The purpose of soil vapor testing is to determine if soil gas from the vadose zone could pose a danger to indoor air quality thru vapor intrusion into, for example, a basement. Often, soil gas investigations are coupled with soil sample & ground water testing. The nature & history of the site guide the regulating agency in determining which compounds are tested. Four examples are given by the document:
Vapor intrusion is evaluated when the volatile & toxic chemicals are present in the subsurface that could pose a vapor intrusion risk to proximal existing or future buildings. Soil gas samples are collected to delineate the lateral & vertical extent of subsurface contamination. Open areas at an impacted site are sampled first, & if data trends warrant, sampling progresses towards existing or proposed structures, ultimately as close as possible to their foundations. Characterization should continue until VOCs are "non-detect". Covered areas, such as beneath a paved lot, may be sampled to provide knowledge of VOC accumulation under an impermeable barrier. 100' is considered a safe distance between a building & a contaminant plume. Contaminant vapor is assumed to move at a rate of 25'/year. This allows one to evaluate whether a plume at location x is increasing or decreasing in concentration or in a steady state. In addition, odors, physiological effects on building inhabitants, wet basements & explosive vapors are reasons for conducting indoor air evaluations.