Vapor Intrusion
Vapor intrusion (VI) is a potential concern for sites contaminated with volatile compounds. These compounds are capable of existing in a gaseous phase and can migrate from soil and groundwater to indoor or outdoor air where exposure can occur via inhalation. VI evaluation has been a regulatory priority since the early 2000's; however, the specific evaluation approach required by state and Federal regulators is still evolving. It should be noted; however, that this evolution is primarily in the screening procedures and not in the underlying equations used to calculate volatilization, transfers to indoor/outdoor air, and mixing in the breathing zone.
I have been calculating contaminant vapor generation and transfer to air for the VI problem since 1994. I use my thorough understanding of VI modeling to ensure compliance with the requirements of the various regulatory programs I encounter. Typically, the differences in the regulatory requirements are in the default model assumptions used for the screening phase. For example, the soil types, building construction, building air exchange rates, and other input parameters required by regulators at the screening level are very conservative to ensure protection of human health. Inspection and evaluation of the buildings in question will often show that the default model assumptions are overly conservative. Using site-specific model input parameters will generally show that the actual risks are significantly lower than may be suggested by the screening results.
In addition to managing the screening process, great care should be taken when planning data collection to address the VI pathway. Many regulatory programs jump to indoor air sampling if screening indicates that VI may be an issue. A stepwise approach including refined modeling, additional soil/groundwater characterization, and/or soil vapor sampling should be considered for VI evaluation. Often, direct indoor air sampling will lead to erroneous inclusion of volatile compounds that are not associated with the soil or groundwater impacts. These compounds may arise from other sources such as cleaning products, building process activities, or ambient outdoor conditions, which will not be addressed by remediation.