A Review of Vapor Intrusion Models

Abstract

A complete vapor intrusion (VI) model, describing vapor entry of volatile organic chemicals (VOCs) into buildings located on contaminated sites, generally consists of two main parts-one describing vapor transport in the soil and the other its entry into the building. Modeling the soil vapor transport part involves either analytically or numerically solving the equations of vapor advection and diffusion in the subsurface. Contaminant biodegradation must often also be included in this simulation, and can increase the difficulty of obtaining a solution, especially when explicitly considering coupled oxygen transport and consumption. The models of contaminant building entry pathway are often coupled to calculations of indoor air contaminant concentration, and both are influenced by building construction and operational features. The description of entry pathway involves consideration of building foundation characteristics, while calculation of indoor air contaminant levels requires characterization of building enclosed space and air exchange within this. This review summarizes existing VI models, and discusses the limits of current screening tools commonly used in this field.

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

“…The subsurface measurements that are captured in this database include the subslab vapor concentration c ss , the soil gas vapor concentration c sg , and the vapor concentration c gw calculated assuming equilibrium with groundwater contaminant concentration. The large variation in values of the ratio c ss /c gw as noted by EPA has been further examined in recent publications Yao et al, 2012Yao et al, , 2013bYao et al, , 2013c.…”

Oxidation of trichloroethylene, toluene, and ethanol vapors by a partially saturated permeable reactive barrier

“…The effective gas diffusion is influenced by the pore space tortuosity, which itself depends on porosity and the volumetric air content USEPA, 1993). The effective gas diffusion coefficient of VOC vapor in the permeable reactive layer was expressed as follows (Millington and Quirk, 1961;Pennell et al, 2009;Yao et al, 2013):…”

“…The effective diffusion coefficient of the target compound assumed to be given by a similar equation as for air (Pennell et al, 2009;Yao et al, 2013):…”

Steady-state analytical model for vapour-phase volatile organic compound (VOC) diffusion in layered landfill composite cover systems

“…Falta et al (1989) and Mendoza and Frind (1990) demonstrated that density-driven flux can sometimes dominate gas phase transport of VOCs with high vapour pressures and high molecular weights in an unsaturated zone with high permeability. You and Zhan (2013) Transport of VOCs in the landfill cover system is also shown to be primarily induced by diffusion (Yao et al 2013;You et al 2013;Shackelford 2014;Su et al 2015). Su et al (2015) demonstrated that toluene diffusion might be the main reason for the toluene reduction in the deeper zones of landfill covers.…”

“…Choi et al (2002) concluded that atmospheric pressure changes can induce gas flow in the unsaturated zone to such an extent that the advective flux of organic vapours in unsaturated zone soil gas can be significant relative to the gas phase diffusion flux of these organic vapours. Falta et al (1989) and Mendoza and Frind (1990) Transport of VOCs in the landfill cover system is also shown to be primarily induced by diffusion (Yao et al 2013;You et al 2013;Shackelford 2014;Su et al 2015). Su et al (2015) demonstrated that toluene diffusion might be the main reason for the toluene reduction in the deeper zones of landfill covers.…”