A NOVEL ADVECTION-DIFFUSION-REACTION MODEL OF THE SPATIO-TEMPORAL MERCURY DYNAMICS IN WATER AND SEDIMENTS

Abstract

Mercury (Hg) is a chronic pollutant of global concern known to be transported long distances in the atmosphere into remote ecosystems. Although a part of the Hg emitted naturally comes from geological and geothermal sources, much of it is recycled Hg previously emitted from primary or anthropogenic sources, and subsequently re-deposited to terrestrial and ocean surfaces. As a consequence, a large part of the 2000 t of yearly emissions from natural sources is actually reemission of previously deposited mercury, much of which has an anthropogenic origin. In some instances, it has been discovered that marine sediments contaminated by industrial effluents may be secondary sources of Hg to aquatic ecosystems even though discharge has been strongly reduced or has even ceased. The exchange of Hg between oceanic surfaces and the atmosphere represents an important process for the atmospheric cycling and environmental turnover of this element. According to Mason et al. (1994), the ocean releases about 1/3 of the total global Hg emissions to the atmosphere (about 30% of the total budget of atmospheric mercury on a global scale) and receives about 30-70% of the global atmospheric deposition. Re-emissions from the ocean of previously deposited Hg are dominated by gaseous elemental mercury. Its low solubility and high Henry’s Law constant induce high evasion fluxes from fresh water systems. In this framework, theoretical studies were able to reproduce the spatial-temporal dynamics of mercury concentration in marine ecosystems by using biogeochemical models. Specifically, in recent works, for biogeochemical models an WASP (Water Analysis Simulation Program) approach has been used to discretize a water body in one, two or three dimensions, by means of interconnected zero dimensional boxes, which represent water or sediment compartments. However, this approach does not allow to localize the zones within the zero dimensional boxes where the mercury concentration takes on the maximum value. Moreover, the effects of the seasonal variations of most environmental variables on mercury concentration are not usually taken into account in this kind of model. Finally, the previous analyses include neither the study of the spatiotemporal dynamics of phytoplankton distributions nor one of the mechanism responsible for absorption of mercury within the phytoplankton cell in real aquatic ecosystems. In this context, we introduce an advection-diffusion-reaction model which allows to reproduce the temporal and spatial behaviour of the concentrations of three mercury species (inorganic, organic, elemental) observed in water, pore water and sediments. Moreover, we also consider the partition of methyl-mercury and inorganic mercury into the dissolved phases (water and pore water) and the particulate phases (Suspended Particulate Matter and Sediment particles). The former include both the ionic form and the fraction complexed by the DOC (Dissolved Organic Carbon), while the latter consider the mercury fractions adsorbed by both inorganic and organic matter.