Evasion of Gaseous Elemental Mercury from Forest and Urban Soils Contaminated by Historical and Modern Ore Roasting Processes (Idrija, Slovenia)

Considerable amounts of gaseous elemental mercury (Hg-0) can be released into the atmosphere from Hg-enriched substrates, such as those from former mining areas, posing a potential environmental threat. In this work, Hg-0 fluxes at the soil-air interface under natural vegetation covers were measured in various locations within the Idrija Hg mining area (Slovenia) and its surroundings. Sites were selected in order to compare Hg-0 fluxes from both forest soils heavily impacted by historical ore roasting and urban soils characterised by a different degree of Hg enrichment due to the natural occurrence of Hg in rocks or recent mining and roasting processes. Replicate measurements at each site were conducted using a non-steady state flux chamber coupled with a real-time Hg-0 analyser (Lumex RA-915M). Moreover, topsoil samples (0-2 cm) were analysed for Hg total concentration and speciation. Cinnabar was the predominant Hg form in almost all the sites. Despite Hg-0 being undetectable in soils using thermo-desorption, substantial emissions were observed (70.7-701.8 ng m(-2) h(-1)). Urban soils in a naturally enriched area showed on average the highest Hg-0 fluxes, whereas relatively low emissions were found at the historical roasting site, which is currently forested, despite the significantly high total Hg content in soils (up to 219.0 and 10,400 mg kg(-1), respectively). Overall, our findings confirm that shading by trees or litter may effectively limit the amount of Hg-0 released into the atmosphere even from extremely enriched soils, thus acting as a natural mitigation.