Distibution And Seasonal Variability Of Mercury In Environment-medium And Phragmites Australis In Hengshuihu Wetland

In China,the mercury study area is mostly concentrated in mercury mine area in Guizhou,but few in non-mercury mine area.Mercury can be transported long distances in the atmosphere and then cause lake lake pollution by atmospheric deposition.Therefore the research in freshwater lake in non-mercury mine area is quite necessary.Wetland is in the land/inland water ecotone,with its particularity redox environment,so strongly impacts the mercury transformation processe.Wetland plant is one part of wetland biogeochemical cycle of mercury.Rightnow the study field of wetland mercury is concentrated in non-biological medium like soil,water and atomasphere,but few study in the relationship of mercury with wetland plant.Hengshuihu wetland is one important wetland in China,and Phragmites australis is the dominant specie.According to previous studies of the Hengshuihu wetland,we have found that mercury was the main heavy metal element in sediment and Phragmites australis of Hengshuihu wetland.At the same time,according to Hengshuihu water quality survey,there was excessive mercury content in winter in Hengshuihu water surface,and this meaned seasonal variation of mercury pollution of Hengshuihu wetland.Therefore,this study chosed Hengshuihu wetland in Hebei Province as study area,and chosed Phragmites australis as study object,by using field experiment combining with controlled experiment to analyze mercury distribution and seasonal variation in environment-medium and Phragmites australis in Hengshuihu wetland.Main results obtained as follow:(1)By a field investigation on mercury pollution in Hengshuihu wetland,we found that,the total mercury concentration in atmosphere in the region was(2.9±0.85)ng·m-3,and the total mercury concentration in surface water was(0.081±0.053)μg·L-1,and the total mercury concentration in soil/sediment was(0.027±0.013)mg·kg-1.There were also some features about spatial distribution: the total mercury concentration in surface water of the shore was significantly higher than the center,but the total mercury concentration in soil of the shore was significantly lower than the center.The total mercury concentration in tmospheric and in soil of the shore were positively correlated and then relatively enriched in the place of strenuous human activities.Compared with 10 years ago,mercury pollution in Hengshuihu wetland was reduced from a high ecological risk to a low ecological risk.This meaned mercury would not cause risk of ecological risk in Hengshuihu wetland.(2)Through the continuous monitoring of available mercury pollution in different environmental medium of Hengshuihu wetland,we could find out that,annual average of total mercury concentration in the atmospheric was 3.87 ng·m-3,slightly higher than the background concentration of total mercury in the northern hemisphere.Total mercury concentration in the atmospheric in winter was litter higher than other seasons.Total mercury concentrations in atmosphere changed by season as follow: winter> autumn> summer> spring.Annual average of total mercury concentrations in water surface was 0.075μg·L-1.Total mercury concentrations in water surface of Hengshuihu wetland changed small within the region.In the spring and summer season there was a large variation of different sampling areas,but in the autumn and winter was stabilized.The annual average of total mercury concentration in soil was 0.027 mg·kg-1 and in summer it was slightly higher than background level.By useing accumulation index and potential ecological risk index to evaluate Hengshuihu wetland ecological risk of mercury contamination for all seasons,results showed that in several sampling points in summer mild mercury contamination ecological risk occured.The human activities were the main reason of mercury ecological risk,so should pay attention to the subsequent protection of the environment and avoid severe mercury contamination.(3)Wetland plant is one important part of wetland mercury biogeochemical cycle,so we added the field experiment of typical wetland plant Phragmites australis in the continuous field experiment for mercury contamination.In Hengshuihu wetland,the annual average value of mercury concentration in the leaf of Phragmites australis was 0.030 mg·kg-1,and changed with the season as follow: winter> autumn> summer> spring.Total mercury concentration in winter was significantly higher than other seasons,with excellent positive correlation with the total mercury concentration in atmosphere and the same performance by season.Total mercury concentration in the stem of Phragmites australis could be measured only in autumn and winter,and the measured values in spring and summer were below the detection limit,and the correlations with the environment-medium were poor.The annual average value of mercury concentration in the root of Phragmites australis was 0.0068 mg·kg-1,consistent with the trend of seasonal variation of total mercury concentration in soil.It also had good positive correlation with concentrations of total mercury in soil and atmosphere.The annual average concentration of total mercury in different parts of Phragmites australis presented as follow: leaf> root> stem,and the accumulation of mercury in leaf was much larger than other parts.(4)For further study of the source andthe cumulative effect of mercury in different parts of Phragmites australi,we have set the control experiment.In control experiment,the water source of mercury in the very beginning with the concentration gradient set different appeared visible gradients,but different concentration sets finally tended to a stable value.Total mercury concentration in soil changed a lesser extent over time.The initial fixed exogenous source of mercury in water showed a slight increase,and the latter was essentially flat.The total mercury concentration in Phragmites australis leaf and stem gradually increased with time,and total mercury concentrations in root had less relationship with time,with no obvious trend.Accumulation of mercury in different parts of the Phragmites australis by the time in the control experiment was similar with under natural conditions,but the concentration gradient set of the control experimental in different parts of Phragmites australis showed no significant cumulative effect.Through the correlation analysis of total mercury concentration in different parts of the Phragmites australis and environment multi-medium of field experiments combined with controlled experiments,we could basically draw the conclusion that: the mercury in Phragmites australis leaf and stem mostly come from atmosphere,and mercury in Phragmites australis root mainly come from soil,and waters has no obvious contribution on the accumulation of mercury in different parts of the Phragmites australis.