SOIL GAS SAMPLING
Passive soil gas sampling and screening technology utilizes tubes containing an absorbent material, placed in a matrix near the surface. As contaminants in the soil and groundwater evaporate, gases are sorbed onto the material. The tubes are usually laid out in a predetermined grid, installed about 16 inches below the surface and 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 soil density. Minute quantities of soil gases are sorbed. The tubes are then shipped to a laboratory for analysis. The tubes are used primarily to screening for soil and groundwater contamination.
Limitations and Concerns
The success of the tubes depends upon gas reaching the sampler. Barriers such as dense clay, as well as the depth and type of contaminant, affect the usefulness of this tool.
The results obtained from the samplers are reported in ion flux, rather than concentration. Flux counts are related to concentrations, but they cannot be extrapolated directly.
A quality assurance program for placing sampling devices in the soil must be rigorously followed.
The passive soil gas samplers were initially developed to monitor gases flowing from a source of interest. 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, respectively) 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 Volatile Organic Compounds (VOCs) and Semi-Volatile Organic Compounds (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. One type of collector has been designed to collect chemical agents, such as mustard agent, and their breakdown products. Although primarily used to measure soil gas, passive samplers are sometimes used to detect vapors in buildings.
Technology Development Status
The passive soil gas method was originally developed at the Colorado School of Mines for petroleum exploration. It is widely used for this purpose and is a commercial technology.
Other Resources and Demonstrations
See also Soil Gas Sampling-Active.
Passive soil gas screening to indicate groundwater pathways and contaminant movement is a new application of this existing technology. Naval Air Station (NAS) North Island demonstrated the passive soil vapor survey technique to identify possible discharges of VOCs into San Diego Bay. Lawrence Livermore National Laboratory has used this technique for the rapid screening of groundwater and soil contamination at its two Superfund sites. Both Emflux and Goresorbers were evaluated through the EPA's Superfund Innovative Technology Evaluation (SITE) Program (see the Web links above), and both identified target constituents in soils as well as contaminants in groundwater that were missed by an active soil-gas method. (See http://www.clu-in.org/studio/passsamp_042407/day1/prez/38/38pdf.pdf for a presentation by Gore on the use of its samplers.)
See http://www.osti.gov/bridge/servlets/purl/268317-JwPjA0/webviewable/ for a comparison of passive soil gas samplers at a DOE site.