Soil gas sampling techniques are categorized as active or passive. Active soil gas sampling uses a vacuum to collect gas samples at discrete depth intervals and assesses the soil gas environment at a particular moment at a particular depth. This approach requires detectable vapor-phase compound concentrations, relatively porous soil, and experienced on-site personnel. As the soil gas samples are usually analyzed immediately, an on-site or nearby laboratory is typically required. Active soil gas sampling is generally used for rapid screening of volatile organic compounds (VOCs) in a subsurface with moderately permeable soils. It is generally not effective in detecting semi-volatile organic compounds (SVOCs).

Passive sampling techniques, on the other hand, rely on diffusion and adsorption. Depending on which adsorbent and diffusion membrane are selected, passive sampling techniques can be used to sample for both VOCs and SVOCs. Passive samplers allow for equilibrium to develop between the soil gases and the sorbent over a period of several days to weeks. The exposure of the passive samplers to the soil gas over extended periods concentrates the mass of VOCs and SVOCs adsorbed to the sampler, thereby enhancing contaminant detection sensitivity.

Sampling Modules

Passive soil gas sampling and screening utilizes tubes containing an absorbent material, which is placed in a sampling matrix near the surface. As contaminants in the soil and groundwater evaporate, gases are adsorbed onto the material.

In their original form, passive soil gas samplers consist of two ferromagnetic wire collectors coated with an activated carbon sorbent that is housed in a small glass tube. The tubes are usually installed in a predetermined grid arrangement approximately 16 to 36 inches below the surface and are left in place for just over two weeks. The time period for screening depends on the volatility of the contaminant, the depth of the suspected contaminant, and the soil density. Minute quantities of soil gases adsorb to the carbon and the wire collectors. The tubes are then shipped to a laboratory for mass spectrometry analysis.

The Gore-Sorber Screening Survey, which was developed by W.L. Gore & Associates, Inc. (Gore), is a passive soil gas sampling technology that has been used by ERM for its studies in the past. A typical Gore-Sorber module consists of several granular adsorbent materials housed in a chemically inert, hydrophobic, microporous expanded polytetrafluoroethene (ePTFE) membrane. The microporous structure of ePTFE allows vapors to move freely across the membrane and onto the sorbent material while preventing water and soil particles from entering the sampler.

Application

The passive soil gas sampling technique is a near-surface screening method that can identify a large range of chlorinated and aromatic vapors such as trichloroethylene (TCE) and benzene that are migrating to the surface from the soil or groundwater. This system provides rapid screening of soils and groundwater (i.e., usually less than four weeks) for VOCs and SVOCs. Data from soil gas surveys can be used to establish the extent of contamination at a site and to guide well placement and soil boring programs. Typical applications include soil and groundwater quality characterization at:

• Refineries and fuel storage terminals
• Fire training areas
• Manufactured gas plants
• Solvent manufacturing/distribution facilities
• Dry cleaners
• Airports
• Landfills
• Military sites
• Retail petroleum facilities
• Brownfield sites

Passive soil gas sampling has also proven to be an effective tool during real estate transfer assessments.

According to Gore’s company web site, the Gore^TM Module is designed to identify and quantify a broad spectrum of volatile inorganic and organic compounds ranging from C₂ (ethane) to C₂₀ (phytane). The capability of the module to detect certain polychlorinated biphenyls, pesticides and herbicides has also been demonstrated.

Limitations and Concerns

• As reported by the Center for Public Environmental Oversight, the success of this sampling method depends upon gas reaching the sampler. Thus, contaminant type, contaminant depth and the possible presence of subsurface barriers (e.g., dense clay layers) are variables that determine the method’s effectiveness.
• Samplers detect the presence of certain compounds, but the results are reported in ion flux, rather than concentration. Although flux counts are related to concentrations, they cannot be directly extrapolated.
• The porosity of the membrane material has been found to be inconsistent, presenting problems with relative values from one location to another. Also, the membrane material is prone to tearing during installation.
• Like all sampling programs, use of passive soil sampling techniques requires that a rigorous quality assurance program be followed.

Sources

• United States Environmental Protection Agency (USEPA) Environmental Technology Verification Report, Soil Gas Sampling Technology, W. L. Gore & Associates, Inc. GORE-SORBER Screening Survey August 1998 (http://www.epa.gov/nrmrl/lrpcd/site/reports/600r98095/600r98095.htm)
• Passive Soil Gas Screening, Center for Public Environmental Oversight (http://www.cpeo.org/techtree/ttdescript/petrex.htm)

About the Author

Ajay Pillai is a senior consultant with Environmental Resources Management in New Delhi, India. He specializes in air quality modeling and impact assessments.

Image: Gore^TM Module from W. L. Gore & Associates, Inc.’s company web site.