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Mercury exchange between outflowing and inflowing waters in the Strait of Gibraltar

The Mediterranean Basin is highly heterogeneous with regard to its climatic and oceanographic properties. The appropriate approach for simulating the transport and transformations of Hg in the water compartment requires the use of a hydrodynamic model with additional modules for transport–dispersion and biogeochemistry. In this work, the PCFLOW3D model was upgraded with a biogeochemical module and used for simulation of mercury transport and transformation processes in the Mediterranean. The circulation for the four seasons due to wind, thermohaline forcing and inflow momentum of the main rivers and through the straits was calculated. The results were compared with measurements and the results of another model (POM — Princeton Ocean Model). An acceptable agreement was achieved. The seasonally averaged velocity fields obtained were used to simulate transport and dispersion of mercury.

A new biogeochemical module dealing with the different mercury species: gaseous elemental (Hg⁰), divalent (Hg²⁺), and mono-methyl mercury (MMHg) in dissolved form and bound to particulate matter and plankton was introduced. Exchange of mercury at the boundaries (bottom sediment/water and water/atmosphere) and transformation processes such as methylation, demethylation, reduction and oxidation were taken into account. The transformation rates between the mercury species were described using simple equations, and thus the time and space variable reaction coefficients should be determined from in-situ measurements. Instead, machine-learning tools and classical statistical methods were used to connect the measured sets of geophysical/environmental parameters and concentrations of different Hg species. The provisional annual Hg mass balance established for the Mediterranean showed that exchange with the atmosphere is the most important source/sink of mercury for the water compartment. Therefore, the model was further upgraded with a gas exchange module for Hg⁰. To improve the results of the simulations the PCFLOW3D aquatic model was further linked to the RAMS–Hg atmospheric model which provided real-time meteorological data, deposition and concentrations of mercury in the atmosphere.

Simulations with the integrated modelling tool were performed and the results were compared to the measurements. Acceptable agreement of the average concentrations down the water column for both total mercury (HgT) and elemental mercury (Hg⁰) was achieved. Agreement of Hg⁰ concentrations near the surface was good; thus exchange with the atmosphere can be simulated with relatively high reliability. Agreement of simulated MMHg concentrations with measurements was not satisfactory, which is probably due to poor understanding of the processes of MMHg formation and its dependence on environmental factors, which have, so far, not been taken into account in the modelling.

In view of the satisfactory modelling results obtained for HgT and Hg⁰, a simulation of management scenarios, particularly the policy target (PoT) scenarios for 2010 and 2020, was performed. The results of these simulations were further used to establish the mass balance of HgT in the Mediterranean Sea.

Mercury concentrations in waters, total suspended matter, plankton and sediments of the Adriatic Sea were determined by emission and absorption spectroscopy. The Adriatic Sea, part of the Mediterranean basin, is located between the eastern coast of Italy and the western coast of Yugoslavia and Albania. Concentrations of dissolved mercury range from 2.03 to 6.75 ng l⁻¹, in total suspended matter from 0.09 to 15.35 ng l⁻¹, in plankton from 0.02 to 0.14 μg g⁻¹ dry wt., and in sediments from 0.02 to 8.63 μg g⁻¹ dry wt. The highest values of mercury were observed in the northern Adriatic. This area is characterized by very shallow waters (20–40 m) and elevated values of water turbidity (0.1–12.0 NU (Nephelometric Units)) caused by suspended particulate matter originating from sediment resuspension and from the contribution of the most important Italian rivers. The mercury levels decrease from north to south, where they reach values comparable with those of the other areas of the Mediterranean basin.

A critical review of existing concentrations of the potential pollutants Hg, Cd, Pb, PCBs and DDT in marine waters, sediments and biota of the global ocean indicates that the highest concentrations are usually found in the most densely populated and industrialized regions which are often located near major river estuaries. In general, present concentrations, particularly those in edible marine organisms, do not give rise for alarm; however, in some cases national and international concentration limits have been exceeded which has caused some concern for human health. In some ‘hot spots’ (e.g. Minamata Bay, Hudson-Raritan Estuary, Los Angeles Bight, Ems Estuary) where measures have been taken to eliminate the sources of contamination, a significant reduction in concentrations has occurred. Reliable temporal data are generally too sparse and have not been collected for a sufficient period to make accurate predictions about the environmental half-life of the contaminants; however, the widespread occurrence of persistent organochlorine residues of PCBs and DDT in remote areas far from known input sources suggests a long residence time in the ecosystem. Spatial data on a global scale are also limited; therefore, it is difficult to draw firm conclusions about long-term consequences of the shift in use of DDT and other chlorinated pesticides towards tropical areas and the southern hemisphere. However, the few reliable data available from these regions indicate concentrations of some organochlorine compounds in marine matrices that are as high or higher than those reported for the northern hemisphere. More data from a much wider area are needed in order to clarify this trend.