Mercury Distribution Characteristics And Its Mass Balance In A Multifunctional Urban Wetland

With the development of urbanization in China,the number of urban constructed wetlands has also increased significantly.Compared with natural wetlands,urban wetlands have many irreplaceable ecological functions,such as regulating urban climate,providing urban landscape entertainment,protecting urban biodiversity,urban flood control and flood discharge,pollutant purification and degradation,etc.They have ecological and social values,and are the ecological systems on which urban sustainable development depends.However,various pollutants in constructed wetlands,including nitrogen and phosphorus pollutants,organic pollutants,heavy metal pollutants and so on,have certain potential hazards to human life.As one of the stable,highly toxic and bioaccumulative heavy metal pollutants,mercury（Hg）can produce more toxic methylmercury（MeHg）by reacting with abundant humic acid and soluble carbon in water under redox conditions,making wetlands mercury-sensitive ecosystems.Especially in recent years,China has attached great importance to the construction of urban ecological civilization,and urban wetland has become an important part of the urban environment.However,urban wetland research lags behind urban development,affecting urban ecological security and social and economic health.As the ecosystem service function of urban wetland is obviously different from that of rural wetland,there are some problems such as strong disturbance by human activities,smaller area,weakened function and aggravated pollution.These characteristics make the migration and transformation of mercury in urban wetland waters different from other ecosystems.It is not clear what impact the construction and maintenance of urban wetlands have on the distribution and transformation of environmental mercury.Especially,sewage reception is an important ecological function of urban wetlands.Because of the complexity of urban sewage quality and sewage treatment process,the impact of urban sewage discharged into urban wetlands by sewage treatment plants on mercury content in accepted water needs further study.Therefore,it is necessary to study the distribution,migration and transformation of mercury in urban wetlands.In view of this,this paper takes an urban wetland park in Chongqing as an example to study the content of total mercury and methyl mercury in water,sediment and plants,analyze the distribution characteristics of mercury in urban wetlands and its source-sink relationship,and provide scientific support for ensuring the ecological and environmental safety of wetland parks.The main results are as follows:（1）The concentration of THg in effluent from sewage treatment area ranged from 1.98 to 38.03ng·L^-1(average 9.10±5.84 ng·L^-1),the concentration of MeHg in effluent ranged from 0.09 to 0.84ng·L^-1(average 0.34±0.08 ng·L^-1),the concentration of THg in deep lake purification area ranged from 0.37 to 85.69 ng·L^-1(average concentration was 6.76±2.29 ng·L^-1),and the concentration of MeHg ranged from 0.04 to 1.47 ng·L^-1(average content was 0.35±0.17 ng·L^-1).From the seasonal distribution,THg concentration was higher in summer than in other seasons,and peaks appeared in August at all sampling points.From the vertical distribution,the vertical distribution of total mercury and methyl mercury in water body is consistent,showing that the surface layer is lower than the deep layer.（2）The source-sink behavior of THg and MeHg in different systems is different.The average removal rates of THg and MEHg were 69.2%and 58.8%respectively.For the whole urban wetland park in this study,total mercury can be reduced by 155.50 g per year,methyl mercury can be reduced by 1.65 g per year,which is the sink of total mercury and methyl mercury in water.（3）The concentration range of THg in the sediments of the urban wetland is 13.792⁶21.624μg·kg^-1,with an average of 160.201±109.758μg·kg^-1,and the concentration range of MeHg is0.021¹5.553μg·kg^-1,with an average of 1.561±1.836μg·kg^-1.According to the flow direction the content of THg along the flow direction increased slightly except in autumn,while the content of total mercury decreased gradually in spring,summer and winter,and the content of MeHg increased in the other three seasons（spring,summer and winter）,except in autumn.From the vertical distribution of the profile,THg and MeHg contents tend to enrich in the shallow layer and decrease with the increase of depth.Especially,in the 12¹6 cm segment,the peak value appeared at 14 cm.（4）Sediments from the urban wetland are an important source of mercury in the water.The range of THg diffusion flux at sediment-water interface is 0.510¹49.037 ng·m^-2·d^-1,and the contribution rate ranges from 0.45%to 97.7%.The range of MeHg diffusion flux is 0.0389¹0.994ng·m^-2·d^-1,and the contribution rate ranges from 0.18%to 12.8%.（5）The average THg and MeHg content of Canna glauca was 31.058±27.306μg·kg^-1 and 0.103±0.059μg·kg^-1 respectively.The average THg and MeHg content of Iris pseudacorus L was 31.942±17.750μg·kg^-11 and 0.097±0.069μg·kg^-11 respectively.The average THg and MeHg content of Cyperus alternifolius L was 44.046±35.772μg·kg^-11 and 0.256±0.487μg·kg^-11 respectively.The average THg and MeHg content of Thalia dealbata Fraser was 92.571±154.061μg·kg^-11 and0.126±0.097μg·kg^-11 respectively.This indicates that different kinds of plants have different adsorption capacity for total mercury and methyl mercury.The content of THg in these four plants was as follows:Thalia dealbata Fraser>Cyperus alternifolius L>Iris pseudacorus L>Canna glauca L.The content of MeHg in these four plants was as follows:Cyperus alternifolius L>Thalia dealbata Fraser>Canna glauca L>Iris pseudacorus L.The average THg concentration of roots was78.519±56.579μg·kg^-1,the average MeHg concentration was 0.197±0.165μg·kg^-1.The average THg concentration of stems was 17.730±9.066μg·kg^-1,the average MeHg concentration was0.073±0.056μg·kg^-1 and the average THg and MeHg concentration of leaves was 35.328±5.374μg·kg^-11 and 0.126±0.031μg·kg^-11 respectively.This indicated that the mercury enrichment ability of different parts of the same plant was also different.The order of THg content in different parts was the same as that of MeHg content.The order of THg content in different parts was root>leaf>stem.（6）According to the BCF,the enrichment ability of the four plants to total mercury is shown as follows:Cyperus alternifolius L>Thalia dealbata Fraser>Iris pseudacorus L>Canna glauca L.According to the TF,the translocation ability of total mercury in these four plants is as follows:Canna glauca L>Iris pseudacorus L>Cyperus alternifolius L>Thalia dealbata Fraser.According to the difference of mercury enrichment characteristics of plants,Canna glauca L belongs to the enrichment type.Others belong to the root hoarding type.To sum up,the four plants selected in the urban wetland park,namely,banana,calamus,windmill and relay flower,can control mercury pollution,beautify the landscape and increase ecosystem diversity to a certain extent.