Summa Variations

 

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:

  1. When USTs or fuel pipelines are / were present, fuel oxygenates (MTBE, ETBE, DIPE, TAME, TBA) should be included for analysis.
  2. At oilfield sites where hydrocarbons are detected in soil "fixed & biogenic" gases (O2, CO2, CH4) should be included for analysis.
  3. At petroleum contaminated sites (including oilfields), dairies, wetlands, landfills or other sites where applicable (ex. coal containing areas, intertidal areas, septic zones, fill areas) methane / hydrogen sulfide should be determined.
  4. Sites that used chlorinated solvents with 1,4-dioxane stabilizer should test for these compounds (as well as the chlorinated breakdown products). 1,4-Dioxane may require SIM mode analysis & to achieve the document mandated 1ug/L RL.

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.