Gaseous mercury evasion from bare and grass-covered contaminated soils at the Isonzo River alluvial plain (North Eastern Italy)

Emissions of gaseous elemental mercury (Hg0) from soil surface represent a relevant component of the global biogeochemical cycle of this element. Terrestrial environments characterised by a high mercury (Hg) content in the substrate due to natural enrichments and/or anthropogenic activities may be an important source of Hg0 for the atmosphere [Eckley et al., 2016]. The alluvial plain of the Isonzo River (NE Italy) suffered a significant contamination due to the historical extraction of the cinnabar ore from the upstream Idrija mine (Slovenia). The magnitude and extension of Hg contamination of soils in this area have been recently described in detail [Acquavita et al., 2021], although an assessment of Hg0 releases from the soil is still lacking. In this work, Hg0 evasion fluxes at the soil-air interface were measured by means of a non-steady state flux chamber coupled with a real-time Hg0 analyzer (Lumex RA-915M) at 6 sites within the Italian sector of the Isonzo plain in summer. At each site, distinct sets of measurements were performed at regular time intervals during the diurnal period, both on bare and grass-covered soils, together with monitoring of soil and air temperature and incoming UV radiation. Moreover, soil samples were analyzed for organic matter content and total Hg concentration. Gaseous Hg fluxes (range=48.2÷944.6 ng m-2 h-1) were comparable with values reported for other sites impacted by mining activity worldwide [Agnan et al., 2016] and generally resulted 1.5- to 3-times higher for bare than grass-covered soils at each site, confirming the role of vegetation cover in influencing Hg release to the atmosphere. Overall, the amount of Hg0 fluxes at different sites seems to be related to total Hg content of soils (range=1.98÷28.93 mg kg-1), although the correlation was significant only for bare soils. Finally, Hg0 fluxes were found to track the incoming UV radiation during sampling, likely due to Hg photo-reduction on surface and soil heating. Acquavita A., Brandolin D., Cattaruzza C., Felluga A., Maddaleni P., Meloni C., Pasquon M., Predonzani S., Poli L., Skert N., Zanello A., (2021). Mercury distribution and speciation in historically contaminated soils of the Isonzo River Plain (NE Italy). Journal of Soils and Sediments, doi: 10.1007/s11368-021-03038-2. Agnan Y., Le Dantec T., Moore C.W., Edwards G.C., Obrist D., (2016). New constraints on terrestrial surface-atmosphere fluxes of gaseous elemental mercury using a global database. Environmental Science & Technology, 50, 507-524, doi: 10.1021/acs.est.5b04013. Eckley S.C., Tate M.T., Lin C.J., Gustin M., Dent S., Eagles-Smith C., Lutz M.A., Wickland K.P., Wang B., Gray J.E., Edwards G.C., Krabbenhoft D.P., Smith D.B., (2016). Surface-air mercury fluxes across Western North America: A synthesis of spatial trends and controlling variables. Science of the Total Environment, 568, 651-665, doi: 10.1016/j.scitotenv.2016.02.121.