PASSIVE SOIL GAS SCREENING

Description

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. Passive soil gas samplers, in their original form, consist of two ferromagnetic wire collectors coated with an activated carbon sorbent housed in a small glass tube. 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 sorb to the carbon and the wire collectors. The tubes are then shipped to a laboratory for mass spectrometry analysis. The tubes act as a screening device for contamination of soil and groundwater. At one demonstration location, samplers were placed onshore and offshore, in sediments beneath 10 to 55 feet of water.

Another version of the system can identify and quantify a broad range of volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), petroleum hydrocarbons, polynuclear aromatic hydrocarbons, and other compounds present at depths to more than 200 feet. Glass tubes with a cartridge consisting of a sorbent sealed in a fine-mesh screen are inserted into the soil. After an agreed upon time, the tube is removed from the soil and the cartridge is thermally desorbed and analyzed in the laboratory. A computer model predicts periods of maximum soil gas emission for geographic locations to optimize sampling efficiency.

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 porosity of the membrane material has been found to be inconsistent, presenting problems with relative values from one location to another. In addition, the membrane material is prone to tearing during installation.

Applicability

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 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. One type of collector has been designed to collect chemical weapons, such as mustard agent, and their breakdown products.

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.

Web Links

http://www.nasni.navy.mil/nelp/9offsamp.htm

http://www.etd.ameslab.gov/etd/technologies/projects/esc/technologies/passivesoil.html

http://www.epa.gov/ORD/SITE/reports/600r98095/600r98095.pdf (Goresorber)

http://www.epa.gov/ORD/SITE/reports/600r98096/600r98096.pdf (Emflux)

Other Resources and Demonstrations

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. A fact sheet, Petrex Passive Soil Gas and Sediment Vapor Sampling, was prepared and is available from the Navy. Lawrence Livermore National Laboratory has used this technique for the rapid screening of groundwater and soil contamination at its two Superfund sites. (Petrex was sold to Gore, which makes a product called a Goresorber). The Emflux (http://www.emflux.com/) collector manufactured by Quadrel is also used. 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.