The Idrija Mercury (Hg) mine in Slovenia, the world’s second-largest, operated for over 500 years before closing in 1995. Mercury from the mine was historically transported to the Gulf of Trieste (GoT) via the Idrijca and Isonzo rivers, leading to significant contamination. Despite extensive studies on Hg speciation and distribution during and shortly after the mine’s operation, there is a lack of recent data assessing the mine’s impact on Hg behavior in the GoT, 25 years post-closure. Current contamination levels, speciation, and bioavailability of Hg species in the GoT remain largely unaddressed. Diffusive Gradients in Thin-Films (DGT) is a useful technique to assess and preconcentrate Labile Hg (LHg) and Labile monomethylmercury (LMeHg) in situ in GoT. The labile fraction of Hg is regarded as the potential bioavailable fraction in aquatic systems. A preliminary sampling was conducted at various sites around the mouth of the Isonzo River, focusing on water and sediment. One site with high Hg concentrations in both water and sediment and significant freshwater input from the Isonzo River was selected for detailed investigations of seasonal variations and Hg methylation in sediments.   In the sediment solid phase, the THg concentrations vary from 6.0 mg kg^-1 to 9.5 mg kg^-1, which are at the same level as the historical ones measured in the 1990s. In this station, a high labile (thus bioavailable) fraction was also seen in water and Dissolved Gaseous Mercury (DGM) represented up to 71 % of Dissolved Hg (DHg) and up to 25 % of Total Hg (THg) in water. In all stations, Particulate Hg (PHg) dominates Hg concentrations, even though the Suspended Particulate Matter (SPM) levels are low (1.5 mg L^-1 to 5.5 mg L^-1). Significant seasonal variations in Labile Hg (LHg) were found in bottom waters, with 4 times higher concentrations in October (up to 2ng L^-1) compared to 0.5 ng L^-1 in June and higher levels of Labile MeHg (LMeHg) were also found in October (0.04 ng L^-1) compared to June (0.02 ng L^-1). In sediment porewater, mobilization of both LHg and LMeHg occurs just below the sediment surface interface, and LHg is also much higher in October, with values up to 120 ng L^-1. Benthic fluxes may highly contribute to the Hg concentrations in the bottom waters of the GoT.

mercury, historical contamination, Gulf of Trieste, water and sediment, DGT