United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-92/170 October 1992
EPA Project Summary
Soil Vapor Extraction Column
Experiments on Gasoline
Contaminated Soil
Michael E. Miller, Tom A. Pederson, Carole A. Kaslick and George E. Hoag
Soil vapor extraction (SVE) is a tech-
nique that is used to remove volatile
organic compounds from unsaturated
soils. Air is pumped through and from
the contaminated zone to remove vapor
phase constituents. In this work, labo-
ratory soil column experiments were
conducted using a sandy soil residu-
ally saturated with gasoline to evaluate
the performance of SVE under con-
trolled conditions. Both vapor extrac-
tion and aqueous leaching of the soil
columns were conducted. The progress
of the vapor extraction event was con-
tinuously monitored by an in-line total
hydrocarbon analyzer (THA). Perfor-
mance of vapor extraction was evalu-
ated by a series of soil chemical
analyses including total petroleum hy-
drocarbons, headspace measurements,
and extraction techniques with quanti-
fication by GC/FID and GC/MS. Con-
taminant levels in aqueous percolate
were compared before and after SVE.
After 60 pore volumes of water flow
through a column, the percolate from
the contaminated soil still contained at
least 100 mg/L of total hydrocarbons.
Vapor extraction of contaminated soil
reduced total hydrocarbons by 99.96%,
and subsequent aqueous leaching re-
sulted in percolate concentrations of
3.7 mg/L initially and 0.6 mg/L after 60
pore volumes of water flow.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, 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).
Introduction
SVE is an innovative technology used
to remove volatile compounds from un-
saturated soils. The basic principles of
SVE technology are straightforward. By
inducing air flow from and through the
subsurface vadose zone, vapor phase
contaminants are flushed from the soil
pores. Extraction wells are the usual route
by which contaminants are recovered from
the subsurface.
The process consists of the following
steps. Contaminant laden air is withdrawn
through extraction wells under a vacuum
created by an above-ground blower or
vacuum pump. The air flow is controlled
by ball or butterfly valves and is monitored
by vacuum gauges. The extracted vapor
stream passes through an air/water sepa-
rator to remove moisture and protect the
blower. After the blower, the air stream
passes through a heat exchanger to con-
trol the relative humidity and improve the
efficiency of the subsequent vapor treat-
ment operation. Injection wells are optional
as a means to enhance air flow. A cap or
surface seal placed over the treatment
area to control the vapor flow path is also
an optional component of the system. The
seal, which could be as simple as plastic
sheeting, serves to induce the air to flow
in a horizontal manner as opposed to a
vertical flow pafhway that may result from
air being drawn from the surface only near
the extraction well.
Vapor extraction is most effective in re-
moving compounds that exhibit significant
volatility at ambient temperatures in soil
(i.e., vapor pressure greater than 0.5 mm
of mercury at 20°C and a dimensionless
Gjj/0 Printed on Recycled Paper
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Henry's Law constant greater than 0.01).
This includes most gasoline constituents
and solvents. Additionally, the mobility of
the compound due to its affinity for the
soil organic matter and small pore spaces
affects the success of the treatment.
Compounds strongly sorbed to the soil
matrix tend to be more difficult to extract.
Although SVE has been used with great
success at numerous sites, the behavior
of the contaminants remaining in the soil
has not been completely assessed. The
efficacy of SVE as a remedial technology
for extraction of gasoline contaminated
soils was investigated using laboratory
scale soil column experiments. One di-
mensional air flow rates were controlled
within a small soil sample with distinct
boundaries. Other parameters controlled
Included moisture content, chemical con-
taminants, and soil characteristics.
The experimental program had three
main objectives:
* Determine the limitations on gasoline
removal from soil using vapor extrac-
tion under controlled, optimized labo-
ratory conditions.
• Evaluate the aqueous mobility of the
constituents in contaminated soil and
those that remain in the soil following
SVE.
• Assess the appropriateness of avail-
able analytical techniques to measure
son contaminant levels.
Experimental Methods
Stainless steel soil columns were 29.8
cm (1 ft) in length with a 10.8 cm (4.25 in.)
internal diameter. A rigid, porous Teflon
disk sat at the bottom of the column and
supported the soil sample; a second po-
rous disk rested on top of the soil. Stain-
less steel endplates were bolted to either
end of the columns, and Won* gaskets
were used to create an air- and water-tight
seal. The columns terminated at each end
in needle valves, which could be closed to
isolate the system. The internal volume of
each column was 2.5 L
The columns were weighed to the near-
est gram following each manipulation. Four
identical columns were packed with Con-
necticut sand; 97% of those particles were
between 0.1 mm and 5 mm in diameter,
with an average diameter of 0.5 mm. The
sand was predominantly quartz (35%) and
feldspar (35%) and had a surface area of
1.9 m^g. As the soil was added to the
columns in 2.5 cm lifts, the columns were
vibrated to achieve maximum compaction.
The process of soil addition and shaking
were repeated until the columns were full
(approximately 4.7 kg of soil). The in-place
bulk density was 1.9 g/cm3, and the po-
rosity was 30% as determined by mass
difference (columns empty, packed with
soil, and saturated with water).
The packed columns were saturated
with water containing 100 mg KCN/L to
prevent microbial activity, drained to field
capacity moisture content, saturated with
gasoline, and allowed to drain to residual
saturation. At various stages of the ex-
periment, a column was opened, the soil
was poured into a stainless steel bowl,
transferred to sample vials and jars, and
analyzed by the battery of techniques listed
in Table 1. One column was sacrificed for
soil analysis after residual saturation with
gasoline.
For headspace analysis, 1 g of soil was
spiked with ~ 500 ppm of""a~,a,~d-~
trifluorotoluene in a 10 mL Teflon-lined,
septum-sealed vial and was heated in a
water bath at 90°C for 20 min. This treat-
ment was assumed to displace all volatile
compounds in the sample into the
headspace. A 200-u.L sample of the
headspace was injected directly into a GC/
FID to obtain a value for headspace vola-
tiles.
Chromatographic peaks eluting between
2-methylpentane and 1,2,4-trimethylben-
zene, defined as the gasoline range or-
ganic (GRO) compounds, were summed.
The area of the internal standard was
subtracted from this total area. The sub-
sequent sample peak area was directly
correlated to a five point calibration curve
utilizing the peak area summation of a 10-
compound mixture containing 2-
methylpentane; 2,2,4-trimethylpentane;
heptane; benzene; toluene; ethylbenzene;
m-xylene; p-xylene; o-xylene; and 1,2,4-
trimethylbenzene. Varying measured
amounts of this mixture were added to
clean soil, and 1 g portions were analyzed
as above to create the calibration curve
for soil GRO concentration.
The final two columns were vapor ex-
tracted simultaneously at a flow rate of
2.1 L/min (4.5 scfh). Air, supplied by com-
pressed air cylinders, passed through an
activated carbon column to remove hydro-
carbons, and then through a flask of water
via a diffuser stone. The humidified air
split into two equal streams and flowed
first through flow meters, then the soil
columns. The column exhaust air flowed
to a sequential sampler. The vapors from
one column, then the other, were sent
every minute to a THA for continuous
monitoring of total hydrocarbon levels.
After 6.5 days of vapor extraction, one
column was sacrificed for soil analysis;
the other received 60 pore volumes of
rainwater flow as above. The soil from this
final column was analyzed at the conclu-
sion of the experiment.
Results and Discussion
Measurements are recorded for a soil
column residually saturated with gasoline,
a column following aqueous leaching, and
a vapor extracted column. Multiple values
for any measurement are replicate sample
analyses.
The gasoline retained in each column
was measured directly by mass difference.
The mass of each column at field capacity
moisture content was subtracted from the
mass of the same column residually satu-
rated with gasoline. Any additional water
that was lost during the drainage of the
excess gasoline was collected in a gradu-
ated cylinder and was also subtracted to
yield the final value. Initial gasoline con-
centrations ranged from 12,700 to 15,300
Table 1. Soil Analytical Techniques-
Measurement
USEPA SW-846
Method No. or Ret
Procedure
• MwMfon of trad* names or commercial products does
not constitute endorsement or recommendation for
use.
Total volatile solids
Total petroleum
hydrocarbons
Gasoline range
organics
Headspace
Semi-volatile organic
compounds
Volatile organic
compounds
160.4
9071/418.1
Enseco-RMAL, 1990
Method described below
8270
8260
Heat to 10&C
then 550°C
Freon extraction,
IP
Methanol extraction,
GC/FID or GC/MS,
integration of peaks
GC/FID
Methylene chloride
extraction, GC/MS
Methanol extraction,
GC/MS
-------
rag/kg. These values were greater than
the results of each of the analytical meth-
ods employed. The discrepancy can be
attributed to volatilization losses during the
transfer of soil samples from the columns
to vials and jars for storage before analy-
sis as well as to losses incurred during
the performance of the analytical methods
themselves.
Aqueous leaching of a soil column re-
sidually saturated with gasoline resulted
in the mobilization of some of the retained
GRO during 60 pore volumes (about 43.2
L) of water flow. In the percolating water
during the leaching experiment, the peak
concentration of GRO was 3,000 mg/L at
the onset of leaching. The concentrations
continually dropped, and after about 5 pore
volumes, they began to level off; however
by the end of the run, they were still as
high as 170 mg/L.
Integration of the GRO concentrations
during the aqueous leaching experiment
yielded a total of 9.8 g of GRO removed.
The original mass of gasoline in the col-
umn was 79 g, so that about 12% of the
contaminants were mobilized in the per-
colating water. Although benzene dropped
below the detection limit of 1.25 mg/L by
60 pore volumes, the levels of toluene,
ethylbenzene, and xylene remained steady
at 70, 5, and 30 mg/L, respectively.
Vapor extraction was conducted con-
tinuously for 6.5 days, with only brief
breaks in the flow to make adjustments to
the system. Vapor extraction of two con-
taminated columns brought the soil GRO
levels down as low as 0.5 mg/kg, a re-
moval of 99.96% of the original GRO.
During the initial 7 min of vapor extrac-
tion, exhaust gas hydrocarbon concentra-
tions exceeded the instrument's THA
maximum limit of detection of 100,000 ppm
(v/v). After this period, hydrocarbon levels
steadily declined with brief periods of in-
creased concentration spikes following
pauses in the flow. The vapor concentra-
tions exiting the two columns were similar
at first during the rapid decline and in the
final stages as the concentrations leveled
off. Hydrocarbon values, however, differed
by as much as 50% during the middle
stages of the extraction process. These
differences can be explained by different
hydrocarbon starting concentrations, since
the column giving rise to the higher levels
had retained 88 g of gasoline before SVE,
whereas the other began with only 66 g.
Final vapor hydrocarbon levels were be-
tween 20 and 40 ppm (v/v).
Integration of the vapor extraction data
yielded a total hydrocarbon mass removal
that was greater than the initial gasoline
mass by about 70%. This is understand-
able since the THA was operated under
circumstances that were best suited for
generation of qualitative data only. The
THA was calibrated only once and for one
concentration range at the start of gas
flow, whereas several concentration ranges
were used during the course of the ex-
periment. Furthermore, the calibration
mixture contained only butanes and pen-
tane; these were not completely repre-
sentative of the complex gasoline mixture.
The longer chain hydrocarbons and aro-
matic compounds also found in gasoline
exhibit greater THA response factors than
the calibration gas compounds. Concen-
trations fell from a maximum of 3.7 mg/L
at 2 pore volumes to a plateau of 0.6 mg/
L after 60 pore volumes.
Conclusions
The gasoline contaminated sandy soil
was susceptible to aqueous leaching of
the hydrocarbon components. Alter 60
pore volumes of water flow, however, the
majority of product remained in the soil
and significant concentrations of BTEX
were still found in the percolating water.
SVE was effective in removing gasoline
from the soil, with a reduction in the GRO
of 3.5 orders of magnitude. After this pe-
riod, the vapor extracted soil still contained
low levels of water-mobile contaminants.
GRO was the most reproducible of the
analytical techniques tested, and
headspace analysis by GC/FID recorded
the highest concentrations.
Soil column experiments provided a well
controlled, effective measurement of the
soil processes occurring in the aqueous
leaching and vapor extraction of gasoline
contaminated soil.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3409 by COM
Federal Programs Corporation under the
sponsorship of the U.S. Environmental
Protection Agency.
•U.S. Government Printing Office: 1992— 648-080/60108
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Michael £ Miller, Tom A. Pederson, and Carole A. Kaslick are with Camp Dresser
andMcKee, Inc., Cambridge, MA 02141. George E Hoag is with University of
Connecticut, Starrs, CT 06268.
Chl-Yuan Fan is the EPA Project Officer (see below).
The complete report, entitled "Soil Vapor Extraction Column Experiments on
Gasoline Contaminated Soil," (Order No. PB92-226430/AS; Cost: $19.00,
subject to change) will be available only from:
National Technical Information Sen/be
6285 Port Royal Road
Springfield,VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
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