USE OF PASSIVE SOIL GAS SURVEYS TO EXPEDITE AND FOCUS SITE INVESTIGATION ACTIVITIES AT CONTAMINATED SITES

Soil gas sampling involves the collection of vapors from soil pore spaces in the vadose zone that are collected and analyzed to determine the presence and concentration of materials capable of partitioning into the vapor phase under ambient conditions, such as volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs). Passive soil gas sampling involves the use of sampling units, housing an adsorbent, which are deployed in the subsurface for a specified period of time, retrieved and analyzed. During deployment, organic vapors migrating through the subsurface are “passively” collected onto the adsorbent.

W. L. Gore & Associates, Inc. developed a sampling unit, the GORE™ Module, with an adsorbent housed in a GORE-TEX® membrane. The microporous, hydrophobic membrane protects the adsorbent from water and soil particles, enabling accurate and sensitive soil gas sampling. ERM has utilized GORE™ Modules to expedite site investigation activities. Three case studies are discussed.

The first site is a former research and development facility where shallow groundwater is impacted with trichloroethene (TCE). However, a source area was not clearly identified. A soil gas survey identified a potential source area beneath a portion of the site building. Subsequent soil sampling confirmed these results.

The second site is a former manufacturing facility where groundwater is impacted with tetrachloroethene (PCE) and TCE. Multi-year groundwater sampling data revealed seasonal fluctuations in PCE and TCE concentrations. A source area was never identified up-gradient of this plume. A soil gas survey identified a single location where a soil-borne source may exist. Subsequent excavation activities confirmed the presences of impacted soil at this location.

The third site is an existing maintenance facility where groundwater is impacted with petroleum constituents. Groundwater analytical data from a newly installed monitoring well identified chlorinated solvents (PCE, 1,1,1-trichloroethane [1,1,1-TCA], and 1,1-dichloroethene [1,1-DCE]). A soil gas survey identified PCE and 1,1,1-TCA in soil gas adjacent to and cross- gradient from the newly installed monitoring well. Subsequent groundwater sampling confirmed these results. A source area in soil has not yet been identified.