Vapor Intrusion - Models and Their Value

Vapor Intrusion- Models and Their Value

The problem of vapor intrusion into structures built atop contaminated properties has received increasing attention over the last decade. On the national level, new guidance from the US EPA is imminent, but in the meantime different states have adopted a wide range of policies to address the problem. Many approaches to characterization are empirical, based upon measurements at impacted locations. Since these can involve alarming and intrusive measures involving many residents, some emphasis has been placed on screening, using simple mathematical descriptions of the problem. With the vastly increased availability of powerful fluid dynamics codes, these models can guide more of the site investigation work, as well as evaluate possible possible mitigation strategies. Examples will be given of how such tools can be used to assess both petroleum-based and chlorinated solvent-based situations, based upon recent research carried out in Brown University’s Superfund Research Program. 

The approach involves solving the equations of vapor transport in the subsurface below the structure of interest, which is typically the soil of the vadose zone. The transport is dominated by diffusion, though entry into the structure may be strongly influenced by local advective transport near the building foundation, as a result of small pressure differences between ambient and the indoors. The indoor environment is typically modeled using a CSTR model. Subtle factors can have a very large influence on predicted indoor air exposures. These include effects related to the resistance of a capillary zone in the case of a chlorinated solvent source in the groundwater, or the form of the assumed biodegradation rate law in the case of petroleum contaminants. The models point to what might represent reliable or problematic field measurement strategies. Also, once a reliable site model is developed, the efficacy of mitigation methods such as subslab depressurization, may be more reliably examined.