Distribution and ecological risk of heavy metals in sediment across the Dongting Lake basin
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Abstract:
Heavy metals in sediments were one of the most harmful and longlasting sources of pollution in the lake basin.The distribution characteristics,source analysis,change trend,occurrence form,adsorption,release rules, and risk assessment of heavy metals in the sediments of Dongting Lake have already been studied by domestic scholars.Based on the sampling investigation of the three lakes and four rivers in the Dongting Lake and the previous research results,the pollution status and possible pollution sources of heavy metals were analyzed comprehensively,which provided a basis for the control of heavy metal pollution in the Dongting Lake. In August 2019,thirtynine and fourteen sampling sites were set up to collect sediments from the lake and four rivers.The samples were cryopreserved to the laboratory.Aqua regia extraction and inductively coupled plasma mass spectrometry (ICPMS) were used to detect the heavy metal content in sediment samples.The heavy metal pollution in Dongting Lake was evaluated by the ground accumulation index and the ecological risk assessment method. [JP3]The average mass fraction of heavy metals were Zn（121.01 mg/kg）>Cr（66.39 mg/kg）>Ni（33.34 mg/kg）>Pb（31.14 mg/kg）>Cu（30.57 mg/kg）>[JP]As（16.58 mg/kg）>Cd（2.88 mg/kg）,respectively.The spatial variation coefficient of Cd was 159.91%.The average content of Cd in the three lakes has decreased in recent 10 years,especially in the West Dongting Lake and the South Dongting Lake,which was up to 19%~74%.However,the East Dongting Lake was relatively more stable and the heavy metal content was still high.According to the ground cumulative index value mean,the pollution of 7 kinds of heavy metals showed.According to single heavy metal hazard index value,the comprehensive potential ecological risk of heavy metal elements was as follows:East Dongting Lake (high)>West Dongting Lake (medium)>South Dongting Lake (low);The inflow rivers were Zishui (high)>Yuanjiang (high)>Xiangjiang (high)>Lishui (low).The overall comprehensive ecological risk index value of Dongting Lake was 308.35,which was at a high ecological risk level. Conclusions(1) The spatial distribution of seven heavy metal elements in the surface sediments of Dongting Lake and the rivers was significantly different.The contents of elements were 0.51,0.57,0.51,0.45,0.29,7.72 and 0.34 times higher than the background values.Compared with other lakes in China,the content of Cd in Dongting Lake was at a higher level,and the contents of the other six heavy metals were at an intermediate level.(2) The results of the two methodslike the ground accumulation index,and the ecological risk assessment method,had shown a Cd for the most serious heavy metal pollution damage elements in East Dongting Lake and water of the lake district,respectively,and the most serious area of sediment pollution in rivers was seriously polluted and high potential ecological risk in Dongting Lake valley. Ni,Cr,Cu and Zn were heavy metals with moderate pollution and low risk.Pb and As were heavy metals with mild pollution and low risk.(3) According to the correlation analysis and principal component analysis,the seven kinds of heavy metal elements may be divided into three categories,including Cd and Zn,respectively,according to the situation of Dongting Lake basin′s natural and human activity,Cd probably from pesticides,fertilizers and sewage may also come from steel manufacturing,paint,etc.,Zn may come from zinc mining or rock weathering and erosion of minerals,and As were the third category and may belong to natural sources.Based on the results of this study,it was suggested that the next step should be the regional classification of pollution levels in Dongting Lake.The environmental protection inspectors and other relevant departments are urged to strengthen the control of Cd input from sources inside and outside the river basin.Comprehensive measures of ectopic or in situ treatment should be adopted.The ecological restoration of Dongting Lake sediments was carried out to eliminate the heavy metal pollution of sediments and improve the overall ecological environment level.
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