Heavy Metals Pollution In The Sediments Of Typical Suburban Lakes In Yunnan-Guizhou Plateau

In recent years,with the rapid development of social economy such as industry and agriculture,the problems of lake deposition and pollution are becoming more and more serious.However,due to the limitation of monitoring data,the understanding of the long-term change of lake environment and its relationship with human activities is not clear enough,which restricts the understanding of the rule of lake environment change.Lake sediments faithfully record the information of lake environment evolution and human activities in the basin,which is a good carrier for the analysis of lake environment evolution and its driving factors.There were obvious regional differences in the impact of human activity types and intensities on lake environment in the watershed.Suburban lakes are greatly affected by urban development,especially under the influence of high-intensity human activities,such as heavy metal pollution and eutrophication.This paper selected the typical suburban lakes——Dianchi Lake and Qionghai Lake,based on the dating of ²¹⁰Pb and ¹³⁷Cs,this paper discusses the evolution characteristics of the sedimentary environment of Dianchi Lake and Qionghai Lake in the past hundred years by analyzing the susceptibility,grain size,major metal elements?Al,Fe,Ti,and other elements?and trace metal elements?As,Cd,Cr,Cu,Ni,Pb,Hg and Zn?and other indicators of the sedimentary cores.The characteristics of the evolution of sedimentary environment in the past century and the history of heavy metal concentration in Dianchi Lake and Qionghai Lake were studied,and the relationship between the spatio-temporal changes of heavy metal pollution and its relationship with human activities in the lake basin,the potential ecological risk of heavy metals in surface sediments were evaluated.The main results are as follows:?1?The grain size composition of Dianchi Lake and Qionghai Lake was mainly composed of fine particles?<63?m?,and the average concentration of clay was 43%.Before the 1950s,the clay concentration,magnetic susceptibility and the concentrations of major metal elements in the sedimentary core of Dianchi Lake were relatively stable,which indicated the material input in the basin was weakly affected by human activities and precipitation;since the 1960s,the above indicators in the sedimentary core gradually decreased,and the sedimentary rate increased,reflecting the increase of input of coarse particles caused by the enhancement of human activities?such as reclaiming land from lakes,deforestation,rapid development of agriculture,etc.?.Multi-core analysis showed that the evolution trend of sedimentary environment in Dianchi lake of different lake areas had strong similarity.The change of clay and magnetic susceptibility in Qionghai Lake was relatively complex,and it was gradually increasing.In the 1970s,the sedimentary environment changed significantly,especially the increase of sedimentary rate,which may be related to human activities such as reclaiming land from lakes,but it needs further study.?2?According to the results of cluster analysis,the metal elements in Dianchi Lake and Qionghai Lake can be divided into two groups.In Dianchi sedimentary core,Group I includes Al,Ti,Cu,Cr and Ni,group?includes As,Cd,Hg,Pb and Zn.Concentrations of all elements were generally constant until the 1950s.;since the1950s,the concentrations of the group I of elements have gradually decreased,which was related to the increase of the input of coarse-grained materials;the group?of elements had an increasing trend in varying degrees,reaching the highest value around 2000,the overall trend has been stable or gradually decreasing in recent years.In Qionghai sedimentary core,the group I includes Cd,Pb,Zn,Cu and As,which were increasing gradually in recent years;the group?includes Al,Cr,Ni,Ti and Hg,whose concentrations were stable before the 1970s,and then fluctuated to some extent.?3?The variations of metal elements and grain size composition in Dianchi Lake and Qionghai Lake indicate that the heavy metal concentration was obviously affected by the sedimentary material,the change of its concentration does not fully reflect human pollution.According to the results of enrichment coefficient,As,Cd,Hg,Pb and Zn were typical pollution elements in Dianchi Lake.The pollution began in the1960s and gradually increased,the peak value of the anthropogenic flux appeared in the mid-1990s to 2000.In recent years,with the implementation of pollution control measures in the basin,Cd,Hg,Pb and Zn has remained relatively stable and decreasing values of As.Cd,Hg and Pb were typical pollution elements in the sediments of Qionghai lake,the pollution began in 1960s-1970s.The anthropogenic metals flux had an obvious peak in 1970s,which was consistent with the variation of deposition flux,and may be related to the input of anthropogenic metals in the soil.Since the 1980s,the anthropogenic metal flux has not increased with the expansion of Xichang City and the economic development of the lake basin.The beginning time of heavy metal pollution in the sediment of Dianchi Lake was earlier than that of Qionghai Lake,which was related to the earlier industrial start of Kunming City.?4?Since the 1960s,the stocks of anthropogenic As,Cd,Hg,Pb,Zn of different lake areas in Dianchi Lake were 0.23-2.5 g/m²,28-155 mg/m²,1.3-26 mg/m²,0.49-2.4 g/m² and 2.4-37 g/m²,respectively,the northern lake area was higher than the southern lake area,which was similar to the spatial variation trend of anthropogenic metal concentration in surface sediments.The concentrations of anthropogenic As?Cd?Hg?Pb?Zn in the surface sediments of Qionghai Lake were 0.2-6.4 mg/kg,0.25-1.24 mg/kg,18-55?g/kg,5.1-13.4 mg/kg and 6.9-62.2 mg/kg,respectively.The high values were mainly in the northwest lake near Xichang City.The spatial variation of anthropogenic metal concentrations in Dianchi Lake and Qionghai lake sediments reflected the profound impact of urban development on the lake environment.Regional comparison showed that Cd was a typical heavy metal pollution element of the lakes in the Yunnan-Guizhou Plateau,and that the pollution of heavy metals in the sediments of suburban lakes was significantly higher than that of other types of lakes.?5?Based on the potential ecological risk index method and sediment quality standard,combined with heavy metal pollution degree?EF value?,the ecological risk of As,Cr,Cu,Ni,Pb and Zn in the surface sediments of Dianchi Lake and Qionghai Lake was low,while Hg and Cd have high potential ecological risk,among which the risk degree of lake areas adjacent to the city was relatively high.