Mass balance of mercury in the Mediterranean Sea

The Mediterranean Basin is rich in mercury from natural and anthropogenic sources. During an extensive research from 20022005, measurements of different mercury forms were carried out in the Mediterranean Sea by the Italian research vessel Urania as a part of the Med Oceanor and MERCYMS projects funded by the EU Framework programme. Further on, transport and fate of mercury forms was simulated in water and air compartments by two different models, and an approximate coupling of both models was performed. The new 3-dimensional model PCFLOW3D was completed with the first version of a biogeochemical module which deals with various mercury species in dissolved and particulate forms. Exchange with the bottom and the atmosphere, methylation, demethylation, reduction and oxidation were taken into account. Based on the integrated hydrodynamic-atmospheric model and results of field measurements the mass balance of total mercury in the Mediterranean Sea was determined in order to estimate main sources and sinks and simulate future trends of mercury contamination in the Mediterranean. The following terms were taken into account: sources of mercury from major rivers and from point sources, exchange with the Atlantic Ocean and the Black Sea, estimated natural sources from geotectonic active areas in the sea, exchange with bottom sediments, and evasion to and deposition from the atmosphere. The main conclusions are summarized as follows: Based on the measurements, total mercury concentrations in the Mediterranean Sea in 2005 was about 1.5 pM. Thus the total mass of HgT in the Sea was about 5400 kmol (1080 t). The outflow of HgT exceeds the inflow for about 41 kmol/year. This indicates that the Mediterranean Sea is slowly recovering from mercury pollution. However, this also shows that the Mediterranean is a net source of HgT for the atmosphere and the connecting seas. The most important sources are (in kmol/year): atmospheric deposition (115), river inflow (68.5), and the bottom sediment (80) which includes estimated sources from geotectonic faults in the sea, while point sources of mercury pollution are estimated to 12.5 kmol/year. The main outputs are: evasion to atmosphere (249), and burial in the bottom sediments (55). The net outflow through Gibraltar is about 8.5 kmol/year, while the Black Sea seems to be a very small source of HgT to the Mediterranean. Some of the important terms in the mass balance equation were obtained by rough estimates. For this reason the prediction for 2020, based on policy target scenarios, can only be given in a wide range. It is expected that HgT concentration will decrease for about 3 to 12% compared to 2005. (c) 2006 Elsevier B.V. All rights reserved.