Distribution Characteristics And Ecological Risk Evaluation Of Polycyclic Aromatic Hydrocarbons In Xiangxi River Basin

The three gorges project has brought enormous social and economic benefits in flood control,power generation and shipping,but its impact on the ecological environment of the river basin cannot be ignored.Xiangxi river is the first tributary of the three gorges dam.In recent years,many experts and scholars have carried out studies on PAHs pollution in the reservoir area,but most of them focus on water and sediment,and few studies on PAHs pollution in the fluctuation zone.In this paper,the distribution,source and environmental ecological risk assessment of PAHs in the sediment,the fluctuation belt and the upper margin soil of Xiangxi Bay were systematically analyzed.With phenanthrene as the target contaminant,batch equilibrium method was used to systematically studied the adsorption kinetics and thermodynamics of the characters of soil deposited in Xiangxi river.The research has acquired following conclusion:1)The distribution characteristic of PAHs in Xiangxi River Basin soilSoil samples were collected from June 2018 to March 2019 at different areas（sediment,fluctuation zone and upper edge of the fluctuation）,the contents of PAHs were determined.The results showed that the concentrations of PAHs in sediments ranged from 28.6 to 613.4ng/g,with an average value of 395.7 ng/g;the concentrations of PAHs in the fluctuation zone ranged from 456.15 to 1420.23 ng/g,with an average value of 805.34 ng/g;and the concentrations in the upper edge of the fluctuation zone ranged from 367.08 to 2232.72 ng/g,with an average value of 992.67 ng/g.For soil samples,the spatial distribution of∑PAHs contents are as follows:the upper edge of the fluctuation zone>the fluctuation zone>the sediment.PAHs in soil of Xiangxi River Basin concentrations present seasonal change of summer>winter>spring>autumn,the horizontal transect zone is higher near the Xiakou town and the Yangtze River estuary.2)Soil physical and chemical properties and their correlation with PAHs contentThe p H value of soil in Xiangxi River Basin is neutral and slightly alkaline,the content of organic matter in each region are as follows:the sediment>the upper edge of the fluctuation zone>the fluctuation zone,the total phosphorus and nitrogen contents are sorted from high to low:the sediment>the fluctuation zone>the upper edge of the fluctuation zone.The correlation between 5 rings PAHs and p H value in sediment,5～6 rings PAHs and organic matter in sediment and the fluctuation zone were positive（p<0.05）,but the correlation between PAHs and total phosphorus,total nitrogen were significant negative.Furthermore,there was a significant positive correlation between single PAH and total PAHs（p<0.05）.3)The adsorption behavior of phenanthrene on soils from Xiangxi River BasinAccording to adsorption kinetic experiments,the adsorption of phenanthrene in soils reached equilibrium in 24 h basically,and the adsorption process can be well described by pseudo-second-order kinetic model.Compared with Linear model and Langmiur model,the isotherm modeling indicated that the adsorption behavior of phenanthrene on the adsorbent could be described better by Freundlich model.The thermodynamical parameters:?<0,?<0,?<0,demonstrating that adsorption process of phenanthrene by soil was a spontaneous exothermic reaction which was mainly physical adsorption.The main driving force for adsorption is the enthalpy change,elevated temperature will inhibit the process,reduced adsorption capacity of adsorbent soils.4)The source and ecological risk assessmentBy the relative abundance method,PAHs of sediments in autumn was originated from oil release and and the other seasons are mainly from wood and oil mixed combustion.The PAHs isomer ratio method analysis also showed that PAHs in sediments fluctuation zones and their upper edge soils were mainly from combustion in four seasons.The Nemero pollution index method and standard of agricultural soil in Canada shows that the ecological risk of sediment in Xiangxi River Basin was low,the ecological risk of the fluctuation zone and the upper edge should be taken seriously.The ecological risk assessment with Ba P toxicity equivalent method showed that the distribution of TEQ_{Ba P}varies from region to region in different seasons,sediment sampling points of high TEQ_{Ba P}were in summer and winter,the fluctuation zone and the upper edge were in winter and spring.According to safety threshold model analysis,most of PAHs monomers in the soil of Xiangxi River Basin are at medium risk level.The total PAHs assessment risk of sediment was gradeⅡat low risk,fluctuation zone and its upper edge soil were all at gradeⅣwith medium risk level.The order of safety threshold in each region is as follows:the upper edge of the fluctuation zone=the fluctuation zone>the sediment.PAHs have potential carcinogenic risks in sediments,fluctuation zones and their upper soil for ingestion and skin contact.The order of carcinogenic risk values in each region is upper edge of the fluctuation zone>fluctuation zone>sediment.in different exposure ways according to the order from large to small is Dermal>Ingestion>Inbalation,and adults are at greater risk from skin contact than children,and children are at greater risk from ingestion than adults.Winter risk is the highest in all regions.