Flux measurements of gaseous elemental mercury (GEM) from the geothermal area of “Le Biancane” natural park (Monterotondo Marittimo, Grosseto, Italy): Biogeochemical processes controlling GEM emission

Diffuse soil fluxes of gaseous elemental mercury (GEM) were measured in a hydrothermal area (20,000 m2) located inside “Le Biancane” natural park, north of Monterotondo Marittimo village (Larderello-Travale geothermal field, Central Italy), using a static closed chamber (SCC) coupled with a Lumex® RA-915 M portable analyzer. At 89 sites, GEM and CO2 fluxes (ϕGEM and ϕCO2, respectively) and temperature at the soil-air interface were measured. The ϕGEM values showed no correlation with those of ϕCO2 and soil temperatures, suggesting that secondary chemical-physical processes affected these gases as they move toward the surface. Formation of Hg-bearing minerals from primary GEM and secondary Hg2+, as well as secondary GEM production likely occurred through processes mediated by microbial activity, particularly in those areas characterized by soils affected by strong air contamination (CO2/N2 ratio < 0.1), where high ϕGEM and low ϕCO2 values were measured. The total output of the measured gas species was equal to 1.6 × 10−6 and 3.15 t day−1 for GEM and CO2, respectively, while the computed GEM specific output was of 8.0 × 10−5 g day−1 m−2, i.e. in the same order of magnitude of other hydrothermal areas worldwide.