Gaseous mercury emissions from forest and urban soils heavily impacted by past mining in the Idrija mining district (Slovenia)

The historical mining and ore roasting activities lasted for about 500 years at Idrija (Slovenia), the second largest mercury (Hg) mine worldwide, have caused a widespread contamination of the surrounding environments through direct losses and discharges of residues and atmospheric depositions (Gosar & Teršič, 2012). Substrate enriched in Hg can release notable amounts of gaseous elemental mercury (GEM) into the atmosphere (Agnan et al., 2016), potentially contributing to widen the spatial distribution of this metal and to increase the exposure to Hg of inhabitants through inhalation. This study is focused on the evaluation of GEM fluxes at the soil-air interface within both the urban area of Idrija and its surroundings. Site selection was aimed at comparing emissions from substrates subject to different Hg supplies. The selected urban soils were characterised by different degree of Hg enrichment due to its natural occurrence in underlying bedrock or variable atmospheric depositions related to ore roasting in modern furnaces. In addition, GEM fluxes were measured at an ancient roasting site in the forests surrounding Idrija subject to considerable Hg supplies during ore processing in XVI-XVII centuries. Measurements were performed during the summer season, the most favourable for GEM evasion (Floreani et al., 2023). A non-steady state flux chamber coupled with a real-time GEM analyser was used to evaluate fluxes from soils with the natural vegetation cover performing replicate measurements at each site. Topsoil Hg content and speciation were also assessed through thermo-desorption technique. Overall, GEM fluxes observed from undisturbed soils ranged between 70.7 and 702 ng m-2 h-1 with the highest average value obtained for the natural enriched site at Idrija. The spatial variability of fluxes was mainly related to total Hg content in soils. However, a more developed herbaceous vegetation may limit GEM releases at the atmospherically influenced site near the most recent furnace despite the greater availability of Hg bound to organic matter, which is potentially more available for reduction to GEM and evasion. This is especially evident considering the low fluxes observed at the forested ancient roasting site despite the extremely high Hg concentrations in soils (up to 10,400 mg kg-1). Results from this study highlight that shading by vegetation can significantly limit GEM re-emission to the atmosphere even in extremely contaminated areas. However, a significantly high flux (2466 ng m-2 h-1) was recorded at the ancient roasting site on a bare soil surface under an uprooted tree, suggesting that care should be taken in forest management practices to avoid potential strong re-emission of GEM related to direct exposure of these heavily contaminated soils to solar radiation.