Three-dimensional modelling of mercury cycling in the Gulf of Trieste

The Gulf of Trieste (Northern Adriatic) is subject to mercury pollution fro m a former mercury mine in Idrija, located along a river which transports m ercury-contaminated sediments into the Gulf. Concentrations in suspended an d bottom sediments are up to two orders of magnitude higher than in the cen tral and southern Adriatic. Extensive research has been carried out on meas urements and modelling of the transport and fate of mercury in the Gulf. Tw o- and three-dimensional models have been developed to include the influenc e of the significant advective transport due to currents. Wind, thermohalin e forcing, and the Soca river momentum are the most important forcing facto rs. A two-dimensional model simulated the transport of non-methylated and m ethylated mercury in dissolved, particulate and plankton fractions. Mercury processes included the input of atmospheric mercury, sedimentation, reduct ion, methylation and demethylation. The model simulations gave basically wh at were proper trends of the phenomena; quantitatively the measured and com puted results are mainly within a factor of three. To simulate the non-unif orm distribution of parameters over the depth, an existing three-dimensiona l (3D) hydrodynamic and transport-dispersion (TD) model, PCFLOW3D, was adap ted and applied. As it was found that most mercury transport is related to suspended sediment particles, a new 3D sediment transport module was also d eveloped and included in the model. Three cases are presented: one describi ng the simulation of TD of dissolved total mercury; another the simulation of the TD of particulate mercury in the Gulf during a river flood; and the third simulating sediment transport in the Gulf during a period of strong E NE wind. Comparison with measurements was only partly possible, but mainly the computed and measured results were within a factor of two and proper tr ends of the phenomena were obtained by the simulations. The combination of modelling and measurements has resulted in some interesting conclusions abo ut the phenomenon of the transport and fate of mercury in a coastal sea. (C ) 2000 Elsevier Science B.V. All rights reserved.