| 其他摘要 | The mercury distribution and speciation in Puding and Dongfeng reservoirs, Wujiang Watershed, China were investigated to understand environmental geochemical cycles of mercury in aquatic system. There are several main research topics in this thesis, namely, (1) Revealing the mercury speciation, seasonal and spatial distribution in water. (2) Revealing how water physicochemical compositions (depends on the watershed characteristics) influence the distribution of mercury species. (3) Studies on mercury speciation and distribution in sediments and pore water. (4) Studies on mercury fluxes, transformations and mass balance in Puding and Dongfeng reservoirs. (5) Evaluation on the methylmercury source behaviors of the reservoirs regarding the dam effect on the river system, as to understand the environmental geochemical cycles in reservoir-river system. Through the detailed studies reported in this thesis, some main conclusions were obtained as follows:
1. The concentrations of all mercury species in the summer (rainy season) campaign for the Puding Reservoir and Dongfeng Reservoir are generally significantly higher than those in winter and spring (dry season) campaigns. In Puding Reservoir, the average concentrations of THg and TMHg in the summer campaign were 4.48 and 0.32ng/L, 2.53 and 3.05 times higher than in the other two campaigns, respectively. In Dongfeng Reservoir, the average concentrations of THg and TMHg in summer campaign were 2.94 and 0.33ng/L, 2.53 and 2.64 times higher than in the other two campaigns. Mercury input from the runoff during the rainy season is the vital cause for the seasonal variation.
2. The THg spatial distribution showed that THg concentrations in the inflows via mercury transportation and transformation can become lower in some extent in the downstream water body of Puding and Dongfeng reservoirs, demonstrating reservoirs had potency to reduce THg in the water of the reservoir-river system. Contrarily, the TMHg concentrations become increased after rivers being dammed. Interestingly, in our typical gorge-shaped reservoirs we find elevated water inflow in rainy season may push methyl mercury to “accumulate” in the downstream part of reservoirs.
3. The Pearson’s correlation matrix showed SPM, water temperature and nitrate were found to play an important role in controlling the distribution of mercury species in the reservoir since statistically significant correlations were commonly found between these parameters and mercury species. In the summer season, due to the high volume of precipitation, higher runoff carrying nitrate (from fertilizer) together with particulate matter contributed to become an important mercury input of the reservoirs. Meanwhile, the relatively active agricultural activities in the summer season tend to increase the mercury levels in these two reservoirs.
4. THg levels in the whole sediments were averagely 0.198 mg/kg in Puding Reservoir, 0.204 mg/kg in Dongfeng Reservoir, without significant difference, but significantly higher compared to data reported in other uncontaminated water sediments, while lower than Hongfeng Reservoir within in the same watershed. The peak values of methyl mercury in the sediment profiles typically appeared in the zones of sulfate-reducing bacteria activities. It is also found organic matter in sediment may influence the concentrations and distributions of methyl mercury.
5. Both reservoirs showed DHg and DMHg concentrations in the sediment pore water significantly higher than in the upper water. The contribution of DHg diffusion from pore water to upper water in four sediment profiles of Puding Reservoir showed seasonal variation (summer > winter), while the DMHg contribution is higher in winter than in summer. No obvious seasonal changes of the diffusions were found in Dongfeng Reservoir.
6. The result from mass balance study of THg indicated total input and total output of mercury in Puding Reservoir were 19088.2g.a-1 and 19087.4g.a-1 respectively, 17116.5g.a-1 and 22562.9g.a-1 in Dongfeng Reservoir correspondingly. River water input is the dominating THg source for both reservoirs, while water outflows is the dominating THg sink for Dongfeng Reservoir and paticulates sedimentation is the dominating THg sink for Puding Reservoir.
7. Both reservoirs had potency to reduce THg in the water of the reservoir-river system and could increase TMHg level after rivers being dammed. Meanwhile, we also found the cause of TMHg increase is from the pore water of the sediment. |
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