Time-Variable Simulation of Soil Vapor Intrusion into a Building with a Combined Crawl Space and Basement

Abstract

A time-variable one-dimensional model (called ViM for Vapor Intrusion Model)to predict indoor vapor concentrations in a dwelling with a combined basement and crawl space has been developed. ViM predicts vapor concentrations in each of the three compartments. Volatile chemicals that intrude into the dwelling are assumed to originate from soil, groundwater (where an attenuating plume is simulated), or ambient air. Processes included in the model are advection, diffusion, biodecay, and adsorption in the soil column; transport by diffusion and advection into individual crawl space and basement compartments; advection from each compartment into an overlying dwelling space; and exchange of ambient air and indoor air. The time-variable concentration fields are solved by first transforming the partial and ordinary differential equations into Laplace space, solving the resulting ordinary differential equations or algebraic equations, and numerically inverting those equations. This approach was an expedient way of handling the coupling between the subsurface and the dwelling. ViM was applied to a building (Building 20) located at the former Moffett Field Naval Air Station, in Mountain View, CA. The building is a former bachelor officer's quarters. The shallow groundwater beneath the building is contaminated with a number of volatile chemicals, including trichloroethene, cis-1,2-dichloroethene, and trans-1,2-dichloroethene, all of which were simulated. Using indoor air data collected in 2003-2004, and other field data collected prior to that time, the accuracy of the model's predictions was demonstrated. ViM's results were also compared against a version of the steady-state Johnson and Ettinger model (1) that was modified to accommodate a dwelling with a combined crawl space and basement (called the JEM model in this paper). The predictions from the JEM model were consistently higher than the predictions from ViM, but still near the upper range of the observed data.

Analytical modeling of the subsurface volatile organic vapor concentration in vapor intrusion

“…With the growing awareness of the problem as evidenced by numerous various studies of the problem in recent years (e.g. Johnson, 2002; Hers et al, 2003; Karapanagioti et al, 2003; Sanders and Hers, 2006; Brand et al, 2007; Mills et al, 2007; McAlary et al, 2009; Eklund et al, 2012; McHugh et al, 2012), there is a strong impetus to better quantitatively characterize vapor intrusion processes.…”

Simulating the effect of slab features on vapor intrusion of crack entry

“…This issue cannot be avoided for research no matter with focus on soil vapor transport [1–7], indoor air concentration [8–11] or both [12–18]. Two general hypotheses have been used in the analysis of the process.…”

“…Two general hypotheses have been used in the analysis of the process. One is to assume that contaminants enter through a permeable concrete slab [8–9, 18], and the other involves assuming existence of a crack or cracks in the slab as the main entry pathway for soil vapor [1–7, 12–14, 19–21]. The use of the former is limited, largely due to the generally accepted low permeability of the typical concrete slab, while the latter was developed for radon intrusion studies, and later widely employed in chemical vapor intrusion studies [22–24].…”

Impacts of changes of indoor air pressure and air exchange rate in vapor intrusion scenarios

“…Several vapor intrusion studies have reported sensitivity analyses on various parameters for different scenarios (Johnson, 2002; Mills et al, 2007). It has been noted in various settings (Johnson, 2002), that the indoor air exchange rate can be important in certain circumstances, but not in others.…”