Spatial Differentiation And Influencing Factors Of PAHs In Urban Soil Columns Of Shanghai

As the important“source”and“sink”for polycyclic aromatic hydrocarbons?PAHs?in urban environment,especially as the predominant growth place of crops,urban soil act as an important role in urban environment system and closely linked to human health.The research on PAHs in soil around the world were mainly focused on the surface soil until now.However,PAHs in surface soil are easily transferred to deeper soil through runoff,infiltration and bioturbution and so on.Besides,urban construction activities have changed soil stuctures intensively,which have profound impacts on the distribution and migration of PAHs in deep soil.Hence,it is conducive to figure out the fate of PAHs in soil via investigating the distribution and migration of PAHs in soil columns.In addition,it could provide theoretical support for urban environment management and human health protection.Therefore,we sampled 24one-meter soil columns collected from power plants?P?,industrial parks?I?and environmental companies?E?.The 16 prioritized PAHs were detected after extracting with the accelerated solvent extractor and purifying with the silica gel column.The influencing factors of PAHs vertical migration were emphatically analyzed combined with the physiochemical characteristics of PAHs and soil edatope.The land use regression models?LUR?were built to analyze the impacts of natural elements and human elements on PAHs.The study provided scientific basis for the construction of harmonious city and the maintenance of residents'health through source identification and risk assessment of PAHs.The main conclusions are as following:?1?The pollution level of 16 kinds of PAHs,which were prior controlled by US EPA,in soil columns was in the range 91.5 to 46834.5 ng/g.The spatial distribution differentiation showed as power plants>industrial parks>environmental companies in horizontal orientation?ANOVA,p<0.01?.Vertical distribution of PAHs in the undisturbed soil columns showed a decreasing trend of PAH concentrations with depth.The ratios of low-molecular-weight PAHs?LMW PAHs,2-ring and 3-ring congeners?were increased with depth while the ratios of middle-molecular-weight PAHs?MMW PAHs,4-ring congeners?and high-molecular-weight PAHs?HMW PAHs,5-ring and 6-ring congeners?were decreased with depth.?2?Anthropogenic factors?land use and artificial disturbance?and edaphic conditions?BC and TOC contents as well as particle size?were capable of co-controlling the distribution and migration of PAHs in soil columns.BC and TOC showed strong correlations with PAHs,especially with HMW PAHs.The higher affinities between PAH-BC indicated that BC is a dominant determinant for the fate of PAHs in soil columns comparing with TOC.PAHs,BC,and TOC showed stronger correlations in P than in I and E.This may imply that pyrogenic combustion sources,e.g.,power plants,could not only co-generate higher concentrations of PAHs and BC but also produce stronger associations of PAH-BC and PAH-TOC.?3?The PAH concentrations in undisturbed urban soil columns were well fitted to the exponential decay model.The migration ability of PAHs was inferred as the chief controlling factor.The concentrations of 16 kinds of PAHs in the infinite depth soil still accounted for a quarter of the initial concentrations?the concentrations of surface or subsurface soil?.It inferred that the capability of vertical migration of PAHs was limited in soil columns.Besides,the higher vertical decreasing rate constants of MMW PAHs and HMW PAHs compared with LMW PAHs demonstrated that LMW PAHs had a greater penetration depth than MMW and HMW PAHs.According to the results of predicted soil depth when PAH concentrations declined to 30%and 50%of the initial concentrations,MMW PAHs and HMW PAHs possessed lower vertical migration ability compared with LMW PAHs.?4?Based on the Spearman correlation analyses,street and transportation area,administration and public services area,commercial and business facilities area and population were related to PAH concentrations positively,while agricultural areas and the shortest distance from the expressway were negative correlated to PAH concentrations.The correlations between population and PAH concentrations increased with the buffer radii and up to maximum at the buffer radium of 3000 m.The closest correlations between land use areas and PAH concentrations were displayed at the buffer radii of 2000 m.The LUR model revealed that population and agricultural areas were significantly contributed to 4-ring PAHs and 5-ring PAHs.The adjusted r values were 0.709 and 0.729?p<0.01?in 4-ring PAHs and 5-ring PAHs respectively.Leave-one-out Cross Validation?LOOCV?results indicated the correlations between measured PAH concentrations and leave-one-out cross validated PAH concentrations were 0.659 and 0.680?p<0.01?in 4-ring PAHs model and 5-ring PAHs model,respectively.Root mean square error?RMSE?values were 0.485 and0.476,respectively.Both LOOCV and RMSE explained the precision of LUR models.?5?Coal combustion and vehicle sources both were identified as major PAH sources in soil columns from the three functional areas based on molecular diagnostic ratio?MDR?and positive matrix factorization?PMF?model.However,the minor contributions of biomass combustion,coking industries,petroleum spill,and natural gas combustion differed slightly in the three functional areas.There was no notable difference in the PAHs sources from different soil layers.It indicated that PAHs were mainly migrated from the upper soil layers to some extent.?6?Risk evaluations were conducted by the Benzo[a]pyrene-equivalent concentration(BaP_eq)method and the incremental lifetime cancer risk?ILCR?model.The total BaP_eq concentrations and the ILCR values showed a general trend as P>I>E.For most sites,the ecological risks were below the threshold safe value of 600 ng BaP_eq g^-1,and ILCR values were at the negligible risk level in I and E.However,dermal contact to soils,especially the deep soil layers in P,have potential health risks,which should draw enough attention.In summary,our findings supplemented knowledge regarding the vertical distribution and migration of PAHs,showed the predictive power of BC and TOC for PAH fate,and revealed the importance of monitoring and managing urban soil in deeper layers not just surface layers.