Study On Distribution Characteristics And Ecological Risk Assessment Of Heavy Metals In Surface Water And Sediments Of Chaohu Lake In Different Water Periods

Heavy metals have the characteristics of refractory degradation,easy accu mulation and high toxicity.Chaohu Lake is one of the major freshwater lakes in China.It is drinking water source of the surrounding towns and towns in th e lake area and it’s also the strategic backup water source of Hefei city.There fore,it is of great significance to study and analyze the distribution of heavy metal in Chaohu Lake and make risk assessment.In this paper,Chaohu Lake was taken as the research object.Samples were collected in three consecutive water periods of Chaohu Lake area:withered water period,normal water perio d,high water period,and used ICP-MS and ICP-OES for heavy metal in 63 s amples.In order to discuss the distribution characteristic of heavy metal eleme nts in surface water and surface sediment,the occurrence forms in surface sedi ment.The single-factor and composite pollution index methods,the surface sedi ment enrichment factor method,the geoaccumulation index method,potential ec ological risk index method,the risk assessments coding method were used to a nalyze the distribution characteristic and risk evaluation of heavy metal.Throug h the above research,conclusions are as follows:（1）There are certain differences in water quality in different water periods of surface water in Chaohu Lake.The p H in each water period was weakly alkaline,the chlorophyll a,algae density and electrical conductivity were the largest in the normal water period,and the turbidity was the largest in the wither water period.In the surface water of East Chaohu Lake,the average value of Fe,Mn,V,Cr,Co,Cu,Zn,Cd,Pb and As was 181.44,3.41,1.99,2.94,0.13,2.54,13.83,0.04,1.61,0.91μg/L in a hydrological year,respectively.All have reached“Environmental quality standards for surface water”（GB 3838-2002）classⅠwater standard value.Form the analysis of time distribution,Pb is the highest in the normal water period,while high water period is opposite;Cr and As are the lowest in the high water period and the highest in the wither water period;other elements are the highest in the wither water period,next in the high water period,and finally in the normal water period.Form the analysis of spatial distribution,the spatial distribution of each element characteristics are not obvious.According to the evaluation results of the comprehensive index method,the range of withered water period is 0.05～0.11,the range of normal water period is0.09～0.14 and the range of high water period is 0.06～0.18.（2）For surface sediments of Chaohu Lake,the average content of Fe,Cu,Cr,Co,Zn,Cd,Pb,V and As was 37048.41,25.80,81.38,16.55,160.43,0.50,4.19,99.64,13.36 mg/kg in a hydrological year,respectively.Among them,the concentration of Cd,Zn,Co,and As elements were 4.12,1.59,0.66 and 0.48 times higher than the soil background value.From the standpoint of the coefficient of variation,Zn and Cd elements belong to high coefficients of variation（CV>36%）,while the coefficients of variation of other elements are relatively large and the distribution differences are small.From the viewpoint of time distribution,the content of Zn in the whole lake is the highest in the withered water period,the content of Pb in the whole lake is the highest in the normal water period,and the concentration of the other elements in the whole lake is the highest in the high water period.From the viewpoint of dimensional distribution,in general,Cd,Cu,Cr,Co,Zn,Pb,V and As elements are mostly low in the middle and high at both ends.（3）In the three water periods,the heavy metal of Cd was mainly in the concentration of aid-soluble state,and Fe,Cr,Zn,Pb,V and As were foremost existed in the residual fraction in the surface sediments of Chaohu Lake.And during the three water periods,the extractable states of Zn and Cd elements in the West Lake were higher than those in other lake.Most elements show a trend of high in the west and low in the east in water bodies and sediments.（4）The assessment outcomes of enrichment factor method show that the enrichment factors of Zn,Cd in the West Lake area are all greater than 1.5 in the three water periods;The assessment outcomes of the geoaccumulation index method show that Zn,Cd in the West Lake area belong to partial moderate to moderate pollution,and most of their I_geovalues are between 1 and 2;The assessment outcomes of the potential ecological risk index means indicated that the main ecosystem contributing factors of the three water periods is Cd,which the RI of whole lake are 175.55、174.36、230.04,respectively,which are the moderate hazard level.The outcomes of RAC method indicated that the element Cd showed a moderate ecological risk level in the withered water period,normal water period,high water period.Combined with above evaluation results,it can be seen that Cd have a greater influence on the water environment.