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 part describing vapor transport in the soil and the other describing 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.

FIGURE 1

Simulated contaminant concentration profile based on ASU 3-D numerical model (α is groundwater source vapor-to-indoor air contaminant concentration attenuation factor and Q_s is soil gas entry rate) (21).

FIGURE 2

3-D simulation results on the contaminant entry rate to sub-structure release rate for 2m (solid points) and 0.1m (open points) depth foundations. The values in the legend refer source depth (m) and results are shown for various soil permeabilities (k). M_ck refers to the mass entry rate through a foundation crack, and M_released is total contaminant release from the source lying directly beneath the building footprint (49). Any ordinate value different from unity indicates that one basic assumption of 1-D models will lead to errors in estimates of contaminant entry rate.