| A large area of the fluctuating zone was formed due to the completion of the Three Gorges Reservoir.. The major type of land in the fluctuating zone is farmland. There are varieties of pollutants in the fluctuating zone, including mercury, due to the long-term fertilization and irrigation, combined with the effect of surface runoff. The reservoir becomes a fragile ecological zone and a typical mercury-sensitive ecosystem consequently. Therefore, it’s of great significance to study the mercury transformation characteristics of fluctuating zone. So far, researchers had investigated on mercury content in the water and soil of the Three Gorges reservoir, the characteristics of adsorption desorption of the soil mercury in the fluctuating zone, and the migration and enrichment of mercury in aquatic food web. However, researches on the mercury transformation in soils in water level fluctuating zone of Three Gorges Reservoir Region are still insufficient. Thus, this study employed the soil from the reservoir area which was collected in Zhong County. The samples were collected periodically during the indoor simulation research, environmental factors on-site was determined, and the trend of mercury species and methyl mercury were analyzed, as well as the effect of the environmental factors. The results are as follow:(1) The methyl mercury of water increased at first and then decreased with the prolongation of flooding time, and the trend of the upper-soil was quite similar but with, lag to a certain degree. During the first impounding, the methyl mercury of low-mercury area and high-mercury area reached the peak of0.26ng/L and0.40ng/L by day25and day20respectively. Duringthe second impounding, the methyl mercury of low-mercury area and high-mercury area reached the peak of0.57ng/L and2.05ng/L by day25and day10respectively:. When there was high mercury in the soil, the content of methyl mercury in the water was higher correspondingly. The methyl mercury levels would increase significantly when the flooding condition occurred repeatedly.(2) During the impounding, the content of aqua soluble species, exchangeable species, acid soluble species decreased at first and then increased. The content alkali soluble species increased continuously. The auricome soluble species decreased continuously. The aquaregia soluble species increased at first and then decreased. The changes of aqua soluble species, exchangeable species and aquaregia soluble species were most noticeable, and the change of mercury species of surface soil was more obviously than the underlying soil. In this study, there was excessively significantly negative correlation between pH of soil and aqua soluble species, exchangeable species, auricome soluble species, positive correlation between pH and aquaregia soluble species, positive or negative correlation between pH and alkali soluble species. There was excessively significant positive correlation between auricome soluble species and organic matter of soil. There was significantly positive correlation between Eh and alkali soluble species.(3) During the flooding, the content of methyl mercury in the surface soil increased at first and then decreased. During firstflooding, the content of methyl mercury in low-mercury and high-mercury soil reached the peak of52.24ng/kg and128.26ng/kg by day10and day25respectively. During second flooding, the content of methyl mercury in low-mercury and high-mercury soil reached the peak of57.40ng/kg and340.95ng/kg by day5and day20respectively. The content of methyl mercury in the underlying soil increased continuously during the flooding time, and the content of methyl mercury was higher than that of the surface soil. The content of methyl mercury in the water level fluctuating zone of three gorges reservoir region increased continuously and the cumulative effect of methyl mercury in high-mercury soil was more significant. There were significantly positive correlation between the content of methyl mercury and the pH and organic matter of soil, significantly negative correlation between the content of methyl mercury and the Eh.(4) After the water drained out, the variation of mercury species in the soils in the water level fluctuating zone of three gorges reservoir region was not quite obvious, and was different from the period of flooding. Aqua soluble species and acid soluble species increased at first and then decreased. Exchangeable species, alkali soluble species and auricome soluble species decreased continuously. Aquaregia soluble species increased continuously. The trend of subsoil was similar with the topsoil, but not obvious. In this study, during the dry period, there was significantly negative correlation between auricome soluble species and the pH of soil, and significantly positive correlation between aquaregia soluble species and the pH. The relation between the organic matter, moisture rate of soil and mercury species was not significant.(5) After the water drained out, the methyl mercury in the soils in the water level fluctuating zone of three gorges reservoir region changed as follow:the content of methyl mercury in both topsoil and subsoil increased continuously at first and then decreased sharply. By day20of the first time drying, the content of methyl mercury in the topsoil and subsoil of the low-mercury soil and the high-mercury soil were:10.23ng/kg and14.70ng/kg,67.97ng/kg, and78.58ng/kg respectively; By day20of the second time drying, the content of methyl mercury in the topsoil and subsoil of the low-mercury soil and the high-mercury soil were:20.28ng/kg and23.65ng/kg、200.77ng/kg and231.69ng/kg respectively. During the period of drying, there was significantly negative correlation between pH and methyl mercury, and significantly positive between organic matter, moisture rate of soil and methyl mercury. |
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