Occurrence and speciation of mercury in the recent sediments of the western coastal area of the Gulf of Trieste (northern Adriatic Sea): is the legacy of historical mining still present?

Mercury (Hg) contamination in the northern Adriatic Sea (Italy) is still an issue of environmental concern due to historical mining at Idrija (Slovenia) which was the second largest Hg mine worldwide (Covelli et al., 2001). The Isonzo/Soča River inputs continue to convey mercury-enriched particles into the coastal area where the element was accumulated in the sediment compartment and transported longshore towards the nearby Marano and Grado Lagoon (Covelli et al., 2007). This study is focused on the surface sediments from the coastal area in front of the eastern sector of the Marano and Grado Lagoon, where a nearshore depositional system made up of relict and active migrating sandbanks extends up to 2 km seawards from the beachfront (Bezzi et al., 2021). The primary aim of this research is to evaluate the occurrence and distribution of Hg in the surface sediments, their grain size composition and potential relationships between Hg concentrations and the prevalence of distinct grain size fractions. Since sediments may also act as a secondary source of contamination due to biogeochemical processes at the sediment-water interface and resuspension events induced by both natural and anthropogenic factors, Hg speciation through thermal desorption technique (Petranich et al., 2022) was also performed to identify the main Hg compounds. A total amount of 122 surface sediment samples was collected and analysed for grain size composition and total Hg (THg). Mercury (Hg) speciation analysis was applied as well on a subgroup of samples which were selected following both statistical and geographical criteria. Sand is the most common mean grain size in the central sector of the investigated area whereas the eastern and western ends are dominated by the silty fraction. The concentration of THg in the sediments varies widely, ranging overall from 0.16 to 59.1 µg g-1. Overall, the highest concentrations of THg were observed in the sediments showing a prevalent very fine sand fraction, most likely due to the occurrence of detrital form of Hg (cinnabar particles). This evidence was confirmed by the speciation analyses which identified cinnabar (α-HgS) and metacinnabar (β-HgS) as the main Hg species in sediments thus testifying to a generally scarce mobility of the element present as a sulphide. This evidence supports the hypothesis that the effects of a resuspension event would be negligible in terms of increase of dissolved Hg in the water column and we could expect a rapid settling back of Hg to the sea bottom in association with the sediment particles.