Mercury contamination of coastal sediments as the result of a long-term cinnabar mining activity (Gulf of Trieste, Northern Adriatic sea)

The Isonzo River mouth has been the source of Hg in the Gulf of Trieste (Northern Adriatic sea) since the sixteenth century, making this shallow basin one of the most contaminated marine areas in extent of time and amount of metal accumulated. The occurrence and behaviour of total Hg (range 0.064-30.38 µg g-1; average 5.04 µg g-1; median 3.10 µg g-1, n=80) and related size fractions in sediments of this coastal area were investigated in detail. The relationship between total Hg and fine silt-clay (< 16 µm) fraction has provided information on the hydrological and mineralogical fractionation process affecting this element, when compared to other heavy metals associated to fluvial inputs. Hg contents are very high along the littoral zone of the northern (Italian) sector where this metal is present in detrital form (cinnabar) in sandy-silty sediments near the river mouth and the surrounding beaches. Within the sediments belonging to the Gulf area, Hg is bound either to fine particles or adsorbed onto surface of clay minerals and/or partially complexed by colloids and organic matter. Recent accumulation of mercury in a 70 cm long 210Pb dated core, collected in the central part of the Gulf, was also compared to other heavy metals (Fe, Cr, Cu, Mn, Ni and Zn). A preliminary estimate of Hg enrichment shows that the first 50 cm of sediment in the central sector of the Gulf of Trieste are noticeably contaminated, reaching a maximum of up to 25 fold above the preliminary proposed natural regional background of 0.17 µg g-1. The vertical trend is well correlated to historical data of Hg extraction activity at the Idrija mine.