Investigating The Influence Of Source Type?soil Texture And Capillary Fringe In Petroleum Vapor Intrusion Risk Assessment Based On Numerical Model

The rapid development of urbanization strategy such as"shifting from labor-intensive industry to service economy","retreat from the city into the industrial park"has caused numerous contaminated sites.While resolve these contaminated sites effectively must depend on scientific and reasonable health risk assessment.According to the "Technical guidelines for risk assessment of contaminated sites" published by ministry of environmental protection,vapor intrusion,Le.,volatile conpounds diffused from the polluted soil or groundwater and entered the building is the main exposure pathway to human,this process has determined the repair target and corresponding risk control measures about volatile pollutants.Petroleum vapor intrusion(PVI)will be easier influenced by surrounding environment in vapor migration so that it should be studied further.In this research,two vapor source types as LNAPL and dissolved phase were selected as study object,the influence of the environmental factors such as soil texture and capillary fringe effect has been investigated by developing the numerical PVI model.In addition,the PVI database reported by U.S.EPA(Environmental Protection Agency)also has been used to validate the simulating results.This study have obtained the vital environmental factors that will influence PVI risk and improved the accuracy of PVI model,the existed vertical risk screening distance also has been revised and provided a certain technical basis for risk screening in PVI.The primary conclusio ns were as followed:(1)In the research for the LNAPL source,the numerical model to simulating the PVI with different scenarios were conducted for three soil types(sand,sandy loam and clay)and two source depth(3 m and 8 m),thern the analytical calculations,and the statistical analysis of PVI database reported by EPA were performed to investigating the role of soil texture in PVI,and validating the 15 feet(4.6 m)vertical risk screening distance proposed by EPA.The results indicated that vapor biodegradation will be affected by soil texture and the vapor concentration will obtain the maximum attenuation when soil type is clay.In the anaerobic zone,the normalized vapor concentration profiles in different soil type scenarios were similar,so that soil texture hardly has influence when biodegradation is weak.Whereas in the aerobic zone,the fine-grained soils will attenuated more rapidly because it's higher moisture content and shorter diffusive reaction length.These results were also agree with the PVI database.Thus,the vertical risk screening distance can be corrected as 3 m and 5 m for fine and coarse-grained soils,respectively.(2)In the research for dissolved phase source,the numerical model simulating vapor originated from the contaminated groundwater has been constructed for three soil types as sand,sandy loam and clay.The model has simulated three different scenarios to investigate the effect of capillary fringe which above the groundwater level,i.e.,homogeneous soil which do not consider capillary fringe;vapor source at the groundwater level;and vapor source located at 1 m above the groundwater level These results will used to validate the 6 feet(1.8 m)vertical risk screening distance recommended by EPA.For the source located at groundwater level scenario,vapor concentration attenuation has significantly larger than homogeneous soil scenario,the main reason is that capillary fringe effect can decreasing vapor upward diffusion rates and enhancing aerobic biodegradation.In addition,all of the results in this scenario were basically not exceeding the indoor air risk screening level,so that in this scenario there has high probability that PVI risk will not emerge,this is also verified the recommended screening distance of EPA.But when source located 1 m above the groundwater level,indoor vapor concentration has increased substantially and sand has the highest value which similar to the homogeneous soil scenario.Therefore,ornce the groundwater level has fluctuated and form the residual vapor source in the upper soil,the smear zone caused by that will greatly increasing the PVI risk,the applicability of screening distance will also greatly reduced,this situation should be more concentrated in site investigation.