Gaseous mercury evasion from bare and grass-covered soils contaminated by mining and ore roasting (Isonzo River alluvial plain, Northeastern Italy)

Abstract

High amounts of mercury (Hg) can be released into the atmosphere from soil surfaces of legacy contaminated areas as gaseous elemental mercury (Hg0). The alluvial plain of the Isonzo River (NE Italy) suffered widespread Hg contamination due to the re-distribution of Hg-enriched material discharged by historical cinnabar mining at the Idrija mine (Slovenia), but an assessment of Hg0 releases from the soils of this area is still lacking. In this work, Hg0 fluxes at the soil-air interface were evaluated using a non-steady state flux chamber coupled with a real-time Hg0 analyser at 6 sites within the Isonzo River plain. Measurements were performed in summer, autumn, and winter both on bare and grass-covered soil plots at regular time intervals during the diurnal period. Moreover, topsoils were analysed for organic matter content and Hg total concentration and speciation. Overall, Hg0 fluxes tracked the incident UV radiation during the sampling periods with daily averages significantly higher in summer (62.4 ± 14.5–800.2 ± 178.8 ng m−2 h−1) than autumn (15.2 ± 4.7–280.8 ± 75.6 ng m−2 h−1) and winter (16.9 ± 7.9–187.8 ± 62.7 ng m−2 h−1) due to higher irradiation and temperature, which favoured Hg reduction reactions. In summer and autumn significant correlations were observed between Hg0 fluxes and soil Hg content (78–95% cinnabar), whereas this relationship was not observed in winter likely due to relatively low emissions found in morning measurements in all sites coupled with low temperatures. Finally, vegetation cover effectively reduced Hg0 releases in summer (∼9–68%) and autumn (∼41–78%), whereas the difference between fluxes from vegetated and bare soils was not evident during winter dormancy due to scarce soil shading. These results suggest the opportunity of more extended spatial monitoring of Hg0 fluxes particularly in the croplands covering most of the Isonzo River alluvial plain and where bare soils are frequently disturbed by agricultural practices and directly exposed to radiation.