Biogeochemical Characteristics Of Mercury In Changshouhu-a Typical Tributary Reservoir Of Three Gorges Region

Mercury and its compounds(methyl mercury) have strong nerve toxicity, lipid solubility and high biological enrichment characteristics. The pollution problem of mercury in environment has been widely concerned after Japan’s "Minamata disease" incident. Smith first discovered the methyl mercury content in new reservoir fish rise beyond the world health organization recommends health standard(≤1.0 mg kg-1, wet weight) in 1970 s. After that, a series of study found in the new reservoir environment, mercury will be absorpted through the activation, methylation, the food chain transmission. The biological enrichment coefficients of methyl mercury in fish is usually within 104 ~ 107. Even the methyl mercury concentration in water is low, high nutrition level fish may appear very high methyl mercury pollution though the amplification of aquatic food chain. As a result, the reservoir is a typical "mercury sensitive ecosystem". There are a lot of dam in our country. By 2013, the Yangtze River basin has been built for 47000 reservoirs, including 173 large reservoirs, 197.8 billion m3 of total capacity, accounting for 78% of the total capacity of the Yangtze River reservoir. Meanwhile, the three gorges reservoir has the Yangtze River canyon characteristics and it belong external regulation reservoir. The background value of mercury in three gorges is relative higher. The steep mountains, high land reclamation rate, and frequently agricultural activity(cultivated land, The cages to breed fish), makes the environmental mercury problems of reservoir cannot be ignored. Therefore, it is particularly important to strengthen the research of mercury biogeochemical system in reservoir environmental. However,the three gorges reservoir formed by a series of water areas. Taking research of mercury biogeochemical in small watershed was to better understand mercury in the aquatic environment of the basin breach.Based on this, we choose the important tributary in the three gorges reservoir reservoir-Changshouhu reservoir as the research object, though the investigation in situ, sampling and experimental analysis. The research included mercury exchange features of water, sediment, water-air interface, sediment-water interface, the biogeochemical characteristics of fish mercury enrichment, and the mercury quality balance system study in Changshouhu reservoir, so as to provide data support of mercury circulation pattern in tributaries of three gorges reservoir area, as well as provide a scientific basis for activation effect and ecological environment problems that may caused by the impoundment of the three gorges reservoir. The results showed that:1) Water concentration of total mercury and methyl mercury in Changshouhu reservoir different seasons showed obvious difference in the surface and vertical space. Total mercury average concentrations(14.77±12.24)ng·L-1, the total average of methyl mercury concentration(0.41±0.47)ng·L-1. It showed that, on the one hand, various forms of mercury distribution were affected by the intrinsic reservoir properties, such as water temperature, pH, redox environment. On the other hand, it is associated with exogenous input of human activities; Dissolved methylmercury mainly results from mercury methylation in the reservoir water; while the particles state methyl mercury mainly comes from surface runoff. The proportion of MeHg/THg in Changshouhu reservoir water is higher and the proportion of DMeHg/MeHg was higher than 50%. The results show that water environmental conditions were conducive to the mercury methylation in Changshouhu reservior. Appropriate water temperature improved microbial activity, promoting the methylation of mercury. Low temperature is advantageous to the mercury demethylation process or inhibition of mercury methylation. Dissolved methylmercury concentration under the surface of 4 ~ 8 showed peak in the vertical profiles, then its value reduced and jump again at the bottom of lake. Particle methyl mercury concentration peak appeared under the surface of 8 ~ 20 m rather than at the interface between sediment–water. It mainly related to the settlement of the methyl mercury particles in upper water. The DMeHg and DO concentration in summer, spring was significant linear correlation(r =2) Dissolved gaseous mercury in Changshouhu reservoir was affected by directly or indirectly water environment factors, and showed obvious seasonal variation. Annual range of dissolved gaseous mercury was 5 ~ 85 pg·L-1, the average concentration of 26pg·L-1. The average concentration in summe and spring were higher than the autumn and winter concentration. In most of the time all year, the dissolved gaseous mercury in Changshouhu reservoir water surface was supersaturated state. It means that eutrophication water condition was advantage Hg0 generation and release into the atmosphere. Average mercury exchange fluxes is(2.1±6.0) ng·m-2·h-13) Average total mercury of sediment in Changshouhu reservoir was(44.8±16.9)ng·g during a sampling period. Mercury exchange fluxes in summer is highest. Path analysis found that direct effect of water-gas mercury exchange fluxes as follow: mercury concentrations in water、light intensity and ultraviolet intensity; Indirect effects are mainly for the UV intensity, light intensity, wind speed. The indirectly effected in wind by UV higher than the winds itself effected to the influence of water-gas mercury exchange fluxes. Therefore, mercury exchange fluxes is restricted by two aspects: on the one hand, light intensity, UV intensity, water mercury concentration, directly or indirectly affect the water dissolved gaseous mercury generated; on the other hand, wind speed affect the diffusion of mercury concentration gradient over the atmospheric of surface water, thus affecting the mercury exchange fluxes.-1, close to the geometric average mercury in the soil, below the average mercury content in the three gorges reservoir belt soil. Methyl mercury of sediment in Changshouhu reservoir was(0.38±0.41) ng·g-1. The seasonal trends of inorganic mercury concentrations in porewater: autumn > summer > spring > winter. The solid-liquid distribution coefficient KdTHg of total mercury between sediment and porewater average was 4.8×103 L· kg-1. The total mercury solid-liquid distribution coefficient between sediment and porewater of in each season was: winter > spring > autumn > summer. The solid-liquid distribution coefficient KdTHg of total mercury between sediment and porewater average was 7.1×102 L·kg-1, The total mercury solid-liquid distribution coefficient between sediment and porewater of in each season was: winter > spring > autumn > summer. Average DMeHg/DHg(%) of porewater was 23.1%±7.5%, and Changshouhu reservoir belongs to medium productivity of methyl mercury lake. Methyl mercury peak appeared under the surface of 16 cm and 28 cm in vertical water profile. It could be sulfate-reducing bacteria activity expanded to more areas, which leads to an increase of the deep sediment rate of methylation. Diffusion flux of dissolved methylmercury in porewater of the autumn and summer were 28.2 and 30.0 ng·(m2·d)-1, more higher than the value 3.8 ng·(m2·d)-1in winter.4) There a large difference among the total mercury and methyl mercury in different nutrition levels of fish and difference organs in fish body in Changshouhu reservoir. Average tota mercury of fish was(55.9±29.6)ng·g-1, average of methyl mercury was(26.3±19.6)ng·g-15) According to reservoir mercury mass balance model, we estimated the Changshouhu reservoir is total mercury “reservoir”, annual net yields of total mercury in Changshouhu reservoir was 4103g; Changshouhu reservoir was the "source" of methyl mercury, annual net yields of methyl mercury in Changshouhu reservoir was 331g; The annual total mercury input flux of Changshouhu reservoir were: the river input > porewater diffusion >wet deposition in atmospheric > sediment resuspension. Rivers was important total mercury input source in the reservoir. The main transfer process for the input total mercury: the water phase transfer to environmental system, diffusion of porewater transfer to the water phase. The annual total mercury output flux of Changshouhu reservoir was: rivers output > suspended particle settling > water-air exchange > sediments buried > fish absorb. The main transfer process for the output total mercury: water phase transfer to the next, far below the aquatic consumption products standard and food limited standards, and found no excessive mercury of fish in Changshouhu reservoir which discovered in northern Europe, the United States. Total mercury in different nutrition level fish as follows: Siniperca chuatsi> Silurus asotus> Pelteobagrus fulvidraco> Erythroculter ilishaeformis> Myxocyprinus asiaticus> Carassius auratus> Parabramispek inensis> Cyprinus carPio> Grass carp. Total mercury content in different fish body organs as follows: fish muscle > heart > liver > gill > fat fish >brain. Total methyl mercury in different nutrition level fish as follows: carnivorous fish > omnivorous fish >vegetarian fish. Total methyl mercury content in different fish body organs as follows: heart > fat > > liver >muscle >brain > gill > eggs. The ratio of methyl mercury to total mercury in fish was(47.3±17.8) %, the proportion in predatory was(49±8) %, the proportion in omnivorous fish was(47±5)%, the proportion in vegetarian fish was 38%. The enrichment coefficient of total mercury in fish muscle was the highest, followed by the heart. The enrichment coefficient of methyl mercury in fish heart was the highest, followed by the liver. According to the health risk assessment results show that HQ(RfD230) of all ages to eat Changshouhu fish are less than 1, show that there was no exposure risk of methylmercury. It is safe to eat Changshouhu reservoir fish. environment system, water phase transfer to solid phase, water phase transfer to gas phase. The annual methyl mercury input flux of Changshouhu reservoir was: porewater diffusion>river input> wet deposition > sediment resuspension. The diffusion of theporewater is primary source of methyl mercury in reservoir. Second, the river input was the second largest source of methyl mercury in Changshouhu reservoir. The main transfer process of input methyl mercury was: last environment system transfer to the water phase and sediment phase transfer to water phase. The annual methyl mercury output flux of Changshouhu reservoir was: river output > fish absorb> suspended particle settling > sediment buried. Methylmercury output main transfer process: water phase transfer to the next water environmental, water phase transfer to biological phase.