£EPA

INTRODUCTION

United States
Environmental Protection
Agency

Environmental Monitoring
Systems Laboratory
P.O. Box 93478
Las Vegas NV 89193-3478

October 1990

TECHNOLOGY SUPPORT PROJECT

Soil-Gas
Measurement

The term "soil-gas" refers to
the atmosphere present in
soil pore spaces. Volatile
compounds introduced into
the subsurface can be
present in the gas phase or
more commonly, can un-
dergo a transition from a
liquid or sorbed phase (pure
product, dissolved, or
adsorbed to soil) to become
part of the soil atmosphere.
Techniques for measuring
soil gases were developed
early in this century for
agricultural studies and for
petroleum exploration.

Within the last 5 years, soil-
gas measurement has
become an accepted environ-
mental site screening tool.
The technique is rapid, low
cost, and provides a high
yield of information when
carefully applied. Because it
is an indirect measure of

underlying contamination and
because of the potential for
false negative results, the
technique should be used
only for site screening and
not for confirmation.

The fate and transport of
contaminants and their
occurrence and detectability
in the soil gases are very
compound- and site-specific.
Soil-gas technology is most
effective in detecting com-
pounds having low molecular
weights, high vapor pres-
sures, and low aqueous
solubilities. These com-
pounds volatilize readily as a
result of their favorable gas/
liquid partition coefficients.
Once in the gas phase,
volatile compounds diffuse
vertically and horizontally
through the soil toward zones
of lower concentration.

Degradation processes (e.g.,
oxidation or reduction) can
eliminate or transform con-
taminants in the soil atmo-
sphere. The susceptibility of
a contaminant to degradation
is influenced by such factors
as soil moisture content pH,
redox potential, and the
presence of microorganisms
that can degrade the com-
pound. Other site-specific
characteristics affecting
results are: soil type, air-filled
porosity, depth to the source,
barriers to vapor transport,
and hydrogeology. Because
site-specific factors influence
contaminant concentrations
detected in the soil gases, a
quantitative correlation
between soil-gas concentra-
tions and underlying contami-
nation is difficult to general-
ize.

APPLICATIONS

EPA
EMSL-
TSP-
1090x

Soil-gas surveys can be used
to:

•	identify contaminants and
relative concentrations

•	identify sources; indicate
extent of contamination

•	monitor the progress of
cleanups

•	guide placement of subse-
quent confirmatory samples
(soil borings, monitoring
wells)

•	monitor at fixed vapor wells
(long-term monitoring)

•	detect leaks through use of
tracer compounds

Typical primary sources
include surface spills, leaking
tanks, pipes, trenches, dry
wells, or landfills. Contami-
nants from such sources
frequently reach the water
table, causing the groundwa-
ter to become a source of

contamination to down-
gradient sites. The nature of
the source will influence the
vertical and horizontal disper-
sion of gas-phase contami-
nant vapors.

Contaminants detectable in
soil gases include many
common chlorinated solvents
and the lighter fractions of
petroleum products, sub-

stances that are widespread
environmental contaminants.
Of the 25 most commonly
encountered contaminants at
Superfund sites, 15 are
amenable to detection by soil-
gas sampling. Inorganic
contaminants that can be
detected by soil-gas sampling
include radon, mercury, and
hydrogen sulfide.

SELECTED C04IPOUNDS DETECTABLE IN SOIL GASES

Aromatic hydrocarbon*:

Benzenes, toluene, xylenes, naphthalene

ftHnhafin hmiirumrfjirn i

AJipflauc nyaroc&roons:

C, - C10 (e.g., methane, butane, pentane,
iso-octane cydohexane)

Mixtures:

Gasoline, JP4

Chlorinated hydroca/bona:
Chbronwthanas (e.g., chloroform,
ca/bon tetrachloride); chloroethanes;
chtamaihanas (e.g., vinyl chloride, di-, tn-,
and perchloroethene)

Other

C02, CS2, HjS, NOx, radon, mercury
compounds

-------
THE TECHNIQUE

Soil-gas samples can be
collected by active or passive
methods. For active sam-
pling, a probe is driven into
the ground and soil gases are
pumped from the subsurface
into a sample container (e.g.,
evacuated canister, tube,
glass bulb, gas sample bag,
syringe) or through a sorbent
medium. For passive
sampling, a sampler contain-
ing a sorbent with an affinity
for the target analytes is
placed in the ground for a
period of time, and contami-
nants are collected by virtue
of diffusion and adsorption
processes. After exposure,
the passive sampler is
transported to a laboratory for

analysis. The most com-
monly used technique for
analyzing soil-gas samples is
gas chromatography (GC) in
combination with a detector
appropriate to the target
analytes. Analyses can be
done on- or off-site. Soil-gas
samples can also be
screened in the field using
organic vapor detectors,
which provide results ex-
pressed as total hydrocarbon
concentration relative to a
calibration standard.

The design of a soil-gas
survey depends on the data
required (e.g., identifying and
quantifying specific com-
pounds vs. measuring total

hydrocarbon concentration)
and the nature of the contami-
nation. A feasibility study is
recommended whenever
possible, particularly for sites
where little information is
available. Such a study can
be valuable in verifying the
effectiveness of the method at
the site, selecting the appro-
priate sampling and analytical
methods, choosing the best
sampling depth, and optimiz-
ing other operational details.
Because soil-gas surveying is
an intrusive technique,
precautions must be taken to
avoid buried utility lines,
tanks, or other objects.

DATA QUALITY
OBJECTIVES AND
QA/QC

Because soil-gas results
provide an indirect measure
of primary contamination,
data quality objective (DQOs)
for soil-gas surveys and the
QA required need not be as
strict as those for confirmatory
sampling and analysis of soil
or ground water. However,

because most soil-gas survey
objectives require compari-
son of data among points to
determine patterns of relative
concentration, the investiga-
tor must be able to determine
whether differences in value
are real or merely due to poor
method precision. Consis-

tency in procedures is

essential, as are collection
and analysis of replicate and
blank sampies ana recu-ar
checks of instrument calibra-
tion. Materials that come into
contact with sampies should
be inert and easy to decon-
taminate.

SUMMARY

Soil-gas measurement can
be an effective method for
determining the source and
extent of volatile contami-
nants in the subsurface.
Because of the many site-
and compound-specific
factors that can influence
results, soil-gas measure-
ment should be done only by
experienced field investiga-

tors. With proper QA and
judicious data interpretation,

this technique is a useful,
low-cost site screening toci.

SUMMARY OF ADVANTAGES AND LIMITATIONS
OF SOil-GAS MEASUREMENT

Advantage

Rapid
Law art
Rial-tim® results
Minimal asturotnca to sts

Limitations

Indirect maasjrwwt

Interferences (false negatives are a problem)
Application limited to high volatility/low solubility
compounds

REFERENCE

Guidance Document for Soil-Gas Surveying, In preparation under EPA EMSL-LV Contract No.
68-03-3245 by C.L. Mayer, Lockheed Engineering and Sciences Company, Las Vegas, NV, in

press.

FOR FURTHER INFORMATION

Ǥ" I echoology %

O	z

O Jupp°rt Q

v ,ojea J

For further details on soil-gas measure-
ment, contact:

Dr. Phil Durgin

Advanced Monitoring Division
U.S. Environmental Protection Agency
P.O. Box 93478

Las Vegas, Nevada 89193-3478

(702) 798-2623
FTS 545-2623

For general Technology Support assistance,

contact;

Mr. Ken Brown, Manager

Technology Support Center

U.S. Environmental Protection Agency

P.O. Box 93478

Las Vegas, NV 89193-3478

(702) 798-2270
FTS 545-2270
FAX/FTS 545-2637

-------