Soil Heavy Metal Morphology Analysis And Ecological Risk Assessment In The Fluctuation Belt Of Xiangxi River Basin

Due to the regular storage of water in the Three Gorges Reservoir,a large area of water level fluctuation zone has formed in the reservoir area.The distribution,migration and transformation of pollutants are greatly affected by the fluctuation of water level.As a typical tributary of the dam head in the Three Gorges Reservoir area,Xiangxi River basin is susceptible to pollution.Heavy metals,as important pollutants,are extremely difficult to be degraded and can be enriched into organisms or even human bodies through the food chain,thus posing a threat to health and even life.This paper through the field monitoring,sampling,and indoor test methods of Xiangxi Rriver water level fluctuation zone in flooding period(September 2018 to June 2019),and in fall dry period(June 2019 to September 2019)of the study on the characteristics of heavy metal content and distribution,cancel and fall with the soil heavy metal forms is analyzed,from the perspective of the total heavy metal and the morphology of Xiangxi Rriverwater level fluctuation zone in ecological risk assessment,in order to Xiangxi Rriverwater level fluctuation zone heavy metal pollution prevention and control to provide theoretical support.research contents are as follows:Analyze the changes of soil physical and chemical properties before and after flooding of five sample points(XX01,XX02,XX03,XX04 and XX05)along the Xiangxi River.The results showed that the soil in the Xiangxi River fluctuation zone was mainly neutral and weakly acidic before flooding,and the p H of the soil rose to neutral and weakly alkaline after flooding.The content of soil total phosphorus and total nitrogen decreased after flooding,and was greatly affected by human activities after flooding.The changes of soil organic matter before and after flooding were: before flooding < after flooding.The change characteristics of Cr,Cd,Pb,Cu and Mn contents before and after flooding were studied.Cd and Pb had significant changes in mean and range before and after flooding.The mean value of Cd measured in June 2019 was 0.16 mg/kg(range:0.14?0.18 mg/kg),while the mean value of Cd measured in September 2019 was 1.30mg/kg(range: 0.99?1.67 mg/kg).The mean value of Pb measured in June 2019 is 10.88mg/kg(variation range: 6.74-16.29 mg/kg),while that of September 2019 is 27.89 mg/kg(variation range: 25.74?30.56 mg/kg).Cd and Pb in the water level fluctuation zone are both "sink" in the drying period and "source" in the flooding period.Moreover,the contentof Cd in the water level fluctuation zone has exceeded the standard.The correlation between soil physical and chemical properties and soil heavy metal content was analyzed.Correlation analysis of soil physical and chemical properties and soil heavy metal content revealed that soil p H was correlated with multiple heavy metal contents,while total phosphorus,total nitrogen,and organic matter were correlated with single heavy metals,and the correlation would change after flooding.The content of Cr,Cd,Pb,Cu and Mn,the ability to accumulate heavy metals and the correlation between heavy metals in the Cynodon dactylon and soil heavy metals were analyzed.It was found that the five heavy metals measured in the experiment were mainly concentrated in the roots of plants.The content of Cd and Pb in the Cynodon dactylon,and the change of its content is downstream > upstream > middle stream.The content of Cr,Pb,Cu and Mn per unit area increased during the submersion.And Cd goes down per unit area.During the dry period,the content of Cr,Cu and Mn per unit area decreased.The content of Cd and Pb per unit area increased.The enrichment coefficient of Cd in soil from the Cynodon dactylon was higher than that of the other four heavy metals.There was no correlation between Cr in the Cynodon dactylon and Cr in soil before flooding,but there was a significant negative correlation between Cr in soil after flooding(P < 0.05).Cu had a significant negative correlation with Cd in the soil before flooding(P < 0.05),and a very significant positive correlation after flooding(P < 0.01).The content of Cd and Pb in the Cynodon dactylon was significantly positively correlated with Cd in the soil before flooding(P < 0.05),but there was no correlation between Pb and Cd after flooding,while Cd in the Cynodon dactylon was significantly positively correlated with Cr in the soil.The morphological characteristics of heavy metals in the soil of Xiangxi River basin were studied.Down with Cd,Pb,Mn activity is higher,higher percentage of Cr and Cu inactive state,the Cd is given priority to with exchangeable state,Pb,Mn mainly iron,manganese oxide combination state,showed that Cd,Pb,Mn affected by human activities is larger,three kinds of heavy metal and obvious regional distribution,mainly by agricultural non-point source pollution and life waste water impact on residents' home.In xiangxi River Basin,Cr is the most stable,Cd,Pb,Cu and Mn all have the risk of re-releasing into the environment,among which Cd has the worst stability and the highest risk.Four ecological risk assessment methods based on the total amount of heavy metals were used to evaluate the heavy metal pollution of soil in the water level fluctuation zoneof Xiangxi River basin.The methods based on the total amount of heavy metals all point out that Cd is the heavy metal with the most serious pollution in the water level fluctuation zone in the detection area,and the maximum pollution degree reaches the severe pollution.Based on morphological methods,it was pointed out that Pb and Mn were the most seriously polluted heavy metals in the water level fluctuation zone in this area.The heavy metal pollution was ranked as Pb > Mn > Cd > Cu > Cr,and the maximum pollution degree was only moderate pollution risk.It is found that Cd is the most seriously polluted heavy metal in the water level fluctuation zone,and from its morphology analysis,we can also know that the morphology of Cd is mainly exchangeable state and residue state,but the exchangeable state is very unstable and easy to migrate to the environment.The Cynodon dactylon in Xiangxi River water level fluctuation zone has a strong ability to enrich Cd,which can be used for the remediation of Cd pollution in the water level fluctuation zone.