Pollution Characteristic Of Heavy Metal In Sediments In The Three Gorges Reservoir Based On Geochemical Baseline

The impact of large-scale water conservancy projects on the existence of the ecological environment has attracted worldwide attention.Effectively evaluating the relationship between heavy metal pollution in sediments and the construction and operation of water conservancy projects is an urgent problem to be solved.The Three Gorges Reservoir(TGR)has achieved the target of water storage to 175 m for eight consecutive years until October 2017.In order to study the temporal and spatial variation of heavy metal content at different water periods under 175 m operating condition in the TGR,probe the impact of large-scale water conservancy project on the Heavy metal Content enrichment and Establishment of heavy metal pollution evaluation system in water environment of reservoir based on geochemical baselinein the TGR,we collected 226 surface sediment samples from the TGR during four consecutive water periods from June 2015 to June 2017,measured Cr,Ni,Cu,Zn,Cd and Pb concentration by using inductively coupled plasma mass spectrometry(ICP-MS).The conclusions is as follows:The content of heavy metals in dry and tributary of Three Gorges Reservoir is as follows: Cr content is between 52.84~171.12 mg/kg,the average content is 98.46 mg/kg;Ni content is between 25.49 ~ 67.39 mg/kg,the average content is 45.68 mg/kg;Cu content is between 22.26 ~114.47 mg/kg,the average content is 59.28 mg/kg;Zn content is between 62.23 ~ 251.61 mg/kg,and the average content was 161.04 mg/kg;Cd content is between 0.17 ~ 2.13 mg/kg,and the average content is 1.05 mg/kg;Pb content is between 20.19 ~ 108.56 mg/kg,and the average content is 56.68 mg/kg.The average content of each element is higher than that of changjiang sediments.Compares with the Three Gorges Reservoir,the contents of heavy metals are increased and the contents of Cr,Cu,Zn,Cd and Pb are increased in comparison with the results of the low water level operation period after the completion of impoundment.According to the time change,there are 3 kinds of heavy metal content distribution in the sediments of the reservoir: Cr increases with time,and then decreases with the change of Ni and Pb,and the trend of Cu,Zn and Cd increases with time and decreases again.The average content of heavy metals in the sediments of the reservoir is not significantly affected by different water periods,in which the average content of Cr and Cu elements is slightly higher than that in dry season,and the average content of Ni,Zn,Cd and Pb is slightly smaller than that in dry season.From the view of space change,Cr and Ni content increase along the main stream,and Cu,Zn,Cd and Pb increase slightly after the first,then decrease the trend.Used Li as reference element,a baseline model of Cr,Ni,Cu,Zn,Cd and Pb is established by a standardized method,and the baseline values of Cr,Ni,Cu,Zn,Cd and Pb are 99.43 mg/kg,44.89 mg/kg,58.22mg/kg,166.32 mg /kg,1.03 mg/kg and 57.05 mg/kg,the baseline values of each element are higher than those of the changjiang sediments,which conformed to the significance of geochemical baselines.The human contribution rates of Cr,Ni,Cu,Zn,Cd and Pb are 4.49%,9.23%,10.69%,3.85%,9.57% and 9.42%,respectively.The heavy metal content in the sediments of middle,downstream and some tributaries of the main stream is influenced by human activities.Based on the geochemical baseline,the cumulative index method and the potential ecological risk assessment method are used to find that the ecological harm degree of dry and tributary sediments in the Three Gorges Reservoir is slight,and there is no pollution of Cr,Ni,Cu,Zn,Cd and Pb.Based on the geochemical baseline as the reference value is more scientific than that of the Changjiang sediments,which embodies the geochemical migration characteristics and geological development process,and can reflect and distinguish the influence of natural and human activities on the content of chemical elements better.Therefore,in the evaluation of heavy metal pollution in sediments,we should combine the geochemical baseline to get the result of the assessment of the pollution of heavy metals in the research scientifically.