Simulation of the Vapor Intrusion Process for Nonhomogeneous Soils Using a Three‐Dimensional Numerical Model

Abstract

This paper presents model simulation results of vapor intrusion into structures built atop sites contaminated with volatile or semivolatile chemicals of concern. A three-dimensional finite element model was used to investigate the importance of factors that could influence vapor intrusion when the site is characterized by nonhomogeneous soils. Model simulations were performed to examine how soil layers of differing properties alter soil-gas concentration profiles and vapor intrusion rates into structures. The results illustrate difference in soil-gas concentration profiles and vapor intrusion rates between homogeneous and layered soils. The findings support the need for site conceptual models to adequately represent a site's geology when conducting site characterizations, interpreting field data, and assessing the risk of vapor intrusion at a given site. For instance, in layered geologies, a lower permeability and diffusivity soil layer between the source and building often limits vapor intrusion rates, even if a higher permeability layer near the foundation permits increased soil-gas flow rates into the building. In addition, the presence of water-saturated clay layers can considerably influence soil-gas concentration profiles. Therefore, interpreting field data without accounting for clay layers in the site conceptual model could result in inaccurate risk calculations. Important considerations for developing more accurate conceptual site models are discussed in light of the findings.

Evaluation of site-specific lateral inclusion zone for vapor intrusion based on an analytical approach

“…The simulations suggest that in the absence of surface cover, the most significant attenuation is observed for the arrangement shown in Figure 4 (c), the low-med-high one, while ${\frac{C_{ss}}{c_s}true|}_{\mathit{\text{max}}}$ is highest in Figure 4(d), with an opposite high-med-low arrangement, with a variation of 2–3 orders of magnitude. Similar finding was also reported by a previous modelling study involving only vertical soil gas transport [20]. Moreover, the presence of capillary fringe can induce a similar effective diffusivity distribution as shown in Figure 4(c), indicating the conservative predictions by previous lateral transport models in the absence of surface cover [7, 16–17].…”

“…The development, validation and applications of the 3-D finite element model examined here were already presented in former studies [18–20, 28–33]. In the present study, the model is applied only in a steady-state mode for non-degradable contaminants.…”

“…Previous studies showed that such capping effect can increase the contaminant subslab vapor concentration when source plume is beneath the building foundation [18–19]. On the other hand, the spatial variabilities of soil gas concentration identified in field studies implied a possibility that soil heterogeneities may also play a role [20–26]. In EPA’s spreadsheet version of the Johnson-Ettinger (J-E) model, the van Genuchten parameters for 12 kinds of soil type (SCS soils) are provided so that they can be chosen to apply in a multilayer system [27].…”

Influence of Soil Moisture on Soil Gas Vapor Concentration for Vapor Intrusion

“…The assessment of extent and hazard posed by vapor intrusion of VOCs into buildings has received increasing attention in recent years (Murphy and Chan, 2011;Eklund et al, 2012;McHugh et al, 2012;Picone et al, 2012;Turczynowicz et al, 2012;Wang et al, 2012). With *500,000 contaminated sites in the United States presenting uncertain VOC vapor intrusion risk (Schuver, 2007), the assessment of risk from inhalation of these vapors has been a topic of recent discussion, field investigations (Fitzpatrick and Fitzgerald, 2002;Sanders and Hers, 2006;William et al, 2007), and modeling studies (Abreu and Johnson, 2005;DeVaull, 2007;Tillman and Weaver, 2007;Bozkurt et al, 2009;Pennell et al, 2009;Yao et al, 2011). In 2002, the U.S. EPA issued a draft guidance for vapor intrusion assessment (EPA, 2002), and new final guidance is imminent.…”

Simulating an exclusion zone for vapour intrusion of TCE from groundwater into indoor air

“…Parker (2002) and DeVaull et al (2002) emphasized the significance of biodegradation, while Johnson (2005, 2006) then included biodegradation processes into a three-dimensional numerical model to investigate the impact of building construction, degradation rate, and the NAPL source zone architecture on indoor air concentrations. Robinson and Turczynowicz (2005) and Bozkurt et al (2009) also use fully three-dimensional problem geometries, with the latter examining the impact of multiple stratigraphic units on the fate and transport of contaminants through the vadose into the indoor air. Abreu and Johnson (2005) and Lowell and Eklund (2004) focus on the issue of lateral source zone separation which has direct relevance to the issue of defining an exclusion zone.…”