Fluid Flow Model for Predicting the Intrusion Rate of Subsurface Contaminant Vapors into Buildings

Abstract

A new method is presented for calculating a building-specific subslab to indoor air attenuation factor for use in assessing subsurface vapor intrusion to indoor air. The technique includes (1) subslab gas extraction with flow and vacuum measurements and mathematical modeling to characterize the bulk average vertical gas conductivity of the floor slab, (2) monitoring of the ambient pressure gradient across the floor slab with a micromanometer, (3) calculating the volumetric flow of soil gas into the building ( Q), and (4) dividing Q by the building ventilation rate ( Q) to calculate a building-specific attenuation factor. Sample calculations using order statistics from 121 individual tests are comparable to the U.S. Environmental Protection Agency empirical attenuation factors for residential buildings and the U.S. Navy empirical attenuation factors for commercial/industrial buildings. A case study of a commercial building shows encouraging agreement between the attenuation factors calculated via this method and via conventional subslab and indoor air sampling.

A Review of Recent Vapor Intrusion Modeling Work

“…Holton et al (2018) used a 3D numerical model combined with field data to investigate the role of indoor sources showing that indoor sources can generate subsurface soil gas clouds that it may take days to weeks under natural conditions to dissipate following indoor source removal. McAlary et al (2018) presented a fluid model to calculate building-specific attenuation factors using subslab flow and vacuum measurements, building pressure, and building ventilation rates. Later, McAlary et al (2020) developed a mathematical model to assess the radius of influence for subslab venting systems based on mass flux instead of static vacuum distribution.…”

Simplified Approach for Calculating Building‐Specific Attenuation Factors and Vapor Intrusion Mitigation System Flux‐Based Radius of Influence

“…The authors have conducted 176 tests of PFE in commercial/industrial buildings using methods described in Data S1. Each PFE test in the data set was analyzed using the two-layer Hantush-Jacob model to develop T and B values following protocols described by Thrupp et al (1996Thrupp et al ( , 1998 and McAlary et al (2010McAlary et al ( , 2018. Each high volume sample (HVS) test results in one unique pair of T and B values.…”

“…Not all buildings are equally susceptible to VI, so it is valuable to consider BSAFs. McAlary et al (2018) derived a relatively simple equation:…”

Subslab Depressurization Versus Subslab Ventilation: Insights from Recent Research

“…This spreadsheet analysis and interpretation can be performed in real‐time during field data collection which enables data adequacy to be addressed efficiently. The model assumptions and application to subslab gas flow are described in McAlary et al 2018, 2020 and ESTCP ER‐201322 2019.…”

“…Conceptual model for two‐layer flow in response to gas extraction below a floor (after McAlary et al 2010, 2018). …”