Pollution Characteristics,Risk Assessment Of Polycyclic Aromatic Hydrocarbons In Xiangxi River Bay In Three Gorges Reservoir

Since the construction of Three Gorges Dam,the migration and transformation behaviors of pollutants in the reservoir area have changed significantly under the effect of periodic fluctuation of water level.In recent years,the pollution of polycyclic aromatic hydrocarbons(PAHs)in this area has been found due to the frequent influence of shipping and human activities along the reservoir bank.However,it will be further spread under the influence of water level fluctuation,leading to the potential risk for the health and safety of the terrestrial and aquatic ecosystems in the Three Gorges reservoir area.In this study,we explored systematically the environmental characteristics of PAHs in Xiangxi Bay,the largest tributary of the Three Gorges Reservoir.The behavior and source PAHs in Xiangxi Bay were analyzed and their potential risk was assessed using various mathematical models.The results of this study could provide a theoretical basis for the prevention and control of PAHs pollution in Xiangxi Bay.The main contents and conclusions are as follows:1.The distribution characteristics of soil PAHs and its correlation with soil physicochemical properties in Xiangxi Bay Bank Zone;Soil samples from three different areas(upper edge of the fluctuation zone,fluctuation zone and sediment)in Xiangxi Bank Zone were collected from June 2017 to March 2018.The contents of PAHs,basic physical and chemical indexes(p H value,organic matter(OM),total nitrogen(TN)and total phosphorus(TP))and typical heavy metal pollutants(Cu,Pb,Cd,Cr)in these soils were determined,meanwhile,the correlation among PAHs,soil physical and chemical properties and heavy metals were studied.The results showed that 16 PAHs monomers in three regions were mainly middle-low ring PAHs.The variation ranges of total PAHs in sediment,fluctuation zone and the upper soil of fluctuation zone were 10.73-1325.25 ng/g,43.60-2110 ng/g,and 83.09-2476.01 ng/g,with the average values of 234.90 ng/g,464.2 ng/g and 916.71 ng/g,respectively;the total PAHs in the upper edge of fluctuation zone was the largest,followed by the fluctuation zone and the sediment was the smallest.The total PAHs near Xiakou Town and Yangtze River estuary were higher than all other sites.The correlation analysis showed that there was a significant negative correlation between PAHs and total phosphorus,Cd and soil aggregates at the size of 2-50 μm(p< 0.05).There was a significant positive correlation between PAHs monomers and total PAHs(p< 0.01).2.The distribution characteristics of PAHs in water and its diffusion behavior of PAHs at water-sediment interface;Surface water samples from five horizontal zones along the banks of Xiangxi River were collected from June 2017 to March 2018 to determine the concentration of PAHs.The diffusion behavior of PAHs at sediment-water interface was analyzed by fugacity model.The results showed that the concentration of PAHs in surface waters of Xiangxi Bay ranged from 178.00 to 294.53 ng/L.There was a significant positive correlation between 5-6 ring PAHs and turbidity in surface water(p< 0.05).The fugacity model analysis showed that PAHs mainly settled to sediment in summer and autumn,while PAHs were mainly in dynamic equilibrium in winter and spring.There was no risk of secondary release of PAHs in the sediment in Xiangxi River during the monitoring period.Organic carbon/carbon black with higher concentration had lower fugacity fraction,and showed stronger adsorption capacity on PAHs.Furthermore,carbon black had a stronger adsorption capacity on PAHs than organic carbon.3.The distribution characteristics of PAHs in fluctuation zones at different elevation and their response to water level fluctuation;During a period of water level fluctuation,the concentrations of PAHs in the lower layer soils were larger than that in the upper layer soils.The distribution of PAHs in soil at 145-155 m elevation was mainly affected by the fluctuation of water level,while the distribution of PAHs in soil at 165-175 m elevation was greatly affected by the exogenous discharge of PAHs from the reservoir bank.After a period of water level fluctuation,soil p H values and TP contents increased,soil p H values at 165 m and TP contents at 175 m were most sensitive to water level fluctuation;the proportion of soil aggregates larger than 50 μm reduced and the proportion of soil aggregates smaller than 2 μm increased,soil aggregates at 155 m-165 m elevation were most sensitive to water level fluctuation.Soil TOC,organic matter and total nitrogen contents decreased with the rise of water level,soil TOC and organic matter at 155 m-165 m and TN at 145 m elevation were most sensitive to water level fluctuation;the total concentration of PAHs increased,PAHs at 165-175 m were most sensitive to water level fluctuation.Pearson correlation analysis showed that the total PAHs content was significantly negatively correlated with soil aggregates larger than 50 μm(p< 0.05),and 4-ring PAHs was positively correlated with soil aggregates smaller than 2 μm(p< 0.05).There was a significant positive correlation between 6-ring PAHs and 3-ring PAHs(p < 0.05).There was a significant positive correlation among PAHs in soil,sediment,each elevation of the fluctuation zone and the upper edge of the fluctuation zone(p< 0.01).PAHs in surface water only had a significant positive correlation with soil PAHs at 155 m and 175 m of the fluctuation zone(p< 0.05).4.The source analysis and risk assessment of PAHs in Xiangxi Bay;The PAHs isomer ratio method was used in combination with principal component analysis-multiple linear regression method to analyze the sources of PAHs in Xiangxi Bay.The results showed that the main sources of PAHs in surface water of Xiangxi River were oil release in autumn and combustion in other three seasons.The main source of PAHs in sediments,fluctuation zones and their upper edge soils was from the incomplete combustion of organic items in four seasons.Most of PAHs monomers in the four regions of Xiangxi Bay were at medium risk level.The total PAHs of water,fluctuation zone and its upper edge soil were all at grade IV with high risk level in all seasons.The total PAHs in sediments were at grade IV with high risk level in summer and winter;and at grade II with low risk level in autumn and spring.The order of risk based on total PAHs in four regions of Xiangxi Bay was,the upper edge of fluctuation zone >surface water >fluctuation zone >sediment.PAHs have potential carcinogenic risks in sediments,water,fluctuation zones and upper fluctuation zones through ingestion and skin contact.The order of carcinogenic risk values in each region was sediment >surface water >upper edge of the fluctuation zone >fluctuation zone.The risk of skin contact was greater than that of ingestion.