Spatial and temporal distribution of mercury in the recent sediments of the Adriatic Sea

Several sources of anthropogenic mercury (Hg), which date back to the sixteenth century, have been identified in the Adriatic Sea making this basin one of the most contaminated marine areas with regard to duration and amount of metal accumulated. Taking into account 2 complementary datasets, the distribution of total Hg was investigated in surface sediments (0.003-16.940 μg g-1; avg. 0.715±1.929 μg g-1; median 0.102 μg g-1, n=162) and in 7 short cores collected from the Po River delta to the Strait of Otranto (0.002-0.963 μg g-1; avg. 0.171±0.226 μg g-1; median 0.077 μg g-1, n=189). The objectives of our research were to integrate the two datasets in a distribution map to identify anomalies and potential sources at basin scale, and to reconstruct the recent history of sediment contamination. Results show that the coastal sediments of the northern sector are particularly enriched in Hg compared to the area extending from the Po delta to the Otranto Strait. This is due to the presence of multiple sources of contamination, the main one identifiable in the Gulf of Trieste at the north-easternmost side of the basin. Even now the Isonzo River supplies significant quantities of Hg to the sea, mainly in mineral form (cinnabar) as a result of mining activity that was operating at Idrija (Slovenia) for 500 years (Covelli et al., 2001). The nearby Marano and Grado Lagoon is another prominent area of metal accumulation by sources which include but are not restricted to mining (Acquavita et al., 2012). Part of Hg contamination is also associated with the past activity of a chlor-alkali plant whose effluents were deliberately discharged into the lagoon system. The “industrial” Hg was dispersed in this environment mixing with Hg from Idrija mine. Similarly, other sources can be recognised in the Venice Lagoon (the Porto Marghera industrial site) and Po River drainage basin. As we proceed southwards along the Italian coast, the concentrations of Hg decrease showing a significant negative gradient with increasing distances from the emission sources. The sedimentary profiles of Hg show decreasing trends in concentration and flux over the last 20-30 years indicating the beginning of a “recovery” period. The only exception is the core collected offshore Otranto, which still shows a weak increasing trend probably due to a delay in transferring the contamination signal of Hg coming from the northern sector of the basin.