| Polycyclic aromatic hydrocarbons(PAHs)are organic pollutants containing 2 to 8fused benzene rings.They are ubiquitous and produced by the incomplete combustion and pyrolysis of fossil fuels and other organic matter.As an important category of persistent organic pollutants,polycyclic aromatic hydrocarbons pose a serious threat to human health due to their teratogenic,carcinogenic and mutagenic properties,and have attracted worldwide attention.Among them,16 types of PAHs have been included in the list of priority control pollutants by the U.S.Environmental Protection Agency(US-EPA).PAHs which generated by natural processes and human activities can migrate to the aquatic environment through atmospheric deposition,sewage discharge,and surface runoff,and then accumulate in the sediments and release them twice,causing continuous damage to the aquatic ecosystem.Therefore,exploring the distribution,source,migration and transformation laws of PAHs are of great significance for reducing and controlling PAHs pollution and water ecological environment protection.With the rapid development of human society,the synergy of climate change and human activities will seriously affect the water balance in the surface water system,resulting in damage to the ecological functions of rivers and lakes as well as severe water level fluctuations.Affected by cyclical fluctuations in water levels,lakes,rivers and reservoirs will form large-scale drawdown zones.Frequent environmental changes make the ecological environment of drawdown zones extremely fragile and vulnerable to pollution and destruction by human activities.In addition,the unique water-covering change mechanism of the fluctuating zone changes the physical and chemical properties of the soil,thereby affecting the migration process of pollutants between media.At present,the research on the material migration under the driving mechanism of the fluctuating zone mainly focuses on the circulation of elements such as carbon and nitrogen.There are relatively few studies on the migration and transformation law of PAHs under the driving mechanism.Poyang Lake is the largest seasonal throughput freshwater lake in China,and the water level changes seasonally by about 10 m during the year.About 3000 km~2 of fluctuating zone can be formed during the dry season,which provide a wintering habitat for some migratory birds and endangered birds.Poyang Lake receives water and sand from"five rivers".With the acceleration of industrialization and urbanization within the basin,a large number of pollutants are collected here through surface runoff and atmospheric migration,which provides a suitable site to explore the migration law of PAHs under the driving mechanism of the fluctuating zone.The multi-medium fugacity model is an important method to study the environmental process of pollutants,however now the fugacity models all believe that the transmission relationship of each medium is stable and cannot be used for the environmental process simulation of special ecosystems such as the fluctuating zone.Therefore,the establishment of a fugacity model suitable for the environment of the fluctuating zone is of great significance to the further improvement of the theory of pollutant migration.With the support of the National Natural Science Foundation of China"Pollution Characteristics and Migration and Transformation Laws of POPs under Dry-Wet Alternation in Lakes",based on field environmental samples and dual-scene IV fugacity models,this paper analyzes the distribution,sources,influencing factors,migration and transformation laws of PAHs in the fluctuating zone of Poyang Lake.Based on literature research,we put forward a scientific hypothesis:the alternating cycle of submergence(replenishment)and exposure(discharge)using the soil in the water-fluctuating zone as a medium may constitute the"geochemical pump"effect,which drives PAHs from the water body to the atmosphere.First of all,this paper discusses the distribution and composition characteristics of PAHs in various media in different seasons of Poyang Lake,uses the characteristic isomer ratio method and principal component analysis-multiple linear regression analysis to study the source of PAHs,and the contribution rates of different sources have been quantitatively characterized.Secondly,through the measurement of soil physical and chemical properties as well as the collection of geographic data,this paper summarizes the influencing factors of the distribution of PAHs in the soil of the Poyang Lake fluctuating zone,and uses the coefficient of variation coupled principal component analysis and hierarchical cluster analysis to select the most significant influencing factor.Besides,the fugacity ratio was used to preliminarily judge the migration direction of PAHs in the water-level-fluctuating zone soil under different water-covering scenarios.Finally,according to the basic fugacity principle and the migration coefficient equations,this paper established a level IV fugacity model for the two scenarios of exposure and flooding in the drawdown zone,and successfully simulated the accumulation process of PAHs in various environmental media under the drive mechanism in the drawdown zone.Then,the migration law of PAHs is explained through the estimation of net exchange flux.The main conclusions of the thesis are as follows:(1)The concentration of PAHs in the soil and sediment of Poyang Lake is in the range of 1.7-774.6 ng/g,which has a significant increasing trend compared with previous reports,and the pollution level is at a lower-middle level in domestic and foreign aquatic systems.The concentration difference between the dry season and the wet season is obvious,the average content is 109.2 ng/g and 197.6 ng/g respectively,however the composition difference is not obvious,and the HMW PAHs of the 5 ring and 6 are still the main component.The concentration of PAHs in the soil and sediments at periodic and the permanent submerged point is significantly greater than the permanent dew point.The concentration of dissolved PAHs in the water is 4.8-62.0 ng/l,and the suspended PAHs in the interval of 116.8-4651.2 ng/g,both show a tendency that the surface water concentration is greater than the bottom water concentration.The composition ratios of PAHs in the dissolved state and suspended state is relatively similar,which may due to similar environmental process emissions and mutual diffusion effects.The content of PAHs in the atmosphere range from 842.7 to 13397.7 pg/m3,with the fourth ring and below PAHs being the main ones.The pollution level in the first half of the year was significantly greater than that in the second half.The HMW PAHs in the Poyang Lake area tend to be adsorbed in the soil and sediments,while the LMW PAHs are mainly distributed in the atmosphere and water.The PAHs in the study area come from the mixed emissions of petroleum volatilization and combustion.Specifically,the PAHs in the atmosphere mainly come from the combustion of coal and biomass,with a contribution rate of 58%;while the main contributing sources in the soil and water are traffic-related combustion emissions(47%)and gasoline leakage and combustion(37%).(2)The concentration of PAHs in the soil and sediments of the water-level fluctuating zone is significantly correlated with the water content and organic carbon content(p<0.01),and soil p H and redox points were only significantly correlated with four-ring and above PAHs in soil and sediments during wet periods(p<0.05).In addition,PAHs pollution in soil and sediments in areas where urban intensive use of land and densely populated is more serious.Compared with seasonal factors,the occurrence of PAHs in the soil and sediments of the Poyang Lake area is more affected by geographical factors.Among all environmental factors related to location differences,soil moisture is the most important factor,followed by soil organic matter,they have a significant positive linear correlation with the concentration of PAHs.The soil moisture content also reflects the difference in submergence time,which proves that PAHs in the soil of the fluctuating zone are more derived from water transport.At the soil-air interface and sediment-water interface,the diffusion and migration direction of LMW PAHs is mainly from soil or sediment to atmospheric water;the equilibrium state HMW PAHs is significantly different,and the soil-air interface is mainly from the atmosphere to the soil sedimentation and the sediment-water interface has multiple exchange trends coexisting.The change of submerged and exposed state changes the migration direction of HMW PAHs in the soil at the periodic submerge point,reflecting that the environmental change in the fluctuating zone may be some PAHs are the driving factors for phase-to-phase migration.In addition,the net change in the concentration of PAHs in the fluctuating zone soil at different periods has the characteristics of accumulation during the flooding period and discharge during the exposure period,which reflects the driving mechanism of the fluctuating zone uses the soil as a carrier to promote the release of PAHs from water to the atmosphere.(3)The level IV fugacity model of exposuring and submergence scenarios established in this paper is suitable for simulating the accumulation and migration process of phenanthrene,pyrene,benzo[a]pyrene and benzo[ghi]perylene in various environmental media under the condition of cyclical overlying water changes in the fluctuating zone.At the permanent dew point,the PAHs in the atmosphere showed a decreasing trend over time,the concentration of phenanthrene and pyrene in the soil decreased significantly,while the HMW PAHs gradually accumulated.The concentrations of the four target substances in the atmosphere of the permanent submerged point decreased to varying degrees.The pyrene in water showed an increasing trend while the other monomers mainly flowed out,and the PAHs in the sediments gradually accumulated over time.The PAHs in the atmosphere at periodic flooding points fluctuate in a short-term and establish a new balance when the water overlying conditions change.The PAHs concentration in the soil shows a trend of slowly decreasing during the exposed period and significantly increasing during the submerge period.The simulation results confirmed again that the influence of the change of the state of water overlying on the occurrence of PAHs.The measured concentrations of soil and sediment in the Poyang Lake area have verified the validity of the model.In addition,the emission rate of PAHs in the environment,the target octanol-water and octanol-gas partition coefficients,and the initial soil concentration are parameters that have a greater impact on the model results,and its more accurate data sources can effectively improve the accuracy of the model.Phenanthrene,pyrene,benzo[a]pyrene and benzo[ghi]perylene in the soil of the water-level fluctuating zone are mainly net outflow during the exposuring period,with an average total net outflow of 0.1 kg/km~2;the submergence period is mainly based on the accumulation process,the average total net accumulation is 0.98 kg/km~2.This result indicates that the soil in the water-level fluctuating zone has become a medium for water to transport PAHs to the atmosphere under the mechanism of periodic submergence,resulting in a"geochemical pump"effect. |
PDF Full Text Request