United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/600/SR-93/140 October 1993
EPA Project Summary
Behavior and Determination of
Volatile Organic Compounds in
Soil: A Literature Review
Volatile organic compounds (VOCs)
are the most common and the most
mobile subsurface contaminants en-
countered at Superfund and other haz-
ardous waste sites. VOCs can be toxic,
mutagenic, or carcinogenic. Accurate
measurement of soil volatile organic
compound (VOC) concentrations is cru-
cial to site investigation, evaluation, and
remediation efforts at Superfund and
other hazardous waste sites contami-
nated by VOCs. Soils that are contami-
nated with VOCs are potential reser-
voirs of long-term ground-water con-
tamination.
At the request of the U.S. EPA, a
literature review was conducted to
present and assess literature research
results pertaining to the problems and
inconsistencies observed in the sam-
pling and analysis of soil VOCs by SW-
846 Methods 8240/8260. This report
summarizes the current literature per-
taining to (1) the fate and transport of
soil VOCs and, (2) the sampling and
analysis of soil VOCs by SW-846 Meth-
ods 5030/8240/8260 using purge-and-
trap/gas chromatography/mass spec-
trometry (PT/GC/MS).
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 documented in a
separate report of the same title (see
Project Report ordering information at
back).
Fate and Transport
Nonpolar VOCs are sorbed predomi-
nately by soil organic matter in moist or
wet soil. Soil sorption exhibits an initial
phase of fast uptake, followed by slow
continued sorption or diffusion of VOCs
into soil microsites. Desorption studies
show a similar rapid desorption phase pre
ceding an extended slow release phase
Soil water retains VOCs in proportion to
compound-specific Henry's Law constants
VOC vapors are adsorbed by soil miner
als in dry soil and the quantities adsorbed
are 2 to 4 orders of magnitude greater
than sorption in a wet soil. Contamination
by nonaqueous-phase liquids (NAPLs) re
suits in a residual saturation fraction, de
scribed as tiny portions of NAPL held by
capillary forces in soil pores, which
changes in composition over time by physi
cochemical weathering. The size of the
residual NAPL fraction is related to the
soil porosity.
Biodegradation of naturally occurring
VOCs (such as petroleum products) readily
occurs under aerobic conditions. Microor
ganisms also degrade halogenated aro
matics (such as chlorobenzene) aerobi
cally, but more slowly than the naturally
occurring VOCs. Halogenated aliphanes
(such as chloroform and TCE) are de
graded far more slowly than the other
compounds by microorganisms or abiotic
processes and degradation occurs mainly
under anaerobic conditions. Degradation
of halogenated aliphanes, however, has
been observed in soils containing sub
stantial amounts of biodegradable carbon
compounds, presumably by co-metabo
lism.
VOCs move in soils by diffusion and
advection. Vapor diffusion, density-driven
NAPL vapor advection, and gravity-driven
NAPL advection are the most important
mechanisms for movement. The move
ment of two fluorocarbons by diffusion in
deep sediments in Texas progressed ap-
proximately 44 m vertically in 40 years
(time since manufactured). Movement of
carbon tetrachloride (a dense solvent) 177
m into the ground water at a site in Idaho
(time of travel unknown) is believed to be
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caused by density-driven vapor advection.
Movement of benzene, toluene, and xy-
lene (solvents less dense than water) 24.4
m vertically in less than 7 years at a
California site has been attributed to grav-
ity-driven NAPL advection.
Sampling and Analysis
Substantial volatile and degradative
losses of soil VOCs have been docu-
mented to occur from sample preserva-
tion and subsampling steps of SW-846
Methods 8240/8260. Soil samples stored
cold (4°C) have maximum holding times
of less than 3 days before the concentra-
tion falls below the 90% confidence limit
of the initial value. Laboratory soil trans-
fers create VOC losses that widely vary
by compound and soil, but losses aver-
age approximately 60%.
Immersion of soil samples in methanol
has been shown to reduce VOC losses
during sample storage and preparation for
analysis. Although the analytical sensitiv-
ity of methanol-preserved samples is less
than that of soil/water samples analyzed
by purge-and-trap preparation, soil-VOC
concentrations in methanol-preserved
samples were 1 to 3 orders of magnitude
greater than soil-VOC concentrations in
collocated samples analyzed by low level
PT/GC/MS. This implies that much of the
existing data of soil VOCs analyzed by
SW-846 Method 8240 could be 1 to 3
orders of magnitude below values obtained
in properly preserved samples or obtained
by field analysis.
To a large extent, erratic recovery of
same-day spikes and loss of analyte dur-
ing storage has impeded the accurate as-
sessment of soil-VOC measurement er-
rors. Quality control samples or perfor-
mance evaluation materials (PEMs) are
not available for soil VOCs. Recently, va-
por fortification of small (2 to 3 g), dry soil
samples (four compounds spiked onto two
soils) has established low relative stan-
dard deviations among samples and stor-
age of at least 3 weeks without measur-
able sample loss. The technique does not
calculate spike recoveries but creates
stable and reproducible concentrations of
VOC-contaminated soils. It is limited to
small aliquots of dry soil. Another option
for PEMs might be samples immersed/
preserved in methanol.
Current analytical methods that utilize
purge-and-trap techniques to remove soil
VOCs are not sufficient to extract all en
trapped VOCs (also referred to as re
sidual, nonequilibrium, or slowly desorb
ing VOCs).
Field (static) headspace techniques of
fer a rapid means of quantifying soil VOCs
with some restrictions. One restriction is
that the detection limit is not as bw as can be
achieved with a PT preconcentration step
After the compounds of interest are identi
fied, however, options for detectors other
than the mass spectrophotometer allow
for extremely low detection limits. Another
restriction is that in soils that are high ir
organic matter or soils that have a large
fraction of slowly desorbing VOCs, PT ex
traction may be more thorough than soi
headspace, thus necessitating laboratory
corroboration of field data.
The information in the full report was
funded wholly or in part by the U.S. Envi
ronmental Protection Agency under Con
tract No. 68-CO-0049 to Lockheed Envi
ronmental Systems and Technologies
Company, Las Vegas, NV.
The EPA Project Officer, Brian A. Schumacher (see below), is with the Environ-
mental Monitoring Systems Laboratory, Las Vegas, NV 89193-3478.
The complete report, entitled "Behavior and Determination of Volatile Organic
Compounds in Soil: A Literature Review," Order No. PB94-100 153/AS; Cost:
$27.00, 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
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