| Volatile organic compounds?VOCs?usually refer to a group of organic pollutants with corresponding volatility and boiling points between 50-260?.A large number of studies have shown that VOCs are important precursors of ozone and secondary organic aerosols and have an important impact on the chemical composition of the atmosphere.In addition,epidemiological investigations have found associations between VOCs exposure and increased cancer risk as well as poor reproductive outcomes.Among various types of volatile organic compounds,BTEXS and volatile halogenated hydrocarbons?VHCs?have been widely detected,which become a key issue in the field of environment.At present,researches on VOCs are mostly focused on the atmosphere and water bodies,but the investigation and research on soil VOCs is still very scarce and scattered.Soil is an important component of the natural environment,of which farmland soil with planting functions is the basis of agricultural production.It is worth noting that soil VOCs pollution is often concealed,accumulative,diverse,and highly toxic.Therefore,it is of urgent and practical significance to establish a preliminary list of typical VOCs residues in Chinese agricultural soils and comprehensively evaluate the risks that may occur at this level.In this paper,agricultural soil samples were collected nationwide at similar sampling points in 2013 and 2016.By optimizing the headspace-gas chromatography-mass spectrometry?HS-GC/MS?method,we determined and analyzed the residual concentrations of typical VOCs?including BTEXS and VHCs?in soil,clarified their spatial distribution characteristics,further explored their possible sources,and based on this,used a health risk assessment model to evaluate the health risks by exposuring to soil BTEXS and VHCs.The main results and conclusions are as follows:?1?BTEXS contamination was commonly detected in both 2013 and 2016 soil samples.In 2013 and 2016,the highest total residual concentrations of BTEXS in the surface soil were 375 and 569 ng/g,respectively.The relative proportion of BTEXS ratios were consistent between two different sampling time,which suggested BTEXS pollution rarely dissipated or accumulated during past few years.The total concentration of BTEXS was mainly higher in the central,northeast and southwestern regions of China.Toluene was the most abundant contaminant in more than half of the samples.Northeastern China was the “hottest” spot for the toluene residue level.We found that the spatial tendencies of BTEXS were quite similar with some typical organochlorine pesticide?OCPs?,which may be related to BTEXS as an additive for some pesticides.?2?The total concentration of BTEXS could be an essential factor influencing the residue status of individual compound in soils.Soil organic matter,temperature,precipitation,and altitude at the sampling site have a certain effect on the residual distribution of BTEXS.Compared with other BTEXS,benzene displays high water solubility and volatility.Therefore,volatile diffusion is more likely to occur in areas with low altitude,high temperature and high precipitation.?3?All highest concentrations of individual BTEXS compounds were observed in the surface layer.Concentrations of BTEXS were decreased exponentially along the depth of soil columns.Among them,significant decreasing trend were observed for toluene,m,p'-xylene and o'-xylene between top,middle and bottom layers,suggesting that they are more restricted in penetrating deeper into soil column due to lower water solubility and higher log Kow values.?4?The health risk assessment model was employed in this study to assess the deterministic exposure risks of BTEXS in farmland soils.In general,HIs for adults and children at all the sites surveyed were below 1,indicating that non-cancer risks for BTEXS are absent in Chinese arable soils.In addition,the estimated cancer risk levels of benzene to adults and children were all below 10-6,implying that very low cancer risks to both adults and children.?5?In this study,13 of the 18 VHCs investigated were detected,the haloalkanes were dominant in most samples across China.The five most frequently detected compounds included chloroform?CF??99.11%?,1,2-dichloroethane?1,2-DCA??71.43%?,dichloromethane?DCM??65.18%?,chlorobenzene?CB??37.50%?and1,4-dichlorobenzene?1,4-DCB??38.39%?.Among the five main detected substances,the distribution of DCM,CF and 1,2-DCA shared a similar pattern,with North and Southwest China showing the most contamination.Specifically,the pollution is mainly concentrated in Shanxi Province and Chongqing municipality.The hotspots of1,4-DCB were in Northeast China.In addition,the Northeast region also showed high levels of CB in the soil.?6?Commercial solvent volatilization release and industrial production leakage may be the potential major source of soil chloroalkanes.Compared with chloroalkanes,the residual amount of chlorobenzenes in soil was significantly related to the amount of pesticides used?P?0.001?,and the correlation between 1,4-DCB and pesticides was the best?R=0.693,P<0.001?,followed by CB?R=0.607,P=0.001?.This might be another piece of evidence supporting pesticides as a source of VHCs,especially CB and 1,4-DCB,in agricultural soil across China.?7?In general,the typical VHCs in farmland soils may not trigger a non-carcinogenic risk to local residents.Among the five main compounds detected in VHCs,the HIs for CF were the highest both for children and adults.In addition,the carcinogenic risks of the five VHC components in Chinese agricultural soil were all at acceptable levels(<1×10-6)for children and adults.The results of this study can provide data support for us to understand the sources and possible consequences of VOCs pollution in farmland soils of China,with a view to controlling soil pollution more effectively. |
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