Leaching of mercury from soils at extremely contaminated historical roasting sites (Idrija area, Slovenia)

Abstract Mercury (Hg) solubility and reactivity in soils at two extremely contaminated ancient small scale cinnabar roasting sites in the surroundings of the Hg mining area of Idrija (Slovenia) were determined in order to assess the mobility of Hg in the aqueous phase and to evaluate the extent of leaching of Hg into Idrijca River and further downstream. Water leaching experiments were performed on soil and SOM (soil organic matter) samples from historical roasting sites Pšenk and Frbejžene trate. The determined concentrations of leachable Hg in soil samples from the studied areas range from 16 to exceptionally high concentration of 18,000 μg/kg, representing 0.0002 to 1.1% of total Hg in these samples, while in SOM samples leachable Hg concentrations range between 13.3 and 6000 μg/kg, which corresponds to 0.0017–0.074% of total Hg determined in SOM. The soluble Hg concentrations in investigated soil profiles range from 183 to 18,000 μg/kg (0.038–1.7% of total Hg). On the average, more than 90% of soluble Hg occurs in a non-reactive complex bound form, suggesting the preferential binding of Hg to humic matter. Soluble Hg in studied soil profiles generally increases with depth. The obtained results show that Hg is effectively transported to deeper soil layers, mainly as soluble organic complexes. It is estimated that there is still about 10 kg of soluble Hg stored in soils of the investigated roasting sites, which is continuously leached to surface waters and deeper into the soil.