Transformation And Release Of Mercury In Water-Level-Fluctuating Zone Of Three Gorges Reservoir Area, China

Mercury and its compounds (especially alkyl mercury) have very strong biological toxicity. And the rate of biological enrichment and amplification were higher than other heavy metals. Meanwhile, their biological half-lifes in brain organ were also much longer. The problem caused by mercury pollution has been aroused public concern. The successful holding of10th International Conference on Mercury as a Global Pollutant (ICMGP) in Halifax, Nova Scotia, Canada in July24-29,2011. And the Intergovernmental Negotiation Committee (INC) organized by UNEP Ad Hoc Open Ended Working Group on Mercury has also been held three times since2010. All of these events do suggest that the focus of international research on mercury have been rise gradually in recent years.The ecosystem of reservoir was very sensitive to mercury. It is an indisputable fact that the mercury concentration in fish will higher than other water body after the reservoirs were built. It has already become one of hot topics of mercury research. The largest reservoir in china, the famous Three Gorges Reservoir, is located at the middle reach of the Yangtze River. The water level of the Three Gorges Reservoir fluctuates from145m in summer (May-September) to175m in winter (October-April), and the water-level-fluctuation zone (WLFZ) with area of350km2were formed. On the one hand, WLFZ is an effective sink for mercury due to its integration and accumulation of mercury from both the terrestrial and aquatic environments (the surface water of adjacent river). And mercury from industrial activities and domestic consumption in the uplands may be accumulated in the WLFZ by flooding. On the other hand, the higher Hg concentration in sediment will also influence the quality of overlying water.Periodic fluctuation of reservoir water caused the sediment could face a prolonged period of dry and wet alternate. And there will be a series of physical, chemical, mechanical changes (such as pH, Eh, electrical conductivity, microorganisms, organic matter etc.) in sediment and overlying water. There is, however, something that eventually will have a much bigger impact on Release, Movement and Transformation of Mercury in Water-Level-Fluctuating Zone of Three Gorges Reservoir Area, China.Up to now, the research about mercury problem in WLFZ of Three Gorges Reservoir Area were just confined to Hg concentrations in water, the distribution of Hg in fish body and investigate the Hg concentrations in flooding soil. However, the speciation of mercury and its transformation in sediment in WLFZ, the characteristic of release from interface and its ecological effects were very insufficient. It's exactly because of this, our research were carried out based on field investigation, field sampling, and laboratory experiments. In this thesis, the author attempts to make a systematic analysis of mercury exchange fluxes on water/air, sediment/air and sediment/water interface in WLFZ. And the speciation of mercury and its transformation in sediment in WLFZ were also included.The result showed that the mercury exchange fluxes on soil (sediment)/air surface were difference with water level fluctuated in different sampling periods. The highest value of mercury exchange fluxes on soil (sediment)/air surface was28.17±36.17ng/m2h (emission, November), and the minimum value of mercury exchange fluxes on sediment/air surface was-6.80±12.35ng/m2h (deposition, January). In general, the mercury exchange fluxes on soil (sediment)/air surface in WLFZ were higher than emission from forest soil and background soil at home and abroad, and lower than emission from sediment in wetland, soil in floodplain and soil in agriculture field. There were trend of diurnal variation characteristics of mercury exchange on soil (sediment)/air surface in WLFZ in sunny day. At the daytime, a sharp increase in flux starting in the morning and peak emission at midday, then decreased gradually with sunlight decreased. At the nighttime, mercury exchange fluxes on soil (sediment)/air surface in WLFZ have been stabilized and its value remained at0ng/m h fluctuation. There were non-significant trend of diurnal variation characteristics of mercury exchange on soil (sediment)/air surface in WLFZ in cloudy day. Mercury exchange fluxes on soil (sediment)/air surface in WLFZ were dominated by emission at daytime in warm season. On the contrary, mercury exchange fluxes on soil (sediment)/air surface in WLFZ were dominated by deposition at daytime in cold season. Except November, mercury exchange fluxes on soil (sediment)/air surface in WLFZ were dominated by deposition at nighttime.Solar radiation was the most important factor which influenced the mercury exchange fluxes on soil (sediment)/air surface in WLFZ at daytime. The regression analysis reveals that mercury emission from soil (sediment)/air surface increased line (November), logarithmic (January) and power (May and July) with raised solar radiation. Temperature was also important factor which influenced the mercury exchange fluxes on soil (sediment)/air surface in WLFZ. The regression analysis reveals that mercury emission from soil (sediment)/air surface increased exponentially with raised air and soil (sediment) temperature. Hg concentration in the air was the most important factor which influenced the mercury exchange fluxes on soil (sediment)/air surface in WLFZ at nighttime. Based on the measured data, we obtain regression equations. And we calculated the mean value of mercury exchange flux from soil (sediment)/air surface was12.77ng/m2h and annual Hg emission from soil (sediment) in WLFZ to the air to be19.53kg.The mercury exchange fluxes on water/air surface were difference with water level fluctuated in different sampling periods. The highest value of mercury exchange fluxes on water/air surface was4.47±36.32ng/m2h (July, four times as great as other sampling periods), and the minimum value of mercury exchange fluxes on sediment/air surface was3.83±18.66ng/m2h (November). The biggest deposition was occurred in January. In general, the mercury exchange fluxes on water/air surface in WLFZ were similar to that of from ocean, estuary, reservoir and lake and lower than emission from large wetland systems at home and abroad. There were trend of diurnal variation characteristics of mercury exchange on water/air surface in WLFZ. At the daytime, a sharp increase in flux starting in the morning and peak emission at midday, then decreased gradually with sunlight decreased. At the nighttime, mercury exchange fluxes on water/air surface in WLFZ have been stabilized and its value remained at0ng/m2h fluctuation. Mercury exchange fluxes on water/air surface in WLFZ were dominated by emission at daytime in warm season and cold season. On the contrary, mercury exchange fluxes on water/air surface in WLFZ were dominated by deposition at daytime in cold season.Solar radiation was the most important factor which influenced the mercury exchange fluxes on water/air surface in WLFZ at daytime. The regression analysis reveals that mercury emission from water/air surface increased logarithmic (November, January and May) and power (July) with raised solar radiation. Temperature was also important factor which influenced the mercury exchange fluxes on water/air surface in WLFZ. The regression analysis reveals that mercury emission from water/air surface increased exponentially with raised air and water temperature. Hg concentration in the air was the most important factor which influenced the mercury exchange fluxes on water/air surface in WLFZ at nighttime. Based on the measured data, we obtain regression equations. And we calculated the mean value of mercury exchange flux from water/air surface was14.45ng/m2h and annual Hg emission from sediment in WLFZ to the air to be22.21kg.The concentrations of total mercury (THg) in sediment in WLFZ were higher than the background value of sediment under the rivers and lower than that of sediment in other Hg polluted area. The concentrations of methyl mercury (MMHg) in sediment of WLFZ in July and September were0.128±0.028ng/g and0.031±0.027ng/g, respectively. MeHg:THg ratio in sediment of WLFZ in July and September were0.296±0.154%and0.069±0.081%, respectively. MeHg:THg ratio were difference with different type of sediment. And the results indicate that flooding sediment was bigger than semi-flooding sediment, and the value of drying sediment is least. In general, lower methylation potential was found in sediment of WLFZ.The Hg contents in each fraction of sediments in July were different, Hg-e (46.99%) ranked first, followed by Hg-s (36.28%), Hg-o (9.57%), Hg-w (4.49%) and Hg-h (2.67%) in that order. In other words, Hg was mainly bound up with Fe and Mn oxides and amorphous organ-sulfur in July. The Hg contents in each fraction of sediments in September were also different, Hg-s (46.47%) ranked first, followed by Hg-e (34.73%), Hg-o (14.18%), Hg-h (2.40%) and Hg-w (2.21%) in that order. In other words, HgS was the main fraction of sediments in September.With the flooding sediment exposed to the air, percentages of Hg in Hg-e of sediments were decreased and that of Hg-w and Hg-h were increased. It's called"spillover effects". With the waves and slope runoff bathed the drying sediment, Hg-s was preserved and Hg-w and Hg-h were taken away by erosion, causing HgS was becoming the main fraction of sediments. The existence of "spillover effects"was proved by the results of simulation testing in lab. The results of simulation testing in lab showed that with the water content in sediment decreased, percentages of Hg in Hg-e of sediments were decreased and that of Hg-w and Hg-h were increased. The mobility and bioavailability of Hg in flooding sediments were higher than that of in drying sediment. The process of flooding sediment exposed to the air, the mobility and bioavailability of Hg will increased. Meanwhile, when drying sediments were flooded, the mobility and bioavailability of Hg will also increase.In the flooding period, MeHg:THg ratio in sediment of Treatment A (shallow sediment) and Treatment B (deepwater sediment) were0.41±0.29%%and0.74±0.52%, respectively. There existed statistically significant difference between the two methods with independent sample T test. In other words, shallow reservoir sediment has higher methylation potential than that of deepwater reservoir sediment. There existed no statistically significant difference among reactive mercury, dissolved mercury and methyl-mercury in overlying water with paired sample T test. There also existed no statistically significant difference among reactive mercury, dissolved mercury and methyl-mercury in pore water with paired sample T test. However, there existed statistically significant difference between overlying water and pore water for reactive mercury, dissolved mercury and methyl-mercury with paired sample T test.In other words, there existed gradients at the interfaces between overlying water and pore water for reactive mercury, dissolved mercury and methyl-mercury. The diffusion flux of reactive mercury for Treatment A and Treatment B were12.79±5.06ng/m2d and13.24±3.68ng/m2d in different experimental flooding duration, respectively. The diffusion flux of dissolved mercury for Treatment A and Treatment B were154.65±47.12ng/m2d and160.23±56.19ng/m2d in different experimental flooding duration, respectively. The diffusion flux of methyl-mercury for Treatment A and Treatment B were7.61±3.39ng/m2d and7.79±4.56ng/m2d in different experimental flooding duration, respectively. There existed no statistically significant difference between Treatment A and Treatment B for the diffusion flux of reactive mercury, dissolved mercury and methyl-mercury with paired sample T test.The contribution rate of reactive mercury in pore water to that of in overlying water was0.002-0.011%. The contribution rate of dissolved mercury in pore water to that of in overlying water was0.002-0.017%. And the contribution rate of methyl-mercury in pore water to that of in overlying water was0.015-0.143%.