United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89193
Research and Development
EPA/600/S8-87/036 Jan. 1988
x°/EPA Project Summary
Soil Gas Sensing for
Detection and Mapping of
Volatile Organics
Dale A. Devitt, Roy B. Evans, William A. Jury, Thomas H. Starks,
Bart Eklund, and Alex Ghalsan
The sensing of soil gas for detection
and mapping of volatile organics is a
relatively new technique that deserves
greater attention. Soil organic vapor
monitoring has been shown to be a cost
effective means of delineating the size
and movement of organic contami-
nants in the subsurface. It has also been
shown to provide immediate informa-
tion of the lateral extent of soil and
ground-water contamination and to
minimize and more accurately predict
the number and location of conven-
tional monitoring wells that must be
drilled.
Literature on the technique for map-
ping soil and ground-water contamina-
tion has been increasing, but compre-
hensive reviews of the method have
been limited. This document is meant
to be a primer on the current state-of-
the-art of soil gas sensing as it relates
to the detection of subsurface organic
contaminants. It is hoped that such a
document will better assist all those
individuals who are faced with assess-
ing the extent of contamination of our
soil and ground water.
The document begins by outlining
many of the parameters (water solubil-
ity, microbial influence, ground-water
flow, etc.) that must be considered by
the scientist before utilizing soil gas
sensors in a field monitoring program.
Next, the complex soil, air, water, and
hydrocarbon system is addressed with
an overview of the important processes
involved in the transport and fate of
organic contaminants in the soil. Addi-
tional sections address the correct
sampling and analytical methodologies
for monitoring volatile organics in the
subsurface, covering such sampling
methods as headspace, ground probe,
flux chamber and passive sampling
techniques. Analytical methods include
organic vapor analyzers (OVAs) and gas
chromatographs with a variety of
detectors. A statistical treatment of soil
organic vapor measurements is also
included to ensure that soil organic
vapor monitoring programs address the
requirement for data precision. The
statistical section also gives greater
insight into understanding the spatial
patterns of soil organic vapor measure-
ments. Finally, case studies are
included to give the unfamiliar reader
examples of the design, procedures,
and results of soil organic vapor mon-
itoring programs that have been suc-
cessful in delineating the size and
lateral extent of subsurface organic
contaminants.
This Project Summary was devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory, Las Vegas,
NV, to announce key findings of the
research project that is fully docu-
mented in a separate report of the same
title fsee Project Report ordering
information at back).
Introduction
Interest in the measurement of con-
centrations of volatile organic com-
pounds in the pore-space gases of soil
was stimulated by enactment of Super-
fund (the Comprehensive Environmental
Response, Compensation, and Liability
Act, or CERCLA) and by the November
1984 reauthorization of RCRA (the
Resource Conservation and Recovery Act
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of 1976) which directed the U.S. Envi-
ronmental Protection Agency (EPA)
to promulgate standards for underground
storage tanks to include provisions for
leak detection. Soil gas monitoring has
been shown to be a useful technique for
mapping organic contamination of the
subsurface prior to the use of more
expensive monitoring methods such as
the drilling of wells. Inorganic contam-
ination may be mapped using geophys-
ical methods such as electrical resistivity
When organic contamination is com-
mingled with inorganic contaminants,
organic contamination may be mapped
with geophysical methods; however,
when relatively low levels of organic
contaminants are not commingled with
inorganic contaminants, soil gas moni-
toring is a useful technique to consider
for mapping volatile organics. Table 1
shows that a majority of the contami-
nants found at Superfund sites are
organic comtaminants with volatile
organics being common.
A desired goal in many soil gas
investigations of organic contamination
is to establish a relationship between the
organic vapors sampled in the vadose
zone to the concentration of contami-
nants in the ground water. If a relation-
ship can be established, soil gas meas-
urements can be useful in determining
the location for monitor wells and
assessing the extent of ground-water
contamination. If a relationship cannot
be established, soil gas monitoring may
still be a technique to consider in quickly
assessing the extent of contamination of
the soil, particularly in the shallow zone
being sampled by the probes or passive
samplers.
The increasing interest in the detection
of leaks from underground storage tanks
has further increased the growing inter-
est in soil gas monitoring. Soil gas
measurements may be used to locate the
source of the leaked material as well as
to map the extent of the contamination.
Soil gas measurements may be made
over time within the excavation zone
containing the tank and piping to deter-
mine if a tank system is tight. While
permanent monitoring of tanks with
vapor monitoring in the vadose zone is
a relatively new and promising use of soil
gas measurements, the monitoring of
underground storage tanks for leaks is
not the focus of this document.
The Document
Five important areas related to soil gas
monitoring are discussed in the docu-
ment: (1) site specific parameter consid-
Table 1. Most Frequently Identified Substances at 546 Superfund Sites
Rank Substance Percent of Sites
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Trichloroethylene
Lead and Compounds
Toluene
Benzene
Polychlorinated biphenyls
(PCBs)
Chloroform
Tetrachloroethylene
Phenol
Arsenic and compounds
Cadmium and compounds
Chromium and compounds
1 ,1 ,1 - Tnchloroethane
Zinc and compounds
Ethylbenzene
Xylene
Methylene chloride
Trans-1 ,2-dichloroethylene
Mercury
Copper and compounds
Cyanides (soluble salts)
Vinyl chloride
1 , 2-Dichloroetha ne
Ch/orobenzene
1,1 -dichloroethane
Carbon tetrachloride
33 Ib)
30
28 (b)
26 (b)
22
20 (b)
16(b)
15
15
15
15
14 (b)
14
13(b)
13(b)
12 (b)
11 (b)
10
9
8
8(b)
8(b)
8(b)
8(b)
8(b)
erations, (2) transport and retention of
organics in soil and ground water, (3)
sampling methods, (4) analytical
methods, and (5) statistical treatment of
soil organic vapor measurements. Two
case studies are presented to illustrate
how a soil gas survey may be performed
and how the data may be displayed and
interpreted.
A number of site specific parameters
need to be considered in a soil-gas
survey. These are listed in Table 2 and
covered in further detail in the document.
Further research is required to under-
stand the transport and fate of liquid
phase, gas phase, and dissolved hydro-
Table 2.
Site Specific Parameter.
Considerations
(a) Source Adapted from McCoy and Associates, The Hazardous Waste Consultant, March/
April 1985, Vol 3, Issue 2
Ib) Compounds amenable to soil-gas surveying—based upon Henry's law constant.
A Chemical and Physical Properties of the
Organic Compound
1. Vapor pressure
2. Water solubility
3. Henry's law constant
4. Concentration
5 Organic Distribution coefficient
IKoc)
6. Density
7. Viscosity
8. Viscosity
9. Boiling point
10. Molecular weight
B. Properties of the Unsaturated Zone
1. Air filled porosity
2. Volumetric water content
3. Soil organic matter
4 Soil texture
5 Vapor pressure of water in the soil
pores
6. Shape and size of pores
7. Depth of unsaturated zone
8. Retention
9. Temperature and temperature
gradients
10. Microbial influence
C. Hydrogeologic Properties
1. Ground- water flow (direction,
velocity, gradient)
2. Water table oscillations
3. Lithology of the aquifer
D. Characteristics of the Spill
E. Miscellaneous
1. Rainfall
2. Background water Quality
3 Barometric pressure and wind
4 Promixity to rivers, lakes and
pumping wells
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"carbons in the ground. What is known
about three-phase transport of hydrocar-
bons is described in a separate chapter
in order to provide the reader insight on
transport and fate processes that need
to be considered in making soil-gas
measurements.
A variety of methods have been used
in collecting a soil gas sample for
analysis, and they are briefly described
in a chapter on sampling methods. Soil
gas measurements have been made with
pipes, evacuated canisters, Tedlar bags,
adsorbents, flux chambers, pumps, and
syringes. There is no standard method.
In general, soil gas methods can be
classified into two categories: active and
passive methods. Active methods involve
the pumping of relatively large sample
volumes from the soil whereas passive
methods involve relatively little, if any,
withdrawal of soil gas from the subsur-
face. Further research is needed to
determine which method is best for
obtaining "representative" measure-
ments of soil gas.
A number of analytical methods and
instruments exist for measuring organics
in the collected soil gas samples. Differ-
ing sensitivities exist depending upon the
method or instrument selected and the
compounds present in the sample.
Organic vapor analyzers, field portable
and laboratory gas chromatographs, and
mass spectrometers are several of the
analytical techniques that are described
in a chapter on analytical methodologies.
The calibration of these methods is also
briefly discussed.
The location of soil gas measurements
in site investigations may dramatically
affect the interpretation of soil gas data.
The depth at which measurements are
made is an important factor particularly
with organic compounds that are prone
to biological degradation or chemical
transformation. One of the case studies
presented in the document illustrates
this point with high concentrations of
benzene being present in the ground
water but not detectable in the vadose
zone. The spatial location of soil gas
points is another important factor in the
mapping of organic contaminants. Too
few sample points, or sample measure-
ments made at a higher density in some
areas, can affect the interpretation of the
collected data through the drawing of
inappropriate contour lines around the
data points. Another factor, frequently
overlooked in soil gas measurements, is
the variability in obtaining consistent
measurements from a probe and analyt-
ical method. While all the factors that
can influence soil gas measurements
have some effect on the measurement
process and the interpretation of the
data, some of the factors become more
important as greater emphasis is placed
on the concentration values being
reported from a soil gas survey. Most of
the time soil gas measurements are
relative measurements that are intended
to identify where to drill monitor wells;
however, there is a growing trend to use
soil gas measurements to define contam-
inated zones and to establish site clean-
up plans. If this trend continues, the
evaluation of the factors that influence
soil gas measurements will have to be
examined further, and the chapter on the
statistical treatment of soil organic vapor
measurements was intended to offer the
reader some insights on how these
factors may be evaluated.
Two examples are presented on how
soil gas measurements may be used to
map ground-water contamination. Gas-
oline from a leaking underground storage
tank was mapped in Death Valley,
California, and variables affecting the
measurement of soil gas were studied
near an industrial site outside of Las
Vegas, Nevada. Both studies are used to
illustrate some of the limitations of soil
gas measurements.
Conclusions
Soil gas measurements are proving to
be useful in mapping organic contami-
nation at sites prior to the drilling of
monitor wells, or the colection of soil
samples. Since soil gas measurements
were first made in the early 1900's,
measurements of soil gas have increased
dramatically within just the past few
years to the point where the technique
is often considered as a first step in an
investigation of organic contamination of
the subsurface.
The team of authors who wrote this
primer on the method reviewed the
literature and used their background to
cover five main topics: (1) site specific
parameter considerations, (2) transport
and retention of organics in soil and
ground water, (3) sampling methods, (4)
analytical methods, and (5) statistical
treatment of soil organic vapor measure-
ments. Two case studies are presented
to illustrate how a soil gas survey may
be performed and how the data may be
displayed and interpreted. While the
document was not intended to be a
guidance document for making soil gas
measurements, some guidance is offered
to the reader on important steps and
considerations in the conduct of a soil
gas study.
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Dale A. Devitt is with University of Nevada, Reno, NV 89507; Roy B. Evans
is with University of Nevada, Las Vegas, NV 89109; William A. Jury is with
University of California, Riverside, CA 92502; Thomas H. Starks is with
University of Nevada, Las Vegas, NV 89109; Bart Eklund is with Radian
Corporation, Austin, TX 78766; and Alex Ghalsan is with Research Triangle
Institute, Research Triangle Park, NC 27711.
Jeff van Ee is the EPA Project Officer (see below).
The complete report, entitled "Soil Gas Sensing for Detection and Mapping
of Volatile Organics," (Order No. PB 87-228 516/AS; Cost: $24.95, 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 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 S300
EPA/600/S8-87/036
0000329
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