| Polycyclic aromatic hydrocarbons(PAHs),widely distributed in environmental media,are genetically toxic,mutagenic,and carcinogenic,posing serious hazards to human health.Currently,16 types of PAHs have been listed as priority pollutants for control.As the main source of drinking water in karst areas in southern China,underground river system plays a key role in social and economic development.Due to the hydrogeological structure of the underground river system with "soil upstairs and water downstairs",it is highly susceptible to the influence of external pollutants.Once PAHs enter the underground river system,they will experience a long time accumulation process and eventually cause irreversible risks to the ecosystem and human health.At present,the research on PAHs pollution in karst areas mainly focuses on distribution characteristics,source analysis,ecological risk assessment,etc.,while the research on key scientific issues such as PAHs migration rule in karst underground river system and fine warning of human health risks of PAHs is not in-depth.Based on the above analysis,the underground river system of Qingshuquan in Nanning,Guangxi was selected as the research object to study the migration rules and health risks of PAHs in the underground river system.Firstly,based on the test data,the spatial and temporal distribution characteristics,pollution sources and ecological risks of PAHs in the water environment of underground river system were systematically analyzed.Secondly,based on the environmental behaviors such as rainfall,distribution and diffusion,the migration rules and influencing factors of PAHs with different molecular weight and morphology in underground river system were discussed.Then,a grid fugacity model of various environmental media in the underground river system was constructed to simulate the variation of PAHs in water environment.Finally,the grid fugacity model and human health risk model were used to jointly assess the current health risk level of groundwater PAHs.At the same time,according to different environmental conditions,PAHs pollution status and human health risk levels were determined,and pollution prevention and management measures were proposed.The main conclusions of the paper are as follows:(1)The concentration of ∑16PAHs in groundwater and sediments of Qingshuiquan underground river system ranges from 271.58-577.66ng/L and from 354.33-1315.36ng/g.Groundwater PAHs are mainly low-ring(2-3 ring)PAHs,and sediment PAHs are mainly highring(4-6 ring)PAHs.From wet season to dry season,the concentration of PAHs decreased gradually.From upstream to downstream,the concentration of PAHs increased gradually.Compared with similar areas at home and abroad,the PAHs pollution level is at a moderate level.PAHs pollution sources can be divided into three types: grass,wood and coal high temperature combustion sources,coal and petroleum combustion sources and petroleum sources,among which the contribution rate of coal and petroleum combustion sources is the largest,followed by grass,wood and coal high temperature combustion sources and petroleum sources.PAHs in groundwater showed moderate ecological risk,while PAHs in sediment showed low ecological risk.(2)During rainfall,the groundwater was dominated by free PAHs,while the bound PAHs were relatively small.The effects of different rainfall levels on the concentration and composition of PAHs were in the order of moderate rain > light rain > short time no rain > long time no rain,in which dissolved organic carbon(DOC)and suspended particulate matter(SPM)were the main controlling factors controlling the morphology and composition of groundwater during rainfall.Rainfall can affect the chemical migration of PAHs in groundwater from sedimentation to re-suspension in the middle and upper reaches,but not in the lower reaches.As the number of PAHs rings increases,the distribution coefficient Kp also increases gradually,and PAHs are more likely to migrate to sediments.The upstream PAHs were more difficult to be adsorbed by particulate matter,while the downstream PAHs were easier.Total organic carbon(TOC)and particle size are important factors affecting the polyphase distribution of PAHs.With the increase of PAHs ring number,the fugacity fraction ff decreases,and the sediment changes from source to sink,and the groundwater changes from sink to source.TOC affects the diffusion behavior of PAHs.With the increase of TOC content,PAHs in groundwater are more likely to diffuse to sediments.(3)The grid construction method solves the defect that the traditional fugacity model assumes that pollutants are uniformly distributed in the medium.On the basis of the grid construction method,combined with the fugacity model principle,structure,calculation formula and calculation process,a grid multi-media fugacity model of karst underground river system is established.The model matches with the characteristics of high heterogeneity of karst system and greatly improves the simulation accuracy.The variation laws of Nap,Phe,Pyr and Ba P were simulated by using gridded fugacity model.The concentration of monomer PAHs was high in the downstream and low in the upstream.With the increase of ring number,the distribution of the four monomer PAHs in groundwater and sediment became more consistent.The error values of model verification are less than 5%,and the simulation results are reliable.Emission factor,advection transfer coefficient,half-life,octanol-water partition coefficient,air area and other parameters are the main sensitive factors affecting fugacity model.(4)The grid fugacity model and human health risk assessment model(RAGS model)were used to evaluate and warn the status quo of PAHs health risks in groundwater.The results of current assessment showed that PAHs in groundwater had a mild health risk,and the route of exposure to skin contact was greater than that of oral ingestion,and that of adults was greater than that of children.The results of risk warning showed that with the deterioration of ecological environment,the concentration of PAHs in groundwater and the level of health risk increased gradually.For different populations,the cancer risk of adults changed from mild to moderate,and that of children changed from no risk to mild.The health risk level of the former was greater than that of the latter.For different exposure routes,the oral ingestion route changed from no risk to no-mild risk,and the skin contact route changed from no-mild risk to mild-moderate risk,and the health risk of the former was greater than that of the latter.For non-carcinogenic health risks,they are negligible.If the ecological environment deteriorates,the warning level of health risks in adults will change from blue to orange,and that in children will change from green to blue.Through the above research work,the influence mechanism of different rainfall intensity on the migration regularity of PAHs in the water environment of underground river system was revealed,a grid-based multi-media fugacity model of karst underground river system was established,which solved the construction defects of traditional fugacity model,improved the simulation accuracy,and explored a new quantitative assessment method of human health risk.It solves the key problem of fine early-warning simulation of human health risks,which has important theoretical and practical significance for karst environment protection.Meanwhile,accurate pollution prevention and control can effectively reduce the economic cost of pollution control. |
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