A two‐dimensional analytical model of vapor intrusion involving vertical heterogeneity

Yao, Yijun; Verginelli, Iason; Suuberg, Eric M.

doi:10.1002/2016wr020317

Cited by 31 publications

(18 citation statements)

References 35 publications

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“…This can result in significantly different predictions of subfoundation soil gas concentration profiles than those with a more realistic continuous variation of moisture content. This concern is further confirmed by a newly published study (Yao et al, 2017), which reported that the predictions of USEPA spreadsheet are higher than the full three-dimensional (3-D) simulations by one to two orders of magnitude in scenarios involving groundwater sources. Although the above studies provided some interesting insights into the influence of soil properties such as moisture content and permeability in VI, there has been no comprehensive investigation of the role of soil texture in the soil gas concentration attenuation processes in scenarios involving groundwater sources.…”

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confidence: 60%

Investigating the Role of Soil Texture in Vapor Intrusion from Groundwater Sources

et al. 2017

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Soil texture is believed to play a significant role in the migration of subsurface volatile chemicals into buildings at contaminated sites, an exposure process known as vapor intrusion (VI). In this study, we investigated the role of soil texture in determining the attenuation of contaminant soil gas concentration from groundwater source to receptor building. We performed soil column experiments, numerical simulations, and statistical analysis of the USEPA’s VI database. The soil column experiments were conducted with commercial sand and soils with sand and sandy loam textures. Measured one-dimensional soil gas concentration profiles were compared with numerical predictions. Good agreement between experiments and model results supports the use of the classical multiphase chemical transport equation for simulating contaminant vapor transport from groundwater through the vadose zone. A full three-dimensional numerical model was then used to simulate typical VI scenarios with groundwater sources. Results indicate that, although soil particle texture can play a role in determining subslab-to-indoor air concentration attenuation, there is no obvious relationship between soil particle size and groundwater source-to-subslab except in the case of a shallow contaminant source. This conclusion is consistent with results reported in USEPA’s VI database, in which variation in soil particle size does not affect source-to-subslab attenuation factors but does influence subslab-to-indoor air concentration attenuation factors by an average of about 0.4 order of magnitude. This finding suggests that an appropriate focus of VI site investigation should include the shallow soil beneath the building foundation.

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confidence: 60%

Investigating the Role of Soil Texture in Vapor Intrusion from Groundwater Sources

et al. 2017

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“…Due to such complexity, a full 3‐D finite element model is used to study the influences of such a preferential pathway on both of these measures. The 3‐D numerical model applied here was developed with COMSOL Multiphysics and has been presented in previous publications (Bozkurt et al, 2009; Pennell et al, 2009; Yao et al, 2011, 2012, 2013a, 2013b, 2013c, 2015, 2017a, 2017b). The numerical model includes two modules, Darcy's law and chemical transport in porous media.…”

Section: Methodsmentioning

confidence: 99%

Three‐Dimensional Simulation of Land Drains as a Preferential Pathway for Vapor Intrusion into Buildings

Yao

Mao

et al. 2017

J of Env Quality

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Preferential pathways can be significant vapor intrusion (VI) contributors, causing potentially higher inhalation risk to residents of affected buildings than that arising through traditional intrusion pathways. To assess land drains as a preferential pathway, a three-dimensional model, validated using data from a 4-yr field study, was used to study the roles of subfoundation soil permeability on soil gas flow and indoor depressurization. Results indicated that it is almost impossible for an indirect preferential pathway like a land drain ending in subfoundation soils with a permeability <10−11 m2 to affect indoor air quality if the land drain connects to a source with the same vapor concentration as that of the groundwater source beneath the building. An equation was developed to estimate the threshold permeability. We also found that even after the preferential pathway was identified using indoor depressurization (also known as controlled pressure method [CPM]) and then turned off, the influence of the preferential pathway and indoor depressurization on indoor concentration might last for months, although it may not be significant (i.e., may not exceed one order of magnitude, in this study). In the absence of such a preferential VI pathway, CPM may actually reduce indoor air concentrations of contaminants below those present under natural indoor pressure conditions, due to the emission rate limit determined by the upward diffusion rate from the vapor source. Our study highlights the role of measuring subfoundation soil permeability to soil gas flow in site investigations and warns practitioners about the possible mischaracterization of indoor air concentration after applying CPM in the absence of a preferential pathway.

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“…The EPA-JEM, a modified version of the Johnson-Ettinger model developed by USEPA, treats the capillary zone and unsaturated vadose zone soil as two separate, but uniform, layers (USEPA 2017b). The resulting model predicts attenuation factors that are up to two orders of magnitude higher (i.e., more conservative) than those predicted by more robust, three-dimensional numerical simulations and a corresponding two-dimensional analytical model (CVI2D) (Yao et al 2017a). The latter model employs a continuous soil moisture content profile calculated using the van Genuchten equation (van Genuchten 1980), and this same approach is now routinely used in most three dimensional VI computational models.…”

Section: The Use Of a VI Screening Modelmentioning

confidence: 96%

Examining the Use of USEPA's Generic Attenuation Factor in Determining Groundwater Screening Levels for Vapor Intrusion

Yao

Verginelli

Suuberg

et al. 2018

Groundwater Monitoring Rem

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A value of 0.001 is recommended by the United States Environmental Protection Agency (USEPA) for its groundwater-to-indoor air Generic Attenuation Factor (GAFG), used in assessing potential vapor intrusion (VI) impacts to indoor air, given measured groundwater concentrations of volatile chemicals of concern (e.g., chlorinated solvents). The GAFG can, in turn, be used for developing groundwater screening levels for VI given target indoor air quality screening levels. In this study, we examine the validity and applicability of the GAFG both for predicting indoor air impacts and for determining groundwater screening levels. This is done using both analysis of published data and screening model calculations. Among the 774 total paired groundwater-indoor air measurements in the USEPA’s VI database (which were used by that agency to generate the GAFG) we found that there are 427 pairs for which a single groundwater measurement or interpolated value was applied to multiple buildings. In one case, up to 73 buildings were associated with a single interpolated groundwater value and in another case up to 15 buildings were associated with a single groundwater measurement (i.e, that the indoor air contaminant concentrations in all of the associated buildings were influenced by the concentration determined at a single point). In more than 70% of the cases (390 of 536 paired measurements in which horizontal building-monitoring well distance was recorded) the monitoring wells were located more than 30 meters (and some up to over 200 meters) from the associated buildings. In a few cases, the measurements in the database even improbably implied that soil gas contaminant concentrations increased, rather than decreased, in an upward direction from a contaminant source to a foundation slab. Such observations indicate problematic source characterization within the dataset used to generate the GAFG, and some indicate the possibility of a significant influence of a preferential contaminant pathway. While the inherent value of the USEPA database itself is not being questioned here, the above facts raise the very real possibility that the recommended groundwater attenuation factors are being influenced by variables or conditions that have not thus far been fully accounted for. In addition, the predicted groundwater attenuation factors often fall far beyond the upper limits of predictions from mathematical models of VI, ranging from screening models to detailed computational fluid dynamic models. All these models are based on the same fundamental conceptual site model, involving a vadose zone vapor transport pathway starting at an underlying uniform groundwater source and leading to the foundation of a building of concern. According to the analysis presented here, we believe that for scenarios for which such a “traditional” VI pathway is appropriate, 10−4 is a more appropriately conservative generic groundwater to indoor air attenuation factor than is the EPA-recommended 10−3. This is based both on the statistical analysis of USEPA’s VI da...

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A two‐dimensional analytical model of vapor intrusion involving vertical heterogeneity

Cited by 31 publications

References 35 publications

Investigating the Role of Soil Texture in Vapor Intrusion from Groundwater Sources

Investigating the Role of Soil Texture in Vapor Intrusion from Groundwater Sources

Three‐Dimensional Simulation of Land Drains as a Preferential Pathway for Vapor Intrusion into Buildings

Examining the Use of USEPA's Generic Attenuation Factor in Determining Groundwater Screening Levels for Vapor Intrusion

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