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. 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.
Applicability
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.
Web Links
http://www.etd.ameslab.gov/etd/technologies/projects/esc/technologies/passivesoil.html
http://www.epa.gov/ORD/SITE/reports/600r98095/600r98095.pdf (Goresorber)http://costperformance.org/monitoring/pdf/goresorbersoil_2.pdf
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. 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.)
http://www.clu-in.org/studio/passsamp_042407/day1/prez/38/38bw.pdf
See http://www.osti.gov/bridge/servlets/purl/268317-JwPjA0/webviewable/ for a comparison of passive soil gas samplers at a DOE site.