Input And Output Balance Of Mercury In Different Evolution Reservoirs In Wujiang River Watershed

In order to know the transfer, transformation, biogeochemistry cyclic processes and methylation of mercury in reservoir ecosystem, it is very important to study the mass balance of mercury. In the last decade, the study on mercury circulation in natural water bodies has been rapidly developed in foreign countries, and gradually payed more attention to methylation and biogeochemical circulation of mercury in reservoirs. China has many reservoirs, 48 thousands reservoirs including 137 large reservoirs have been constructed in Changjiang River watershed. With the promotion of hydroelectric construction in West China, many reservoirs are being and will be constructed. However, the study on the mass balance and methylation of mercury in reservoir ecosystem is just begun in China, and less report and basic data can be found. Therefore, it is of great significance for evaluating and preventing mercury pollution in reservoir to strengthen the study on biogeochemical cycling of mercury in China.In this thesis, six reservoirs at different evolutive periods in Wujiang River watershed, i.e. Hongjiadu Reservoir, Yinzidu Reservoir and Suofengying Reservoir belong to primary stage of evolution, Puding Reservoir and Dongfeng Reservoir belong to intermediate stage of evolution, and Wujiangdu Reservoir belong to senior stage of evolution among them, were selected as the research objects, input and output fluxes of total mercury (THg) and methylmercury (TMHg) in these reservoirs were estimated in order to identify the action of "sink" or "source" and study the mass balance. Some main achievements were sumarrized as follows:1. The rivers were the primary source of THg, MHg and total suspended substance (TSS), while the output fluxes of THg, MHg and TSS in reservoirs were mainly outflows. The evidently positive correlationship were obtained between the fluxes of THg in rivers and outflows and the concentration and flow of THg, the flux and concentration of TSS. The fluxes of MeHg and TSS in rivers and outflows were showed evidently postive correlationship between their concentrations and flows, and were controlled by the flows of rivers and their concentrations. The input and output fluxes of THg, MHg and TSS were evidently influenced by their flows.2. From the estimation of input-output fluxes, all of six reservoirs were showed as the "sink" of suspended particular matter and THg. As to methyl mercury, Yinzidu reservoir, Hongjiadu reservoir and Suofengying reservoir behaved as a "sink", while others as a net "source".3. The stocking rates of THg in Puding reservoir, Yinzidu reservoir, Hongjiadu reservoir, Dongfeng reservoir, Suofengying reservoir and Wujiangdu reservoir were 76.9%, 50.6%, 93.6%, 34.6%, 23.4% and -2.3%, respectively; while the stocking rates of particular matter were 86%, 63%, 98%, 73%, 54% and 56%, respectively. The six reservoir showed the "cleaning" action to THg and and particular matter, and Puding reservoir and Hongjiadu reservoir were showed Visibly higher than that in other reservoirs. The stocking rates of THg in downstream reservoirs decreaed gradually and showed weak "cleaning" action if other reservoirs existed upstream.4. In Yinzidu reservoir, Hongjiadu reservoir and Suofengying reservoir belonged to primary stage of evolution, net MeHg fluxes were -37.7 g/yr, -286.3 g/yr and -813.5 g/yr, and the stocking rates of MeHg were 9.7%, 43.7% and 51.3%, respectively, showed as the "sink" of MeHg. But for Puding reservoir, Dongfeng reservoir and Wujiangdu reservoir belonged to the intermediate and senior stages of evolution, net MeHg fluxes were 20.5 g/yr, 433.3 g/yr and 786.2 g/yr, and the generation rates were 6.6%, 43.9% and 58.0%, respectively. It suggested that the methyaltion of mercury would be increased and MeHg would be converted from a "sink" to "source" in the reservoirs in Wujiang River watershed.5. Compared with the concentrations of mercury speciations, the ratios to THg in influent and outflow rivers and the changes of MeHg along water flow, little influence of the reservoirs on the concentrations of dissolved mercury (DHg) and reactive mercury (RHg) was found. However, the ratios of DHg, RHg, and MeHg to THg in outflow rivers increased, which made the increase of MeHg concentration in cross sections in Wujiang River. The transported mecury in the rivers showed the variations of speciation and distribution, which changed the original distribution of mercury and transportation process in the rivers. Moreover, the reservoirs at different evolutive periods showed different effects on the rivers, and the concentration of output MeHg increased with increasing the age of the reservoir.