Evaluation of potential vapor intrusion issues has become standard practice during due diligence for property transactions, Site Investigations, and Phase II Environmental Site Assessments (ESAs). The most recent update of the Phase I ESA (ASTM E1527-13) has a new requirement for the investigator to evaluate the potential for vapor intrusion, which becomes a concern when volatile organic compounds (VOCs) are identified in the soil or groundwater in proximity to a building.
When a site assessment or due diligence identifies the potential for vapor intrusion, there are several options to determine if the vapor intrusion pathway is complete. These options include:
- soil gas sampling
- sub slab sampling
- soil gas flux sampling
- indoor air sampling
At first glance, indoor air sampling appears to be the least expensive approach; it is also the last approach an investigator should take. The problem with indoor air sampling is that most households or businesses have commercially available products (cleaning supplies) or building materials (carpets, caulks, glue) that can act as a background source for VOCs. Additionally, homeowners often store petroleum products in their garage, basement, or mudroom that can also act as a source for VOCs. These background sources can lead to positive detections of VOCs that have nothing to do with the vapor intrusion pathway. If an indoor air sample is collected without first performing a soil gas or sub-slab investigation, it can be difficult, if not impossible, to determine the source of the detected VOCs. This may lead to faulty conclusions of the occurrence of vapor intrusion and costly remedial efforts.
The preferred methods for assessing the vapor intrusion pathway are soil gas sampling or sub-slab sampling. These approaches involve collecting soil gas from the subsurface or from below the slab to determine if there is a source for VOCs to enter the building. If elevated VOCs are detected in the soil gas or sub-slab sample, additional vapor intrusion modeling can be performed to determine if the VOCs could potentially enter the building at concentrations of concern.
If all else fails, it may be necessary to perform indoor air sampling. But, this should be the last resort after exhausting all other options.