Modelling of mercury transport and transformations in the water compartment of the Mediterranean Sea

Abstract 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 (Hg 0 ), divalent (Hg 2+ ), 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 Hg 0 . 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 (Hg 0 ) was achieved. Agreement of Hg 0 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 Hg 0 , 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.