Modeling and field evidence of pressure-driven entry of soil gas into a house through permeable below-grade walls

Abstract

Modeling and field evidence are presented which indicate that soil gas can enter houses with basements at significant rates through permeable below-grade walls. Entry via this previously neglected pathway could result in elevated indoor concentrations of radon and other pollutants. Using artificial depressurization of the basement ( -25 to -30 Pa), field measurements were made of pressure-coupling between a basement and the surrounding soil and of soil-gas entry into the house . A two-dimensional, steady-:-state finite element model of fluid flow through porous media was used to simulate the experimental conditions, assuming air flow occurs through permeable substructure walls. The model predicts a soil-gas entry rate of 2.5 m 3 hr -l. The best estimate of soil-gas entry, based on field tracer-gas studies, was 4 m 3 hr-1 , with a range of 1.5-12 m 3 hr-1 . The soil was modeled with and without a lowpermeability layer just above basement floor level. The layered-soil model explains high pressure-coupling observed at 3-m depth out to 14 m west of the house.

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

“…When considering mechanical dispersion (often simply termed dispersion) (Gelhar et al, 1992) induced by groundwater flow, the most relevant process in the contaminant vapor transport through soils is in the transverse vertical direction (here, the z-direction) (Klenk and Grathwohl, 2002;Olsson and Grathwohl, 2007). For groundwater flow in the unsaturated zone, Atteia and Hohener (2010) suggested modifying the Millington-Quirk equation as shown in Equation (9).…”

“…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.…”

Scale Dependence of Soil Permeability to Air: Measurement Method and Field Investigation

“…By the same criterion, houses, because of their large size, tend to interact with soils at a scale of ~ 3 -10 m [Nazaroff et al, 1987;Garbesi and Sextro, 1989;Turk et al, 1990].…”

The Influence of a Subslab Gravel Layer and Open Area on Soil-Gas and Radon Entry Into Two Experimental Basements

“…In tlle extreme, cracks with a combined area of 1.5 m 2 were found in a house in New Jersey (Turk et al 1991a). In addition to flow tllrough cracks, there may also be significant bulk soil-gas flow through basement walls constructed out of a high permeability material such as hollow concrete blocks (Garbesi and Sextro 1989;Ruppersberger 1991).…”