearing phases, thus promoting the release of Hg²⁺ to the environment. This study highlights the need to integrate Hg speciation with the concentration of total and leached Hg when assessing long-term environmental risk in decommissioned mining areas.