Effect of Vapor Source−Building Separation and Building Construction on Soil Vapor Intrusion as Studied with a Three-Dimensional Numerical Model

Abstract

A three-dimensional numerical model of the soil vapor-to-indoor air pathway is developed and used as a tool to anticipate not-yet-measured relationships between the vapor attenuation coefficient, alpha (indoor air concentration/source vapor concentration), and vapor source-building lateral separation, vapor source depth, and building construction characteristics (depth of building foundation) for nondegrading chemicals. The numerical model allows for diffusive and advective transport, multicomponent systems and reactions, spatially distributed foundation cracks, and transient indoor and ambient pressure fluctuations. Simulations involving different lateral separations between the vapor source and building show decreasing alpha values with increasing lateral separation. For example, alpha is 2 orders of magnitude less when a 30 m x 30 m source located 8 m below ground surface is displaced from the edge of the building by 20 m. The decrease in alpha with increasing lateral separation is greater for shallower source depths. For example, alpha is approximately 5 orders of magnitude less when a 30 m x 30 m source located 3 m below ground surface is displaced from the edge of the building by 20 m. To help visualize the effects of changing vapor source-building separations, normalized vapor concentration contour plots for both horizontal and vertical cross sections are presented for a sequence of lateral separations ranging from the case in which the 30 m x 30 m source and 10 m x 10 m building footprint centers are collocated to shifting of the source positioning by 50 m. Simulations involving basement and slab-on-grade constructions produce similar trends. In addition, when buildings are overpressurized to create outflow to soil gas on the order of 1-3 L/min, emissions to indoor air are reduced by over 5 orders of magnitude relative to intrusion rates at zero building underpressurization. The results are specific to simulations involving homogeneous soil properties, nondegrading chemicals, steady source concentrations and building underpressurizations, and the geometries studied in this work.

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.…”

“…In other words, there is no simple linear dependency observed between the source vapor concentration and the soil gas vapor concentration. The weak dependency can be the consequence of various specific site and sampling conditions, such as sampling locations (Abreu and Johnson, 2005), source concentration distribution (Yu et al, 2009), soil heterogeneity , open ground surface capping effects , transient effects, such as groundwater fluctuations (Picone et al, 2012), barometric changes (Garbesi and Sextro, 1989;McHugh et al, 2012), and so on. On the other hand, there is one major factor that often does not receive sufficient attention-that of soil moisture content.…”

Vapour intrusion from the vadose zone—seven algorithms compared

“…Screening-level algorithms should also not be overly conservative because they might have insufficient discriminative power. There has been little evaluation of the false-negative (or type II) error produced by the algorithms at field sites, with the possible exception of the widely studied Johnson and Ettinger model (JEM) (Fitzpatrick and Fitzgerald 2002;Johnson et al 2002;Hers et al 2003;Abreu and Johnson 2005).…”

Coupling between geochemical reactions and multicomponent gas and solute transport in unsaturated media: A reactive transport modeling study

“…These models show that viscous gas transport can be caused by a variety of processes including the injection and extraction of air or vapor [e.g. Massmann, 1989;Falta et al, 1992], the volatilization of organic compounds [e.g., Falta et al, 1989;Mendoza and Frind, 1990;Gaganis et al, 2004], sustained underpressurization relative to atmospheric conditions in basements of buildings and the adjacent sediments [e.g., Hers et al, 2002;Abreu and Johnson, 2005], barometric pumping [e.g., Massmann and Farrier, 1992], displacement due to infiltrating recharge water [e.g., Celia and Binning, 1992], and temperature changes [e.g., White, 1995].…”