Gaseous mercury evasion from the water-air interface at freshwater environments impacted by different anthropogenic sources

The gaseous exchange of mercury (Hg) at the water-air interface represents an important aspect of the biogeochemical cycle of this element in the environment. Particularly at sites impacted by Hg due to anthropogenic activities, evasion can contribute to reduce the amount of this metal in the water column, but also to widen its spreading in the environment due to the long-range atmospheric transport of the element (Driscoll et al., 2013). However, dynamics of gaseous elemental Hg emissions from the water-air interface at Hg contaminated sites are still poorly characterised. In this work, Hg evasion fluxes were monitored by means of a flux chamber coupled with a real-time gaseous Hg analyzer (Lumex RA915M) in 3 freshwater environments characterized by different contamination legacy: the artificial reservoir of Solkan (SK-Slovenia) along the Isonzo River, where Hg contaminated sediments due to cinnabar extraction activities at the Idrija Hg mine have been accumulated since 1984, date of the dam construction; the dock near a chlor-alkali plant at Torviscosa (TR-Italy); the Cavazzo Lake (CV-Italy), chosen as pristine environment with no known Hg sources. At each site, 6 distinct sets of flux measurements were performed at regular time intervals during the diurnal period in 3 different seasons (spring, summer, and autumn). Moreover, the concentrations of dissolved gaseous mercury (DGM) were monitored during the sampling period together with total dissolved Hg, incoming UV radiation and main water physico-chemical parameters. As expected, at all sites the highest Hg fluxes were found in summer, due to higher irradiation, which favours the formation of volatile DGM through photoreduction, and higher water temperatures, which decrease the solubility of Hg. On average, Hg evasion fluxes found in this study were higher than those reported in literature for other background freshwater environments. Surprisingly, Hg fluxes at CV site were comparable to those of the other sites, likely due to reemission of Hg of atmospheric origin, but further research is needed to confirm this hypothesis. The widest diurnal variability of Hg fluxes was observed at TR site, located inside an industrial complex, and likely related to irregular supplies of Hg through atmospheric depositions and water circulation. Conversely, SK and CV showed a more regular trend, confirming that Hg fluxes are strongly influenced by local conditions and site-specific interaction of different factors. Based on these results, the investigated sites can be considered as secondary sources of Hg for the atmosphere, underlining the importance of direct Hg fluxes monitoring to understand its fate in the environment. Driscoll C.T., Mason R.P. Chan H.M., Jacob D.J. & Pirrone N. (2013) - Mercury as a Global Pollutant: Sources, Pathways, and Effect. Environ. Sci. Technol., 47, 4967-4983.