(6e) A Modeling Study of the Transient Process of the Fate and Transport for Vapor Intrusion

The inhalation of volatile organic compounds vapors that intrude into buildings from subsurface contaminant sources, so-called vapor intrusion, has resulted in recent health and safety concerns. With the approximately 500,000 contaminated sites in the United States having uncertain vapor intrusion risk, the assessment of inhalation of the vapors is a pervasive topic of regulations, field investigations and the fate and transport simulations. While previous simplified steady state transport models predict some parts of the large variation of field measurement, transient studies can be used to illustrate the temporal changes at an actual site.

In this study, a detailed three-dimensional numerical model was used to simulate the transient vapor intrusion process from a changing groundwater source underneath the building basement, based on our previous study on soil moisture. The amounts of time to get to steady state vapor concentration distribution were compared for the different soil properties, which mainly determine the effective total diffusivity parameters for contaminant transfer through soils. The results illustrated that comparing to a dry sandy soil, a wet clay soil required longer time to transport the vapors from the source into the basement. Further, comparing to previous models with homogeneous soil, the low diffusivity at the capillary zone greatly resisted the contaminant transport, and thus elongated the time scale to get to the steady state. The transport model chose tetrachloroethylene as a typical chlorinated subsurface contaminant, and was based on contaminant molecular diffusion and mechanical dispersion.