Transport and dispersion of particulate Hg associated to a river plume in coastal Northern Adriatic environments

The Marano Lagoon, in the northern Adriatic Sea (Italy), has been affected by mercury (Hg) contamination from industrial and mining activities. It has been estimated that 186,000 Kg of Hg were deliberately discharged into the main drainage system (Aussa-Corno River) by a chlor-alkali plant (CAP) from 1940’s to 1984. The lagoon has also experienced a secondary long-term Hg input, originated from mining activity in Idrija (Slovenia), due to the supply of fluvial suspended particles carried by the Isonzo River in the Gulf of Trieste. Since local fishing activities are extensively conducted, there is great concern on the risk posed by potentially harmful effects of Hg to the trophic chain. Present inputs of this metal, both in dissolved (52.4-4.1 ng L-1) and particulate (130.8-3.4 ng L-1) phases, were preliminary investigated in the water column. Although direct discharge of Hg from the CAP no longer exists, the metal is still released from the source area into freshwaters, and its distribution and abundance is controlled by the saltwedge circulation system, which is tide-influenced. Remobilization from bottom sediments can also be a secondary source of Hg into the aquatic environment. A speciation technique, used to investigate the main binding sites and phase associations of Hg in sediments (5.69-0.82 µg g-1), evidenced the presence of mobile (1.8-11%) and potentially available species for methylation processes. The results are particularly important if related to resuspension effects caused by natural events and anthropogenic activities. Preliminary considerations on Hg behaviour in this estuarine environment are reported.