United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV89114
Research and Development
EPA/600/S2-87/027 June 1987
v>EPA Project Summary
Soil-Gas Measurement for
Detection of Subsurface
Organic Contamination
Henry B. Kerfoot and Larry J. Barrows
Two techniques for soil-gas measure-
ments were used at a site in Pittman,
Nevada. Two distinct organic contam-
inant plumes were investigated.
Twenty-three monitor wells had been
drilled across the plumes to provide
ground-water concentrations of ben-
zene/chlorobenzene and chloroform.
One soil-gas sampling technique (a
commercial ' 'passive" sampler)
involved the placement of activated-
charcoal-coated wires approximately 1
foot below the surface of the ground
for nine days. Analysis of the samplers
was made at a commercial laboratory
using pyrolysis and mass spectrometer.
The other technique (an ''active"
sampler) consisted of a pipe being
pounded into the ground and sampled
using a small hand pump and gas-tight
syringes. Analysis of the samples was
made using a field portable gas
chromatograph
The accuracy, precision and repre-
sentativeness of the two soil gas
techniques were evaluated. Closely-
spaced, repetitive measurements were
made in the vicinity of the monitor
wells. Frequent calibrations were made
on the field portable gas chromato-
graph to analyze the accuracy and
precision of the analytical method. The
data from both soil-gas techniques
were compared to the data from the
monitor wells to assess the represen-
tativeness of the soil-gas techniques.
The data from the commercially-
marketed "passive" samplers exhi-
bited a large degree of variability. It was
not possible to map either ground-
water plume using this method at the
Pittman site. The data from the
"active" sampler permitted the chlo-
roform plume to be accurately mapped;
however, this technique, as well as the
' 'passive" technique, were unable to
map the benzene/chloroform plume.
Biodegradation of the benzene vapors
in the vadose zone is believed to be
responsible for the lack of detectable
concentrations. Further studies are
recommended.
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 (see Project Report ordering
information at back).
Introduction
Efforts to map the extent of ground-
water contamination have relied on the
drilling of monitor wells. The proper
location and number of wells is an issue
in site investigations with cost being an
important factor Geophysical tech-
niques have proven to be useful in
locating where monitor wells should be
drilled particularly when inorganic,
electrically-conductive contaminants are
involved. In those instances where
organic contaminants are involved,
particularly when concentrations are
relatively low, traditional geophysical
techniques are less helpful in mapping
the ground-water contamination prior to
the drilling of monitor wells. The sam-
pling of vapors in the vadose zone,
commonly referred to as "soil-gas"
monitoring, has become increasingly
-------
attractive to those who seek additional
techniques and information on the extent
of organic contamination at a site.
The strengths and weaknesses of the
soil-gas technique are still being
researched. The number of practitioners
using the technique at sites during the
past few years have increased dramat-
ically. This together with the growing
number of reported instances where the
technique has been used with some
success would suggest that the soil-gas
technique is a credible technique to
consider in a site investigation along with
more tested, older approaches such as
those found under the general category
of geophysics.
The soil-gas technique, being a rela-
tively new technique, cannot be consi-
dered to be a standard technique with
well-defined quality assurance/quality
control practices being prescribed. Soil
gas is typically collected with adsorbents,
pipes, canisters, and bags, and the
sample is generally analyzed with
organic vapor analyzers, field-portable
gas chromatographs, laboratory gas
chromatographs and mass spectrome-
ters. As with the placement of monitor
wells, the number and placement of the
less-expensive soil-gas measurement
devices is an issue also. Neither standard
sampling and analytical procedures nor
prescribed quality assurance precisions
of the method defines the extent of
ground-water contamination.
The representativeness of the method
of collecting a sample is also an item of
research interest. While the method
appears to be capable in many investi-
gations of being able to map contami-
nants in the ground water, there is some
question as to whether the contaminant
concentrations measured in the vadose
zone can reliably measure the extent of
contamination of the underlying ground
water. In some cases, a soil-gas mea-
surement may simply measure contam-
inants within the vadose zone or con-
taminants that have migrated downward
from a surface spill.
The objectives of the field study
described in this report were to deter-
mine the accuracy, precision and repre-
sentativeness of two common soil-gas
measurementtechniques in mapping the
contamination at a previously studied
site.
Procedure
The site is located at Pittman, Nevada
(Figure 1). Measurements made along
the Pittman Lateral (a large pipe used to
supply water from Lake Mead to the Las
Pittman
Figure 1. Location of the Pittman Site.
Vegas metropolitan area) determined
that two distinct organic and inorganic
plumes were in the ground water.
Twenty-three monitor wells had been
drilled at 200 foot intervals and sampling
of those wells over a period of years
indicated that benzene/chlorobenzene
and chloroform plumes were present in
the shallow ground water aquifer (Figure
2). Soil-gas measurements were made
in the vicinity of several of the wells.
Two techniques were used. A
commercially-marketed, activated-
charcoal-coated wire (Figure 3) was
-------
West
Benzene/Chlorobenzene
East
655
650
645
640
0 500
Scale in Feet
635 630
Stations
0 200
Scale in Meters
625 620
rest Well
615
610
T Water Table
Figure 2. Hydrogeologic cross section along the Pittman Lateral.
buried approximately one foot below the
surface of the ground and left for nine
days. The samplers were returned to the
firm for analysis by Curie-point pyrolysis
and mass spectrometer (MS). The wires
were heated to approximately 400°C
under vacuum in a 1.1 MHz, 1.5 kW
Fisher Curie-point pyrolyzer, and the
desorbed gases were flushed into an
Extranuclear Laboratories Spectra El
quadrapole mass spectrometer where
low-energy ionization (15 eV) was used
to minimize fragmentation. The other
technique used a custom-designed and
manufactured steel pipe (Lockheed Gas
Analysis System-"LGAS") that was
driven into the ground to a typical depth
of approximately four feet, and samples
were obtained using a small hand pump
to evacuate the deadspace. A gas-tight
syringe was used to inject a small sample
of the gas into a field portable gas
chromatograph. The total volume of
sample obtained from the ground was
approximately 75 mL.
A series of tests were conducted to
develop quality assurance procedures for
both soil-gas sampling techniques in
addition to assessing the general ability
of the techniques to map ground-water
contamination. The spatial variability of
soil-gas measurements in a small area
was assessed, the change in concentra-
tion over depth was evaluated, and the
concentration of contaminants over a
series of sample cycles was also eval-
uated at one point for the soil-gas
measurements using the LGAS. Spatial-
variability of soil-gas measurements
made with the activated-charcoal adsor-
bent was also investigated.
Results
Soil-gas measurements made with the
pipe and pump were able to map the
chloroform but not the benzene/chlor-
obenzene contaminant plume (Figure 4).
Measurements made with the activated-
charcoal-coated wire were highly vari-
able and were unable to map either
ground water plume (Figure 5). Although
no measurements of the biological
activity in the vadose zone were made
above the benzene/chlorobenzene
plume, it is believed that the inability to
measure detectable levels of benzene in
the soil gas with either method, any-
where from 1 1 ,/2 feet above the water
table to just below the ground surface,
is due to biological degradation of the
benzene in an aerobic environment.
Further studies in this area are
recommended
Concentrations of chloroform and
carbon tetrachloride increased linearly
with depth above the contaminated
ground water, m agreement with a model
for vertical transport of volatile organic
compounds in the vadose zone by gas
diffusion.
Detailed investigations of the sampling
and analytical procedures for the LGAS
were helpful in obtaining reproducible
results. Teflon components were elim-
inated from the- probe and sampling train
to avoid a "memory" effect in sampling
low levels of organics. A vacuum gauge
was attached 10 the sampling manifold
on the probe to monitor the rate at which
the vacuum diminished to help ensure
that there was a good seal between the
probe and the soil. The gauge was also
helpful in determining when the pores
in the sample tip were completely
clogged. Clogging of the probe and leaks
were found to significantly affect the
measurements that were made to assess
spatial variability in the measurements.
The length of time gas was present in
the syringe also was found to have an
effect on the precision of the measure-
ment. Blanks and frequent use of cal-
ibration gas standards were helpful in
reducing and assessing the precision of
the method.
Conclusions
The ability of the soil-gas probe to
discern not only the center but edges of
the chloroform plume in the ground
water indicates that soil-gas measure-
ments, with proper safeguards, can be
of value in mapping ground-water con-
tamination from some organic contam-
inants. The absence of benzene in the
vadose zone indicates that caution must
be exercised in the use of soil-gas
measurements to map ground-water
contamination. The inability of one
"passive" sampling method (using
activated-charcoal and pyrolysis to map
organic contaminants) should not be
interpreted as a failure of the general
approach. After this study was com
pleted, another "passive" method was
used and the results were good. Further
efforts at developing standard test
procedures for the evaluation of the
adsorption, desorption, and analysis
phases of passive sampling are planned.
Further research is also planned in
evaluating the representativeness of
soil-gas measurements with any method
to contaminant levels in the soil, ground-
water, or both.
-------
-Oo3
S^\
Ground Surface
, \J,iJ r rJ ° J ' Yl/ * l -. •,
*£ rJr>o£ • .o>,( ^ ;=? vy
'^f 0 "' "„ ,'f 0°°»°^ "''".
p^^j:^;, „:•
>r>°ooM>3.,c: '^
^^o^'f^
Sfjcr"?', V
•C, 0. ^-.. olj'
S..5?.rf;^
Charcoal
Adsorbent
Inverted
Glass
Tube
Ferromagnetic
Wire
Threads
l°''^^•O^^^^o^^^' '
Figure 3. Schematic drawing of a PETREX® sampler in place
-------
woo-.
800-
600-
400-
200-
!!
631
629
627
625
623
"HT-
627
-f
619
West
East
600-1
500-
400-
300-
200-
100-
WN'ES
631
WN'ES
629
WN
WN
WN'ES WN1ES
627 625 623
Borehole Identification
621
619
Figure 4. Soil-gas and ground-water chloroform plumes.
-------
Benzene, M/Z = 78
Chlorobenzene, M/Z- 112
Chloroform, M/Z - 83
2000 -,
1500-\
© 7000H
P
£ 500 -I
Borehole Identification
10 •<» "fr
Co ~o>
Co CQ CQ
4000
3000
2000
Borehole Identification
CQ co CQ CQCO co COCQCQ
20000i
70000
5000
0
-Non-detect
> to
-* V.
ID c
C Ct)
b c
•T o
\ 5-
*3) -
n
n\\
}
\
n
1
} I
!"
k.
C
01
«J
0
-------
-------
United States Center for EnvironiTTent.il Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-87/027
------- |