Study On Migration And Transformation Of Polycyclic Aromatic Hydrocarbons In The Soil Of Water Fluctuation Zone Of Xiangxi River Reservoir Bay In Three Gorges Reservoir Area

Since the Three Gorges Dam impounded,a water-level-fluctuation zone（WLFZ）with a drop of up to 30 m and an area of 349 km² has been formed in the reservoir area.As a staggered zone between water and terrestrial ecosystems,the fluctuation zone is vulnerable and unstable due to periodic anti-seasonal water level dispatching in reservoir area.Xiangxi River is the first tributary in front of the Three Gorges Dam,and its ecological environment directly affects the ecological safety of the reservoir area.Polycyclic Aromatic Hydrocarbons（PAHs）are a class of harmful persistent organic compounds.In recent years,although more and more scholars have gradually realized that water level dispatching has made remarkable changes in the distribution characteristics of PAHs before and after reservoir impounding,the influence of water level fluctuation on the distribution of PAHs has been verified by comparing the changes of total PAHs before and after impounding.However,the mechanism of the effect of water level fluctuation on the PAHs distribution in reservoir area is still remained to be explored.The morphological transformation and migration behavior of PAHs plays a key role in the distribution of PAHs in soil and water in reservoir area,which deserves further study.Therefore,we analyzed the morphological transformation and migration behavior of PAHs in the fluctuation zone soil under the background of fluctuation of water level by combining field monitoring and laboratory simulation based on the characteristics of PAHs in the fluctuation zone soil of the Three Gorges Reservoir Area,It provides a basis for pollution control of PAHs in soil of Xiangxi Bay.The main research contents and conclusions are as follows:（1）Environmental Pollution Characteristics of Soil PAHs in the Rising Zone of Xiangxi River Gulf in Reservoir AreaFrom June,2020 to March,2021,samples were collected from the soil of 7 sample zones in the fluctuation zone of Xiangxi River in three quarters to determine the physical and chemical properties of the soil and the content of PAHs,and to conduct risk assessment and correlation analysis on the soil PAHs in the fluctuation zone.The particle size of soils in WLFZ was mainly ranged in 2～50μm.Total phosphorus（TP）and organic matter in WLFZ in different seasons are as follows:autumn>summer>spring;The changes of PAHs in WLFZ in three seasons were mainly manifested as follows:spring（March 2021）>summer（June2020）>autumn（September 2020）.Toxicity equivalent values of low and low ring PAHs in 16PAHs monomers of Xiangxi River fluctuation zone were smaller,high ring PAHs values were higher,TEF values of high ring PAHs were higher,and the most significant contribution values of Ba P and DBA were the highest.The TEQ_{Ba P} values of three seasons in soil of WLFZ are spring>autumn>summer.Soil organic matter had a significant negative correlation with sand（D>50μm）（p<0.05）,and had a significant positive correlation with clay（p<0.05）;each rings of PAHs showed a significant positive correlation with the total amount of PAHs（p<0.05）.（2）The Response of PAHs morphological transformation in the soil of WLFZ to submergence depth and timeIn order to study the response of PAHs morphological transformation of soil in the fluctuation zone to the flooding depth and time during the process of water level change,soil flask experiment in the fluctuation zone was carried out by means of in-situ floating platform device.Dissolved organic carbon（DOC）showed an upward trend as a whole,and the DOC concentrations in overlying water and soil remained in relative equilibrium.Particle size more than 50μm was the main aggregate in soil,accounting for 58.43%of the average soil aggregate.The organic PAHs accounted for the highest proportion,the acid-soluble PAHs accounted for the smallest proportion,and the residual PAHs tended to be stable.The 15 m group had the largest bioavailability and the 5 m group had the smallest bioavailability.PAHs in water and acid soluble state had certain migration ability.Acid soluble PAHs showed very significant positive correlation with soil TOC and clay（D<2μm）（p<0.01）;Soil clay（D<2μm）there is a very significant positive correlation with acid-soluble PAHs and organic PAHs（p<0.01）;There was a significant positive correlation between organic PAHs and soil TOC（p<0.05）.（3）The Characteristics of Soil colloid in WLFZ of Three Gorges Reservoir Area and Its Impact on PAHs MigrationBased on the difference of PAHs distribution in soils at different elevations of the fluctuation zone,the release characteristics of soil colloids in the fluctuation zone and the influence of soil colloids on PAHs migration were explored from the perspective of promoting pollutant transport by soil colloids.When p H>4,the stability of soil colloid tended to be stable at all elevations,and the colloid dispersion was better when p H>4.Ca²⁺had a stronger influence on the stability of colloids than Na⁺.When p H ranged from 6 to 8,the colloids were easy to move,and the influence of Na⁺on the mobility of colloids was lower than that of Ca²⁺.The content of phenanthrene in colloidal leaching solution group was higher than that in non-colloidal leaching solution group.The facilitation of soil colloids at 185 m was more significant than that at 145 m and 165 m elevation.