| 其他摘要 | Some studies have revealed that reservoir is one of the typical Hg-sensitive ecosystems in northern Europe and North America, and special attention has been paid to methylmercury pollution in fishes in newly constructed reservoirs. However, to date, little is known about this kind of MeHg pollution in China. Considering it, this thesis takes six reservoirs in Wujiang River as the object to research, and these reservoirs were divided into three different evolutive periods from different ages. Hongjiadu Reservoir, Yinzidu Reservoir, Suofengying Reservoir belong to primary stage of evolution, and Puding Reservoir, Dongfeng Reservoir belong to intermediate stage of evolution, Wujiangdu Reservoir belongs to senior stage of evolution. Here, we present data from six different reservoirs in Wujiang River Basin, input and output fluxes of total and methyl mercury were calculated in order to identify the action of “sink” or “source” of reservoir to THg and MeHg during different evolutive period. There are three main research topics in this thesis, (1) Characteristics of mercury speciation and distribution in inflows and outflows of six reservoirs; (2) Studies on mercury distribution and speciation in precipitation and wet deposition fluxes of THg and MeHg; (3) Input and output fluxes of THg and MeHg in six different reservoirs. Based on the detailed researches, the main conclusions we have obtained are as follows:
1. The mean annual concentration of total, particulate, dissolved, reactive, total methyl and dissolved methyl mercury in Wujiang River was 3.41±1.98, 2.05±1.73, 1.36±0.44, 0.24±0.11, 0.15±0.06, 0.08±0.03 ng•L-1, respectively. Compared to published data from foreign countries, concentrations of THg were significantly lower than values in polluted rivers, and slightly higher than values in unpolluted rivers. Concentrations of DHg, RHg, TMeHg were slightly lower than those in Hg-impacted rivers, and similar to levels in uncontaminated rivers. Compared to Aha Reservoir, Hongfeng Reservoir and Baihua Reservoir which are also located in Karst region in Guizhou Province, mercury levels were significantly lower.
2. Due to the construction of reservoir, concentrations of THg and PHg in outflow rivers decreased, while concentrations of TMeHg and DMeHg increased, and elevated ratio of DHg, RHg, TMeHg to THg also appeared in outflow rivers. Distribution of Hg along the Wujiang River showed that construction of cascade reservoirs influenced the initial biogeochemical cycling of Hg, and concentration of methyl mercury increased at several sites below reservoir. Therefore, it is possible that MeHg would increase with the evolution of reservoir.
3. The concentration of THg, DHg, PHg, RHg, TMeHg in precipitation ranged from 7.49 to 149, 1.23 to 10.0, 5.76 to 142, 0.56 to 2.94, 0.08 to 0.82 ng•L-1, and PHg was the main form, which accounted for 87% of THg. The seasonal variations of THg, DHg, PHg, TMeHg in precipitation samples were obvious, and their concentrations in spring and winter samples were significantly higher than those in summer and fall. For the spatial distribution, there did not appear to be a geographic trend. During 2006, the mean annual wet deposition fluxes were 34.7±5.80 µg•m-2•yr-1 for THg and 0.18±0.03 µg•m-2•yr-1 for MeHg, which were mainly influenced by the rainfall amount. THg concentrations and wet deposition fluxes in precipitation of Wujiang River were much higher than those data reported in North America and Japan, lower than some urban regions in China (such as Changchun and Beijing), but the concentration and wet deposition flux of MeHg were equivalente to most literature data.
4. In each reservoir, the input fluxes of THg and MeHg from precipitation were highly dependent on rain amount and reservoir area, and concentrations of Hg appeared to have little influence. Mass of THg from riverine inputs was regulated by river flow, while mass of MeHg and SPM were regulated by river flow as well as concentrations. Mass of THg, MeHg, SPM from output of discharged flow were controlled by river flow as well as their concentrations. Due to the basin`s relatively big watershed area relative to surface water area, riverine transport were believed to be the major pathways of water volume and mass of THg, MeHg and SPM to reservoirs, which accounted for around 87%, 80%, 85%, 86% of total inputs. The outflow through the reservoir dam represented 80%, 77%, 86%, 79% of the total outputs in terms of water volume and mass of THg, MeHg and SPM.
5. From the results of input-output fluxes budgets, we found that six hydroelectric reservoirs were a “sink” of suspended particular matter and five hydroelectric reservoirs were a terminal “sink” of THg except Wujiangdu Reservoir. As to methyl mercury, some reservoirs, including Yinzidu Reservoir, Hongjiadu Reservoir, Suofengying Reservoir, behaved as a terminal sink while others acted as a net “source”.
6. The stocking rate of THg in Puding Reservoir and Hongjiadu Reservoir was 56% and 57%, respectively, which was visibly higher than that in other reservoirs, indicating that there was a relatively small “sink” if other reservoirs existed upstream. In Puding Reservoir, Dongfeng Reservoir and Wujiangdu Reservoir, net MeHg yields were +69.4 g•yr-1, +368 g•yr-1 and +857 g•yr-1, and the conversion rate were 13%, 73% and 84%, respectively. It is suggested that net MeHg yields and conversion rate would increase and reservoirs were tend to be a net “source” converted from a “sink” with the development of reservoirs. |
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