Distribution Characteristics,Source Apportionment And Risk Assessment Of PAHs In Surface Soil Of Gas Stations

During the construction,operation,and decommissioning of gas stations,soil environmental pollution,represented by polycyclic aromatic hydrocarbons（PAHs）,may occur,resulting in potential ecological and environmental hazards and human health risks.However,relevant research is relatively lacking.Therefore,this article takes Lanzhou gas station as an example,systematically collecting 41 soil surface samples under cement hard sealing,and using gas chromatography mass spectrometry（GC-MS）to determine the content of 16 PAHs,The characteristics of its composition and structure are analyzed.The spatial distribution characteristics were discussed using the Kriging interpolation method based on GIS.According to the"Standard for Soil Environmental Quality,Soil Pollution Risk Management and Control of Construction Land"（GB 36600-2018）,the pollution degree was evaluated using the single factor pollution index and the Nemero comprehensive pollution index method.The possible sources were scientifically and systematically analyzed using a combination of characteristic ratio method,principal component analysis（PCA）,and positive definite matrix factor model（PMF）methods.The ecological risk and human health risk were assessed using toxicity equivalent method,risk entropy method,and lifetime carcinogenic risk method,respectively.The results show that:（1）The content,composition,and spatial distribution characteristics of PAHs in the surface soil of Lanzhou gas station.The total content of PAHs in the soil of the study area is 2.62～2421.50μg/kg,with an average value of 215.26μg/kg,of which 5 rings（29.29%）>4 rings（28.46%）>6 rings（19.26%）>3 rings（15.73%）>2 rings（7.27%）.The high value areas of PAHs content in the surface soil of the study area are mainly concentrated in the Xigu area.PAHs generated during high-temperature pyrolysis and polymerization production may enter the soil through atmospheric dry and wet deposition.（2）According to the"Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard"（GB 36600-2018）released by the Ministry of Ecology and Environment of China,there are no exceeding standards for 16 types of PAHs in the soil of Lanzhou gas stations,and the overall environmental quality status is good.Compared with the Dutch standard,the single factor pollution index of Nap（1.04）and Fla（1.06）in the surface soil of the study area indicates that they are at a moderate pollution level,while other PAHs are in a low pollution state.The Nemero comprehensive pollution index is 1.13,indicating the presence of light pollution;Compared with Canadian standards,all individual PAHs are at low pollution levels,with a Nemero comprehensive pollution index of 0.19,indicating a lower level of surface soil pollution at gas stations.Therefore,the risk of surface soil pollution at gas stations in Lanzhou deserves attention.（3）The sources of PAHs in the topsoil of Lanzhou gas stations are complex.The characteristic ratio of PAHs shows that the sources of PAHs in the study area are relatively complex,and are jointly affected by coal and biological combustion sources,petroleum combustion sources,and petroleum sources.The PCA analysis results showed that they were derived from traffic exhaust emissions,volatile sources of gasoline and diesel leakage（69.58%）,and coal combustion sources（10.58%）,respectively.PMF analysis shows that the main sources of PAHs are gasoline combustion sources（47.62%）,diesel combustion sources（28.40%）,coal combustion sources（17.18%）,gasoline and diesel volatilization,and oil leakage sources（6.80%）,consistent with the structure of oil products sold by gas stations.It can be seen that the results obtained by different source analysis methods are basically consistent,while the PMF model method provides more detailed and accurate results than traditional source analysis methods.（4）The ecological risk of PAHs in the topsoil of Lanzhou gas stations is not significant.The toxicity equivalent method showed that the range of w(BaP_eq)for 16PAHs was ND～371.671μg/kg,with an average value of 38.604μg/kg.The w(BaP_eq)of seven carcinogenic PAHs is ND～366.450μg/kg,with an average value of 38.302μg/kg.Accounting for 99.22%of the total w(BaP_eq)mass fraction.It can be seen that they have a high toxic equivalence factor and are the main contributor to the toxic effects of PAHs in the topsoil of the study area.The risk entropy analysis shows that Fla,Chry,BkF,BaP,DahA,InP,and BghiP in the study area are at low risk levels,while other PAHs are at medium risk levels.And the RQ of the total PAHs in the study area＿（NCs）values range from 0 to 51.76,RQ＿The（MPCs）value is 0 to 0.78,which is generally at a low risk level.（5）The lifetime carcinogenic risk of PAHs in the topsoil of Lanzhou gas stations deserves attention.The ILCRs values for the three exposure routes of PAHs in the study area were skin exposure>intake>respiration.The average ILCRs values for dietary intake,skin exposure,and respiratory exposure in adults were higher than those in children because the respiratory rate and duration of exposure in adults were higher than those in children.The combined CR value of PAHs exposure in the soil of gas stations in the study area for adults and children,as well as men and women,is 10^-6or less,so it can be considered that there is no risk or the risk can be ignored.（6）Suggestions for the prevention and control of potential pollution risks of PAHs in the topsoil of Lanzhou gas stations.According to the analysis of the pollution characteristics of PAHs in the soil of gas stations,as Xigu District is a typical industrial area in the northwest region,resulting in a high content of PAHs in the soil of gas stations in this area,pollution prevention and control measures should be implemented in this area to reduce the potential diffusion risk caused by the infiltration of PAHs and other pollutants into the soil.According to the source analysis of PAHs in the soil of gas stations,the main sources of pollution in gas stations are the combustion of fossil fuels such as gasoline and diesel,coal combustion,volatilization of gasoline and diesel,and oil leakage.Strengthen the promotion of clean energy and clean production,and explore and study the closed-loop process of gas stations to reduce the generation of pollutants and prevent soil pollution.According to ecological risk assessment and health risk assessment,the soil pollution level of Lanzhou gas station is non pollution or light pollution.Considering the cumulative effect of PAHs,when the pollution intensity is high and the content of small molecular pollutants is high,rainwater and sewage will migrate to underground soil or aquifer,causing potential pollution to soil and groundwater.Therefore,monitoring points should be set up for underground concealed facilities and surrounding areas of gas stations,and the monitoring depth should be lower than the bottom of the concealed facilities and equipment for continuous monitoring.