Pollution Characteristic And Potential Ecological Risk Assessment Of Heavy Metal In Surface Water And Surface Sediments Of East Dongting Lake

Located in the north of Hunan province,by Yueyang city,East Dongting Lake is one of important ecological wetland nature reserve in China.However,because the urbanization process is accelerated around the lake,the population is increasing and the influence of human activities on the lake environment is more and more serious.East Dongting Lake is facing three major environment problems:the lake area declining,water quality deteriorating and ecological system function degradation Due to the rise of mining and chemical industry around the lake,the number of the pollutants are increasing which are discharged into the lake.Then,most heavy metal elements are harmful to human body.If they entered the ecological environment,it was difficult to be biodegraded but easier to be enriched in the environment.Finally,they entered the food chain and endangered human health.Therefore,it is of great significance to comprehensively analyze the pollution status of heavy metals in East Dongting Lake.In this paper,on the basis of previous studies,the ten sampling points were installed in East Dongting Lake,collecting the surface water and surface sediment samples,and using the inductively coupled plasma source mass spectrometer（ICP-MS）for the element content test of Cu,Cd,Pb,Zn,Cr and As.In turn,the spatial distribution,the sources of heavy metals of surface water and the surface sediment and the combined forms and distribution in surface sediment were analysed.The single factor index method,Nemerow comprehensive pollution index method,the geoaccumulation index method,potential ecological risk index method,the ratio of secondary comparison and primary phase and value method and the risk assessment coding method were used to evaluate heavy metal pollution.The main conclusions are shown as follows:（1）In the surface water of East Dongting Lake,the average contents of Cu,Cd,Pb,Zn,Cr and As were 1.85,0.07,4.97,15.73,1.27and 0.65μg/L,respectively.From the concentration of the heavy metals,the water quality achievedⅠclass.From the spatial distribution,the variation level of As was the highest,followed by Pb,Cu,Cd and Cr,and Zn was relatively low.The results of single factor index method showed that Cu,Cd,Zn,Cr and As have no pollution,and Pb was slightly polluted.The results of Nemerow comprehensive pollution index shows that the range of the comprehensive pollution index is 0.20⁰.83 in the research area.The comprehensive pollution index of the sampling points was 4#>2#>5#>10#>1#>6#>9#>3#>7#>8#.（2）In the surface sediments of East Dongting Lake,the average content of Cu,Cd,Pb,Zn,Cr and As was 44.68,5.5,58.51,88.09,139.70and 39.38 mg/kg,respectively.The content mean of Cu,Pb,Zn and Cr were under the screening value of alkaline soil environmental risk,and Cd was 6.8 times of the value,and As was 2 times of the value.The average content of the six heavy metals were all higher than the background value of the sediment in the national river system.Cu,Cd,Pb,Cr and As were 1.3,30.56,2.5,2 and 3.58 times of the background values of the tidal soil in Hunan,respectively.And the average content of Zn were under the background values.From the spatial distribution views,the variation level of Cr was high,and Pb belonged to medium variation level.The distribution differences of Cu,Cd,Zn and As were rarely,and they were mainly from natural action.（3）The heavy metal of Cu,Cd,Pb,Zn,Cr and As in the surface sediments of East Dongting Lake were mainly occured in the residual fraction,but the others were different.The residue state of Cr and As was highly.And Cr had only the residual fraction and the oxidizable fraction,with the proportion of the oxidizable fraction lowing.The content of the three secondary phases of As is relatively low and its toxicity is strongly for human beings.The proportion of residual state of Cu,Pb and Zn is similar,ranging from 65%to 70%.In the secondary phase,the oxidizable state of Pb was highly than others,which leaded to the possibility of the transformation to others,especially aid-soluble fraction.The residue state of Cd was the smallest and the content of aid-soluble state was more higher.According to the speciation proportions of heavy metals in the sediments,the bioavailability was descending order of Cd>Pb>Cu>Zn>As>Cr.（4）The results of the geoaccumulation index method showed that the pollution degree of the heavy metals was Cd>Cr>Zn>Pb>As>Cu.Besides,the comprehensive geoaccumulation index(I_tot)showed that the East Dongting Lake was extremely heavily polluted.And the no.1and no.8 samples were the most polluted in the study area.The results of potential ecological risk index method showed that the potential ecological risk level order of the heavy metals was Cd>Cr>As>Cu>Pb>Zn.The mean RI value in the study area was 500,indicating a medium ecological risk,with Cd as the main contributor.The evaluation results of secondary phase and primary value method showed that the order of ecological risk among the six heavy metals was:Cd>Cu>Pb>Cr>Zn>As.The results of the risk assessment coding method showed that Pb and As showed low risk,Cu and Zn presented medium risk level,and Cd presented high risk.The risk of heavy metals was following as:Cd>Zn>Cu>As>Pb.Cr only existed in residue state and reducible state,further more,there is hardly ecological risk.