Estimated annual mass-balance of suspended particulate mercury in a contaminated lagoon environment.

River has been demonstrated to be still a point source of mercury (Hg) in the Gulf of Trieste (northern Adriatic sea) although the Idrija mine definitely stopped his activity in 1996, after 500 years of operation. Due to the erosion of the mining region, of the Idrijca-Isonzo river drainage basin and of the flood plains, fluvial waters remobilize and transport Hg contaminated sediments downstream to the estuarine zone and the marine environment. The present study aims at verifying the role of riverine suspended particulate matter (SPM) associated to tidal fluxes as an important way of Hg dispersion into the Grado coastal lagoon system all over the year. An annual mass-balance of particulate Hg (PHg) in the Grado lagoon was then attempted through several sampling campaigns of the water column at the corresponding tidal inlet. PHg concentrations (avg. 3.11 ± 2.62 μg g-1, d.w.), notwithstanding the ebb or flood tides, were significantly higher than the local sediment background in the Gulf (0.13 μg g-1). The relative affinity of Hg for the particulate phase in surface waters, expressed as log Kd (=[PHg]/[DHg], L kg-1), was also confirmed by higher average values (5.6–6.7) which are in agreement with those previously reported for Isonzo River freshwaters and Gulf of Trieste surface waters. PHg contents, as dry weight but also expressed by volume (ng L-1), showed the highest values in ebb tide conditions thus suggesting to be originated from the erosion of tidal flats and saltmarshes of the Grado lagoon. When compared to daily rainfalls and river discharge trends, high PHg surface concentrations in flood tide conditions are related to rainfalls events occurred within the Isonzo River basin. Results obtained from the tidal inlet can be used to make an indicative assessment of the amount of Hg bound to SPM which is transported in and out of the lagoon basin following the action of tidal fluxes. A simple estimation provides a negative sedimentary budget for the Grado lagoon basin which loses between 0.16 and 1.01 kg of PHg during a tidal semi-cycle. This conclusion is in agreement with evidences of severe morphological deterioration emerged from recent studies on the lagoon environments which testify a current sedimentary loss from the lagoon into the northern Adriatic sea.