Mercury cycling and transformations at the water-air and soil-air interfaces in a contaminated regional context

The study of dynamics of fluxes of gaseous elemental mercury (Hg0) between atmosphere and natural surfaces in contaminated sites is essential to fully understand the fate of this metal once released in the environment and to assess the potential risks for ecosystems and human health. The aim of this research was therefore to study the processes and environmental factors that affect Hg0 exchanges at water-air and soil-air interfaces in selected environments within the area between Northeastern Italy and Western Slovenia. The study area is characterised by a widespread Hg contamination related to historical Hg mining in Idrija (Slovenia) and, secondarily, past wastewater discharges of a chlor-alkali plant in Torviscosa (Italy). Considering the Hg0 fluxes at the water-air interface, the research was divided into two parts. The first was devoted to study Hg0 fluxes during complete 24 h cycles in coastal environments: to understand the influence of different Hg contamination degree and hydrodynamic conditions, a strongly impacted confined fish farm (Val Noghera, Italy) and an open marine area (Bay of Piran, Slovenia) in a relatively pristine area of the Gulf of Trieste were selected. The second part of the research was focused on evaluating the effect of past Hg anthropogenic supplies from different sources on present-day Hg0 fluxes during the diurnal period: measurements were conducted in freshwater environments impacted by mining (Solkan Reservoir, Slovenia) and industrial activity (Torviscosa Dockyard near the former CAP, Italy) and results were compared with those obtained in pristine site (Cavazzo Lake, Italy) with unknown Hg sources. Hg0 fluxes were evaluated in field by means of a floating flux chamber taking measures at regular time intervals. Dissolved gaseous mercury (DGM) concentrations in surface water layers were also determined as the volatile fraction available for volatilisation together with main meteorological and physico-chemical water parameters. Generally, higher Hg0 fluxes were recorded in more impacted sites than pristine areas for both coastal and freshwater environments and were related to both irradiation and water temperature, which may have favoured Hg reduction to Hg0 and volatilisation. However, stagnant water conditions and stratification strongly limited Hg0 emissions at the fish farm and CAP compared to pristine environments. In terrestrial environment, the Hg0 fluxes at the soil-air interface were measured at six selected sites along the Isonzo River alluvial plain characterised by variable total Hg concentration in surface soils. Several replicate measurements were performed by a non-steady state flux chamber on both bare and grass-covered soil plots in different seasons (summer, autumn, winter) during diurnal hours. Total Hg concentration and Hg speciation in topsoils were also assessed and the main meteorological parameters and soil physico-chemical characteristics were determined. Fluxes of Hg0 recorded at the soil-air interface were higher than those found at the water-air interface and positively correlated with soil Hg content and both irradiation and temperature. Only in winter, no correlation was found between soil Hg content and fluxes, likely due to scarce contribution of predominant cinnabar fraction to formation of Hg0 at low temperatures. Soil shading by active vegetation significantly reduced Hg0 emissions from soil surfaces in summer and autumn, whereas this effect was not observable in winter due to the scarce vegetation development. The relatively high Hg0 fluxes recorded in this study for both aquatic and terrestrial environments suggest that the studied areas can represent a secondary atmospheric Hg source even decades after the phase out of the main anthropogenic contamination sources.