United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas. NV 89193-3478
Research and Development
EPA/600/S4-88/019 July 1988
x>EPA Project Summary
Soil-Gas and Geophysical
Techniques for Detection of
Subsurface Organic *
Contamination
Ann M. Pitchford, Aldo T. Mazzella, and Ken R. Scarbrough
From 1985 through 1987, the Air
Force Engineering and Services
Center (AFESC) funded research at
the U.S. Environmental Protection
Agency (EPA) Environmental Moni-
toring Systems Laboratory in Las
Vegas, Nevada (EMSL-LV) through
an interagency agreement. This
agreement provided for investi-
gations of subsurface contamination
at Air Force Installation Restoration
Program sites. The purpose of these
investigations was to demonstrate
and evaluate inexpensive and rela-
tively rapid reconnaissance tech-
niques which can detect and map
subsurface organic contamination.
This information can reduce the
number and improve the placement
of wells required in an investigation,
resulting in significant savings in
terms of costs and time.
The methods chosen for demon-
strations included active and passive
soil-gas sampling and analysis, and
the geophysical techniques of
electromagnetic induction (EM), and
d.c. resistivity. Field studies were
performed at four Air Force Bases:
active soil-gas measurements were
performed at all sites; d.c. resistivity
and EM measurements were per-
formed at three sites; and passive
soil-gas sampling was performed at
two sites. The techniques of
ground-penetrating radar and com-
plex resistivity were included in the
evaluations using experiences at
other locations. Based on this limited
set of cases and Information from
published literature, general guide-
lines on the application of these
techniques for detecting organic
contamination were developed.
The active soil-gas sampling
technique successfully mapped sol-
vents, gasoline, and JP-4 con-
tamination at the four bases where it
was used. The passive soil-gas
technique was successful in some
cases, but not as successful as the
active technique, and further re-
search on the performance of the
technique is recommended before
the method is used widely. The
geophysical methods were suc-
cessful for site characterization, but
the EM and d.c. resistivity techniques
did not detect gasoline and jet fuel
number 4 (JP-4) contamination
when it was present. The use of EM
and d.c. resistivity for direct
detection of hydrocarbons appears to
be a subtle technique which depends
on a thorough understanding of
background information at the site,
the skill of the instrument operator,
and may depend on the length of
time the spill has been present. The
ground-penetrating radar and com-
plex resistivity techniques were used
successfully at a number of locations
for detecting organic contamination.
This work was conducted from
January 1985 to October 1987.
This Project Summary was
developed by EPA's Environmental
Monitoring Systems Laboratory, Las
Vegas, NV, to announce key findings
of the research project that is fully
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documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
In 1984, the U.S. Environmental
Protection Agency (EPA) Environmental
Monitoring Systems Laboratory in Las
Vegas, Nevada (EMSL-LV) and the Air
Force Engineering and Services Center
(AFESC) entered into an interagency
agreement concerning investigations of
subsurface contamination at Air Force
Installation Restoration Program (IRP)
sites. Organic contamination was em-
phasized in these studies. The traditional
approach to these site investigations
involves the installation of wells and
analysis of ground-water samples. This
approach provides a direct measurement
of the contamination at the locations
sampled. However, information about the
extent and degree of contamination may
be limited by the number, cost and
possible locations of the wells. If
inexpensive, and relatively rapid recon-
naissance techniques could be used as
an aid to selecting the well locations, the
number of wells could be reduced. This
would save money and time.
The interagency agreement initiated
studies at four IRP sites to demonstrate
indirect methods for detecting and
mapping organic contamination in
ground-water and soil. The methods
chosen for evaluation were soil-gas and
geophysical measurements. These
measurement results then were com-
pared to ground water data obtained
during the same study. This made it
possible to evaluate the performance of
the soil-gas and geophysical tech-
niques. However, because of the wide
variety in contaminants and geological
conditions, care must be used when
applying the conclusions developed from
these site-specific studies to other
locations. To help to extend the results
from these studies to other site
conditions, additional examples were
assembled from the literature. Using all
this information, general guidelines were
developed for the use of these
techniques in investigations of organic
contamination of soil and ground water.
Approach
The overall approach to the project
was divided into two parts with activities
in each proceeding concurrently. These
parts consisted of working with a panel of
experts to broaden the ideas, ap-
proaches and experiences being used as
a basis for developing the guidelines;
and performing site investigations to
demonstrate the soil-gas and geo-
physical techniques. The Air Force
Bases (AFBs) selected are listed in Table
1. Each AFB provides differing geology,
climate, depth to water table, and con-
taminants, thus representing a variety of
situations for performing the com-
parisons.
This series of studies was intended to
help formulate a hierarchy of techniques
which could be logically adapted and
applied to detect contamination for a
variety of site conditions. However, the
results from the field studies fit better
into a framework of broad guidelines
rather than into a detailed strategy which
ranks techniques.
Field Study Results
The methods chosen for dem-
onstrations included active and passive
soil-gas sampling and analysis, and the
geophysical techniques of EM and d.c.
resistivity. Active soil-gas measure-
ments were performed at all sites;
resistivity and EM measurements were
performed at three sites; and passive
soil-gas sampling was performed at two
sites. Key results from these in-
vestigations are summarized in Table 2.
The active soil-gas sampling
technique successfully mapped solvents,
gasoline, and JP-4 contamination at all
four bases where it was used. Results
from Robins AFB demonstrated that the
choice of sampling depth can influence
the measurements obtained. At this AFB,
initial sampling at 1 meter revealed very
little contamination as shown in Figure 1,
while additional sampling at 2 meters
located more contamination, which is
shown in Figure 2 Thus, it is important
to perform depth profiles at a number d|
locations during the initial phase of a
study, preferably in regions of known
(quantified) ground-water contam-
ination, in order to select the sampling
depth. Sampling depth is particularly
important at sites where relatively old fuel
spills have occurred, because chemical
or biological oxidation of the petroleum
hydrocarbons can remove fuel con-
stituents from the aerobic soil horizons.
The real-time nature of this method also
represents a significant advantage over
more time-consuming techniques since
the choice and number of sampling
locations can be evaluated as data are
obtained.
Two of the sites investigated with
active soil-gas techniques were also
investigated by a passive technique
which used adsorbent charcoal badges.
At these sites, tests were performed to
determine the feasibility of mapping the
contamination at these sites by selecting
the best exposure times for the badges.
Performing feasibility tests with the
badges was demonstrated to be very
important; an insufficient exposure time
may indicate an area is uncontaminated
when contamination actually is present
Alternately, overexposure of the badges
may result in saturation of the sorbent
which would mask any relative
differences in soil-gas contamination at
the various sampling locations. This
passive soil-gas technique was not as
successful as the active technique in
detecting contaminated ground water
However, contaminated areas were
identified successfully in some cases.
Further testing of the performance of this
technique for a variety of contaminants
and geologic conditions is recommended
before the method is used widely. If on-
site personnel are available to conduct
the sampling, the low analytical cost of
this method has potential for reducing
site investigation costs in some cases.
The geophysical methods were suc-
cessful for site characterization, but the
EM and d.c. resistivity techniques did not
detect gasoline and JP-4 contamination
when it was present. This was attributed
to the natural variations in background
Table 1.
Geology, Climate, and Contaminants at Air Force Base Study Sites
Base
Geology
Climate
Contaminant
Holloman AFB sand, interbedded clay and
Phetps Collins ANGTB karst humid
Robins AFB marine sand humid
Tinker AFB clay humid
gasoline, JP-4, solvents
solvent, JP-4, buried metallic objects
JP-4, solvents
JP-4
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Site and contaminants
Holloman AFB,
BX Service Station,
Gasoline
Robins AFB.JP-4 Spill,
JP-4
Method
Active soil-gas
sampling
EM, d.c. resistivity
Active soil-gas
sampling
Passive soil-gas
sampling
EM, d.c. resistivity
Comment
Compares favorably with ground-water data. Demonstrates movement of contaminants along
utility corridors.
Do not detect organics because of natural variability in soil resistivity. Culture limited extent of
survey.
Compares favorably with ground-water data in spite of 20-year age of spill. Demonstrates
importance of depth of sampling.
Preliminary test has mixed results compared to ground-water data.
Do not detect organics because of natural variability in soil resistivity due to rainfall effects and
Tinker AFB, Fuel Farm
290, JP-4
Active soil-gas
sampling
Passive soil-gas
sampling
EM, d.c. resistivity,
complex resistivity
culture. AFB radar interferes with EM-34 measurements..
Compares favorably with ground-water data; technique effective in clay soil
Preliminary test has mixed results compared to ground-water data. Technique may be
responding to surface contamination at times.
Were not attempted due to high density of buried pipes and tanks, and fences and pipes on
surface.
resistivity which masked any resistivity
anomaly due to the presence of
hydrocarbons. Based on these results,
the use of EM and d.c. resistivity for
direct detection of hydrocarbons appears
to be a subtle technique which depends
on a thorough understanding of
background information at the site, the
skill of the instrument operator, and may
depend on the length of time the spill
has been present. This does not
preclude the use of these techniques in
site characterization. The techniques of
GPR and complex resistivity were not
demonstrated at the AFBs, but their
successful performance in detecting
hydrocarbons has been documented in
the literature. Table 3 summarizes the
general recommendations for application
of the geophysical techniques.
Note that only two techniques, GPR
and complex resistivity, are recom-
mended for routine use in detecting
organic contamination. GPR is com-
mercially available. Complex resistivity,
however, is the subject of several
research efforts, and is not widely
available. The d.c. resistivity and EM
techniques may sometimes be useful at
a site for detection of hydrocarbons, but
the conditions for which this is true are
not now understood. Other techniques
with greater likelihood of success should
be considered first.
Fundamentals for Planning Site
Investigations
To place these results in context,
recommendations for planning a site in-
vestigation also are presented. These
recommendations were prepared in
conjunction with members of the panel of
experts assembled to provide advice to
the project. The recommendations ad-
dress general considerations in design-
ing an investigation, provide examples
and references to similar cases in the
literature, list the steps in planning a
soil-gas investigation, and list issues to
be considered in planning a geophysical
investigation. The issues which should
be considered are presented in series of
questions organized by topic area, in-
cluding hydrology, the use of isotopes,
and water chemistry.
Conclusions
Demonstrations of soil-gas and
geophysical techniques at four AFBs
provided the basis for the development
of broad guidelines for the application of
these methods. The active soil-gas
sampling technique successfully mapped
solvents, gasoline, and JP-4 contam-
ination at the bases. The passive soil-
gas technique was successful in some
cases, but not as successful as the
active technique, and further research on
the performance of the technique is
recommended before the method is used
widely. The geophysical methods were
successful for site characterization, but
the EM and d.c. resistivity techniques did
not detect gasoline and jet fuel number 4
(JP-4) contamination when it was
present The use of EM and d.c. re-
sistivity for direct detection of hydro-
carbons appears to be a subtle technique
which may sometimes be useful at a site
for the detection of hydrocarbons, but the
reasons for this are not well understood.
Other techniques with greater likelihood
of success should be considered first.
The ground-penetrating radar and
complex resistivity techniques have been
used successfully at a number of
locations for detecting organic
contamination.
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Legend
Total Hydrocarbon Concentration
(ng/L) in Soil-Gas
LF-1-2 O — Well Sampling
SG-6 •—Soil-Gas Sampling Location
j 0,000*- —Isoconcentration Contour Line
*
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Legend
Total Hydrocarbon Concentration
(fig/L) in Soil-Gas
LF-1-2 O—Well Sampling Location
•—Soil-Gas Sampling Location
XJ.O6—Total Concentration
10 OOO—lsoconcentration Contour Line
' (H9/U
130.000
130.000
61
590
10 0 10 20
Sacle in Meters
Figure 2. Concentrations of total hydrocarbons in soil gas at JP-4 spill site. Ftobins AFB. Sampling depth: 2 meters.
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Table 3.
Generalized Applications of Geophysical Techniques
Application
Technique
Ground Penetrating Radar
(GPR)
Electromagnetics (EM)
B.C. Resistivity
Complex Resistivity
Seismic Refraction
Metal Detector
Magnetometer
Site
Characterization
yes
yes
yes
yes"
yes
no
no
Conductive
Leachate"
yes
yes
yes
yes"
no
no
no
Metal Obiects"
yes
yes
yes
yes"
no
yes
yes""
Organic
Contamination
yes
possibly
possibly
yes
no
no
no
"In some cases, the organic contamination will be associated with inorganic contamination;
examples include organics in metal drums and mixed organic-inorganic leachate plumes.
"But d.c. resistivity is equally good and much cheaper.
""Ferrous metals only.
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The EPA authors Ann M. Pitchford, Aldo T. Mazzella and Ken R. Scarbrough,
are with the Environmental Monitoring Systems Laboratory, Las Vegas, NV
89193-3478.
Aldo T. Mazzella is also the EPA Project Officer (see below).
The complete report, entitled "Soil-Gas and Geophysical Techniques for
Detection of Subsurface Organic Contamination," (Order No. PB 88-208
1941 AS; Cost: $14.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-88/019
0000329 PS
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