Mercury mobility in harbour sediments: evidence from selective sequential extraction and short-term microcosm resuspension experiments (northern Adriatic Sea, Italy)

The coastal sediments of the Gulf of Trieste have been historically contaminated by mercury (Hg) due to mining activity in Idrija (Slovenia). Several research activities have taken place investigating Hg in this area (e.g. Covelli et al., 2001) but no information has thus far been available to evaluate the possible impact on the water column of the contaminated sediments as a consequence of dredging operations. Sixteen surface sediment samples were collected along the main channel approaching the Port of Monfalcone, which is used for commercial and industrial activities. The channel is expected to be deepened by 1 meter in the near future. From sixteen surface sediment samples analysed for Hg concentrations, selective sequential chemical extraction (SSE, Bloom et al., 2003) was applied to selected samples. The effects of sediment resuspension on the fate of the Hg species were evaluated in the laboratory using a short-term small-reactor system. The experiment was conducted with sediments taken from two sites following the same approach which had already been successfully executed in a previous study. The six sediment samples subjected to the SSE procedure were characterised by Hg mainly related to poorly soluble or insoluble compounds (98.7%), such as sulphides. This was confirmed by the results obtained from experiments conducted in the laboratory over a period of 24 h, where the first 10 cm of the sediments were resuspended in the overlying water (approximately 15 cm). After resuspension, dissolved Hg (DHg) and methylmercury (DMeHg), were detected in only one of the two selected sites. DHg showed concentrations comparable to those observed at the beginning of the experiments (pre-resuspension), varying in a narrow range (2.72-8.17 ng L-1). DMeHg concentrations were < lod (limit of detection) over time and were only occasionally observed (up to 809 pg L-1). In addition, the high values of the partitioning coefficient (KD = [Hg]solid/[Hg]dissolved) confirmed the high affinity of Hg to the suspended sediment particles rather than to the dissolved phase. The consequences of a resuspension event which takes place over a limited time period can be considered of negligible impact due to the scarce mobility of Hg from sediments and to the dilution of Hg species present in the porewaters.