| Mercury (Hg) is a volatile, highly toxic and bioaccumulative pollutants. To understand its geochemical behavior in environmental media is essential for eco-environmental security and human health.Aquatic system is considered to be one of the most sensitive environments for the activation, methylation and bio accumulation of Hg, leading for damage to the organisms. The Three Gorges Reservoir, the world spotlight reservoir, has contributed significantly to the economic development of China. Meanwhile, the change of water environment with its special water level control mode in this reservoir would enhance the bioavailability of Hg and increase the ecological risk of Hg pollution. Currently, many studies has been conducted about the characteristics of Hg in the Three Gorges Reservoir and its water-level-fluctuating zones, but the studies related to terrestrial watersheds have not yet been reported. Many previous studies have shown that terrestrial watersheds have a direct impact on the mercury in the rivers, lakes and other water bodies. However, the Three Gorges Reservoir which contains large proportion of the farming area (about 39.34%) covers a wide water area the biosis and producing activities in agro-ecosystems will input large of Hg to the Three Gorges Reservoir, facilitate the chemical behavior of Hg in the agro-ecosystems and the migration of Hg to the reservoir, or generate direct and indirect impact on Hg environment by altering water environment quality or Hg load of reservoir. Besides, government had taken some effective measures to control the point source pollution, the effect on water environment from agricultural terrestrial environment aggravated relatively. Therefore, it is urgent for us to study the geochemistry of Hg of agricultural watershed in the Three Gorges Reservoir areas, which is meaningful to make a reasonable prediction and assessment for the effect of agricultural watershed to the Three Gorges’ environment and valuable for us to make systematic analysis and administration for the Hg in reservoir.In this study, Wangjiagou, a typical watershed, located in FuLing in Three Gorges Reservoir areas, was selected as research object to assess the environmental geochemistry of Hg in soils and waters in the agricultural environment, as well as the Hg (Hg) pollution level and the capacity of soil Hg in the Three Gorges Reservoir areas. The aim is to evaluate and predict the Hg input load of water from watershed in reservoir reasonably and provide the research foundation to the treatment of Hg pollution in reservoir by understanding the geochemical characteristics of Hg in typical watershed of reservoir. The research results as follows:1. Spatial distribution of Hg in soils in Wangjiagou watershed(1) Used the grid method to sample and analyse, the Hg contents of surface soil (0-5 cm) in this watershed ranged from 9.47 to 94.57 μg·kg-1, and mean value was 34.23±16.23 μg·kg-1, lower than the background value of soils in the Three Gorges Reservoir. The highest Hg contents in surface soils were observed in woodland, followed by farmland, settlement and dry land. The main reason that lead to the difference of Hg values in different land-use type is the external disturbance, vegetational coverage and the runoff scour from rainfall. Moreover, the analysis of Universal Kriging interpolation method showed a weak spatial correlation of soil Hg contents in this watershed. Nature factor, such as soil parent materials and topography, played a key role to affect the spatial diversity of soil Hg in watershed, instead of anthropogenic interference. Additionally, the geo-accumulation index method which assess the Hg pollution level indicated that overall confirmative soil Hg pollution was not found in this watershed. And the potential ecological risk index showed that overall confirmative soil Hg potential ecological risk was not found in this watershed.(2) Surfaces soils from different land-use type (dry land, farmland, woodland) significantly showed Hg accumulation. The Hg contents decreased with the increase of soil depth, and tended to keep a stable value below 60 cm. The Hg concentration in soil between 0-5 cm was approximately 2 to 4 times more than the deep soil, of which woodland was most obvious. In this area, the background value of soil is about 24.13 μg·kg-1.(3) The total Hg capacity of soil in this watershed was 31.54 kg with average Hg inventory level in unit area of 477.78 g-hm-2. And the total Hg capacity of of surface soil (0-5 cm) was estimated to be about 1.73 kg with mean inventory level in unit area of 23.29 g·hm-2. The total capacity of Hg in the Three Gorges Reservoir was then also estimated to be approximately.2739 t, and that in surface soil in this reservoir was about 137t.2. Spatial and Temporal Distribution of Hg in Water in Wangjiagou watershed(1) Total mercury concentration in water ranged from 1.72 to 64.04 ng·L-1 with an average of 13.54±10.55 ng-L-1, while total methylmercury (TMeHg) concentrations ranged from 0 (undetected) to 4.24 ng·L-1 with the average of 0.22±0.42 ng·L-1. Particulate mercury (PHg) was the dominant THg form in these four water bodies, and particulate form also accounted for a major fraction of TMeHg in precipitation and pond, not for wells and streams in which dissolved MeHg was the main form. Water in this watershed could meet the national environmental quality standard of surface water without damage to human living in this region.(2) An obvious Hg distribution pattern in these four investigative water bodies was observed with THg concentration in the order of precipitation> pond> streams> wells and streams> pond> precipitation> wells for TMeHg. The THg temporal distribution showed that THg concentrations in cold season were higher than that in warm season, but the temporal distribution of TMeHg varied with the water types and was also impacted by rainfall. In general, due to the small area of this watershed, there was no obvious spatial distribution of Hg in four water bodies. Water environment factors, external interference and rainfall were important factors to influence the spatial and temporal distribution of mercury in Water.(3) Wet deposition was found to be a significant external source of Hg in this agricultural watershed, while pond water was the main "sink" of mercury in this watershed. However, pond water would change to be the major source for Hg and MeHg, once it was used to be agricultural irrigation. Hg and MeHg could transport with particulate and finally enter into reservoir with runoff, which was a significant pathway for Hg load in the reservoir. However, this similar pathway contributed minorly to MeHg load in the reservoir. |
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