| 其他摘要 | There are three main research topics in this thesis, namely, (1) the foundation and application of methylmercury in natural water by means of the distillation-ethylation combined with gas chromatography (GC)-cold vapor atomic fluorescence spectrometer (CVAFS); (2) a preliminary study on speciation and distribution of mercury in Wujiang River; (3) studies on reservoir effect on the mercury distribution in the Wujiang River and the mercury transformation within the reservoir—a case study from Wujiangdu Reservoir and Dongfeng Reservoir, two different aged hydroelectric reservoirs both located on the up-mid mainstream of the Wujiang River. Through the detailed studies reported in this thesis, some main conclusions were obtained as follows: 1. An analysis technique is established for determination of methylmercury in natural water based on the distillation-ethylation combined with gas chromatography (GC)-cold vapor atomic fluorescence spectrometer (CVAFS). Methylmercury in water sample reacted with sodium tetraethyl borate after distillation, forming volatile methylethyl mercury, catched and concentrated in by nitrogas gas blowing; then determined with GC-CVAFS. The recovery rate of the technique is 88.2%~108.4%, and the average relative standard deviation is 5.4%. The methylmercury detection limit is 0.009ng/L for 45mL sample determined. This method is sufficient to accurately measure methylmercury in most ambient
water, and quite simple, fast compared to other earlier methods in China. 2. The mercury concentration in the Wujiang River was much higher than those reported about other undisturbed rivers in North American and Europe. The average total mercury was 65.9 ng/L in high flow period and 16.9 ng/L in low flow period, respectively. The mercury in Wujiang River was mostly associated with suspended particles, and the particulate mercury constituted on an average of 84% of the total mercury in high flow period, of 52% of the total mercury in low flow period, respectively. The pilot study indicated that Wujiang River is contaminated in terms of mercury in a certain extent. 3. Using input—output budgets, annual net yields of total mercury in Dongfeng Reservoir was - 57785.1 g/yr, constituting 40.2% of the total inputted mercury; while in Wujiangdu Reservoir, annual net yields of total mercury was - 23273.7 g/yr, constituting 14.0% of the total inputted mercury. Both hydroelectric reservoirs were believed to be the "reservoir" of the total mercury. Contrary to the total mercury, the two hydroelectric reservoirs acted as the "source" of the
methylmercury. Annual net yields of methylmercury in Dongfeng Reservoir was +284.7g/yr, accounted for 4.4% of the total inputted methylmercury; while in Wujiangdu Reservoir, annual net yields of methylmercury was +6622.0 g/yr, accounted for 80.1% of the total inputted methylmercury. Wujiangdu Reservoir was a significant methylmercury source compared to the Dongfeng Reservoir. 4. Obviously seasonal changes of total mercury and methylmercury in water body
within the two reservoirs were obtained during the three sampling periods, with the concentrations in spring were generally higher than that of the winter and the summer. In winter, spring and summer, the averaged total mercury levels in water-column in Wujiangdu Reservoir were 9.0 ng/L, 16.0 ng/L and 7.9 ng/L, respectively; while in Dongfeng Reservoir the averaged total mercury levels were 8.2 ng/L, 12.5 ng/L and 9.4 ng/L, respectively. In view of methylmercury, the averaged concentration in Wujiangdu Reservoir was 0.74 ng/L in winter, 0.97 ng/L in spring and 0.89 ng/L in summer, respectively; and in Dongfeng Reservoir was 0.74 ng/L in winter, 1.12 ng/L in spring and 0.38 ng/L in summer, respectively. The methylmercury correlated with the total mercury in both reservoirs, but only significant at the 0.05 level (r=0.451*, n=24, in Wujiangdu Reservoir; r=0.471*, n=27, in Dongfeng Reservoir). The distribution, speciation and levels of mercury within the reservoir water body were governed by several factors, such as the external input, dilution, primary production and the operation type of the reservoir. 5. Total mercury levels in the whole sediments were 254.7±44.3ng/g in Wujiangdu Reservoir and 171.0 ± 23.0ng/g in Dongfeng Reservoir, without significant variations between different seasons or depths. The total mercury levels in both reservoirs were higher compared to data reported in other uncontaminated
reservoirs. Unlike concentrations of total mercury, sediment-phase methylmercury were not as uniform. The transformation of inorganic mercury into methylmercury is most active in the upper 5cm of the sediment profile. Methylmercury contents in the sediments during different seasons were highly
dependent on the microbial activity; thus, methylmercury levels were elevated where nutrient supplies and biological productivity are favorable. The
methylmercury levels in sediments were increased gradually from winter to spring and to summer in each reservoir. The methylmercury levels and the percent of the total mercury that occurred as methylmercury in sediments were higher in Wujiangdu Reservoir than in Dongfeng Reservoir. 6. No obviously seasonal changes of inorganic mercury were observed in pore water. On the contrary, significantly seasonal changes of methylmercury in pore water were observed. The depth profiles trend of methylmercury in pore water was similar to that of the sediment. With regards to the whole three sampling periods, the methylmercury in pore water column were evidently correlated to that of the sediment (r=0.770**, n=75, in Wujiangdu Reservoir; r=0.675**, n=75, in Dongfeng Reservoir). 7. The pore water in the sediment is believed to be the source of the inorganic mercury and methylmercury to the overlying water body. The contribution of inorganic mercury and methylmercury diffused from the pore water to the overlying water were finite according to the Wujiangdu Reservoir and the Dongfeng Reservoir, due to the deep water and the short water residence time. Still, the contribution of methylmercury was greater than inorganic mercury, and that contribution in Wujiangdu Reservoir was greater than in Dongfeng Reservoir. 8. The mercury and methylmercury concentrations in fish collected within the Wujiangdu Reservoir and the Dongfeng Reservoir were both under the national standard. The low level of mercury and methylercury of fish in the two reservoirs maybe due to the too fast growth and the too short food chain. The methylmercury levels and the percent of the total mercury that occurred as methylmercury in fish were higher in Wujiangdu Reservoir than in Dongfeng Reservoir. It can be concluded that the fish methylmercury level in reservoirs among Wujiang River basin is determined not by reservoir's age but by the methylmercury levels in the material at the base of the aquatic food chain. 9. Our studies indicated that the sedimentation of autochthonous organic materials stimulating bacterial activity leads to the methylation of initial inorganic mercury burden of the soils, for the flooded soils poorer in organic carbon. Within a given time, With the development of the reservoir, the enhanced primary production promotes an increase deposition of locally produced organic matter will significantly accelerate the transformation of inorganic mercury into methylmercury. |
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