| The fugacity-based multimedia environmental mathematical model was introduced and applied in this article. The basic concepts, model principle, characterization, researching region and status quo of the mathematical model had been approximately discussed. By using the aquivalence as the equilibrium criterion, the chemical fate of heavy metal compounds such as mercury compounds in water environmental system as Three-Gorges valley could be quantitative simulated through the modified computer program for multimedia environment chemical pollution behavior.The species, interphase transport and transformation of mercury in an aquatic environment system, the influencing factors of Three-Gorges valley and limitations of existing mathematical models of mercury behavior were discussed. According to the actual situation of hydrology, geology and climatology along Three-Gorges, adeveloped multimedia mathematical model was put forward to study the behaviors andchemical fate of mercury. Meanwhile, the prior tasks and directions were studied.The original QWASI model was modified and applied to describe the distribution, reaction rate and persistence time of inorganic (Hg2+), elemental (Hg0) and methyl mercury (MeHg) species between mixed water column and an active sediment layer, and the main transport processes and chemical fate in the Three-Gorges valley system.A sensitivity analysis of the model was conducted and the results indicated that the partition coefficient of water-sediment, density of sediment and suspended particle, the initial concentration of air and reservoir area were very important and should be precise.The results suggested that the three Hg species experienced different fate and persistence, with overall Hg dynamics dominated by the fate of Hg2+ (predominate species). The main conclusions were as follows,(1) Based on the aquivalence as equilibrium criterion, the developed multimedia mathematical model could be applied to describe the behaviors and chemical fate of heavy metal in super long-scale reservoir, such as Three-Gorges Dam.(2) Due to the characteristic of long riverway in Three-Gorges valley, the total Hg concentration of water column and sediment would increase. At the same time, the net transport rates of air-water and water-sediment also would be higher than other situation. The overall Hg persistence time and individual half-time in sediment phasedecreased evidently.(3) As the result of seasonal fluctuation of water, the total Hg concentration of water and sediment would be degressive in sequence of low water, flush time and ordinary level. The system persistence time of total Hg was the longest at the period of low water and the shortest in flush time. By contraries, the half-time of the Hg species were the longest in low water time and the shortest in flush time.(4) The local climate of Three-Gorges valley would change the former temperature and some other characteristics after the dam built. The reservoir would run at a relatively narrow range of temperature. The difference of total Hg concentrations of water and sediment, and the interphase transport rates were not distinct when the effect of changed local climate considered. That's mean the calculated results were averaged by the influence of local climate, and the Three-Gorges reservoir could be treated as a lake properly.(5) Three-Gorges reservoir would be managed by the means, so called "cumulate clear water and sluice mud water". It might result in that the concentrations of total Hg increased in river water column and declined in sediment at the time of flood. The net transport rate between air phase and water phase did not change very sharply, but that of water-sediment would vary distinctly.
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