>EPA
States
invironmental Protection
\gency
;PA/540/R-95/533
Office of Research and
Development
Washington, DC 20460
September 1995
Office of Solid Waste and
Emergency Response
Washington, DC 20460
Bioremediation Field
valuation
Alas:
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Notice
This project has been funded wholly or in part by the U.S. Environmental Protection Agency. It
has been subjected to the Agency's peer and administrative review and approved for publication
as an EPA document. Mention of trade names or commercial products does not constitute en-
dorsement or recommendation for use.
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The Bioremediation Field Initiative
In 1990, the U.S. Environmental Protection Agency (EPA) established the Bioremediation
Field Initiative as part of its overall strategy to increase the use of bioremediation to treat
hazardous wastes at Comprehensive Environmental Response, Compensation, and Liabil-
ity Act (CERCLA, or Superfund) and other contaminated sites. The primary purpose of the
Initiative is to collect and disseminate information on the capabilities of bioremediation
technologies so that EPA and state project managers, consulting engineers, and industry
representatives can make better-informed decisions about applying bioremediation in the
field. Participants in the Initiative include EPA's Office of Research and Development, Office
of Solid Waste and Emergency Response, and regional offices, as well as other federal
agencies, state agencies, industry, and universities.
The Initiative conducts a variety of activities to facilitate the exchange of information about biore-
mediation, including sponsoring technology-transfer conferences on topics related to bioreme-
diation, maintaining an electronic database of information on bioremediation sites nationwide,
and publishing a quarterly bulletin of recent developments in field applications of bioremediation.
In addition, the Initiative provides support to states and regions for intensive evaluation of biore-
mediation at selected sites across the country. The extent of the Initiative's involvement at these
sites varies, from providing support for laboratory feasibility studies, to assisting with field treata-
bility studies, to overseeing and assessing full-scale site remediations.
Sites are nominated for field evaluations through the EPA regional offices or through the
states with concurrence from the regional offices. To date, nine sites have been selected
for performance evaluation of bioremediation: West KL Avenue Landfill Superfund site,
Kalamazoo, Michigan; Libby Ground Water Superfund site, Libby, Montana; Park City
Pipeline, Park City, Kansas; Bendix Corporation/Allied Automotive Superfund site, St.
Joseph, Michigan; Eielson Air Force Base Superfund site, Fairbanks, Alaska; Hill Air Force
Base Superfund site, Salt Lake City, Utah; Escambia Wood Preserving site-Brookhaven,
Brookhaven, Mississippi; Public Service Company site, Denver, Colorado; and Reilly Tar
and Chemical Corporation Superfund site, St. Louis Park, Minnesota.
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Acknowledgments
This document was prepared for the U.S. Environmental Protection Agency's (EPA's) Office
of Research and Development (ORD) and Office of Solid Waste and Emergency Response.
Dr. Fran Kremer, ORD, served as Program Director, and Dr. Gregory Sayles, EPA National
Risk Management Research Laboratory, provided technical direction from EPA for the re-
search conducted at Eielson Air Force Base. Technical direction from the U.S. Air Force
was provided by Dr. Ross Miller and Ms. Catherine Vogel. The work was carried out by
Battelle Memorial Institute, with Drs. Robert Hinchee and Andrea Leeson serving as Project
Managers.
EPA also gratefully acknowledges the technical and financial contributions of those who
collaborated with EPA to conduct this field evaluation. In particular, EPA wishes to acknow-
ledge the additional funding provided by Eielson Air Force Base, the U.S. Air Force
Armstrong Laboratory, and the U.S. Air Force Center for Environmental Excellence. EPA
also acknowledges the assistance of Valerie Overt on, Eastern Research Group, Inc. (ERG),
who provided writing and editing support.
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Eielson Air Force Base
ABSTRACT
This publication, one of a series presenting the findings of the Bioremediation Field Initiative's
bioremediation field evaluations, provides a detailed summary of the evaluation conducted at
the Eielson Air Force Base (AFB) Superfund site in Fairbanks, Alaska. At this site, the Initiative
provided support for an evaluation of bioventing with soil warming systems to stimulate in
situ bioremediation of soil contamination resulting from a JP-4 jet fuel spill. The purpose of
the evaluation was to assess the feasibility of using bioventing technology to remediate JP-4
jet fuel contamination in a cold climate. The evaluation was conducted as a joint effort of
the U.S. Air Force and the U.S. Environmental Protection Agency's (EPA's) National Risk
Management Research Laboratory (NRMRL).
The Air Force and NRMRL operated a bioventing system in a contaminated site at Eielson
AFB. During most of the study, the system was operated as an air injection system—one of
the first such systems ever evaluated. For comparison, the system was briefly operated in
the air extraction mode. Extraction bioventing was found to be much less efficient than in-
jection bioventing.
To evaluate injection bioventing with and without soil warming, the Air Force and NRMRL op-
erated the system in four contaminated Eielson AFB test plots: one in which the soil was
warmed via circulation of heated ground water, one in which the soil was warmed via heat tape,
one in which the soil was warmed via solar heating, and one with no soil warming (the control).
The Air Force and NRMRL conducted a variety of tests to measure soil temperatures,
microbial respiration/contaminant biodegradation rates, and extent of contaminant removal, as
well as to determine whether air injection bioventing generates air emissions. All three soil warm-
ing methods raised soil temperatures and stimulated biodegradation, but the warm water
and heat tape methods resulted in high soil temperatures year-round and respiration/
biodegradation rates two to three times higher than the rates found in the unheated
control. Significant contaminant removal occurred, and no significant air emission problems
were detected.
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FIELD EVALUATION
Purpose of the
Evaluation
Petroleum distillate fuel hydrocar-
bons such as JP-4 jet fuel are gener-
ally biodegradable if indigenous
microorganisms receive an adequate
supply of oxygen and nutrients. Typi-
cally, much of the hydrocarbon resi-
due at fuel-contaminated sites lies in
unsaturated (vadose) zone soils im-
mediately above the water table. To
successfully bioremediate such sites,
adequate oxygen must be provided
to the unsaturated zone soils. To
date, most efforts to bioremediate
fuel spills have focused on soluble
fuel components in ground water
rather than hydrocarbon residues
in unsaturated zone soils.
Conventional bioremediation sys-
tems use water to carry oxygen to
the contamination. When water-
based systems are used to remedi-
ate contaminated soil, however,
oxygen usually remains the limiting
factor. This problem has led re-
searchers to investigate the use of
air as an alternative source of oxy-
gen. Air has two major advantages
over water. First, on a mass basis,
less air than water is needed to de-
liver adequate oxygen. Second, air is
more diffusible than water, facilitat-
ing delivery of oxygen to soils such
as clay that are relatively imperme-
able to water.
Researchers had reason to believe
that moving air through soil could
indeed supply enough oxygen to
promote biodegradation of petro-
leum contaminants. As early as
1981, researchers had begun evalu-
ating soil vapor extraction (SVE)
technology to remediate petroleum-
contaminated soils. The technol-
ogy involved moving air through
contaminated soils at high rates
to promote volatilization of the
contaminants. Although SVE tech-
nology was designed to promote
volatilization, researchers found that
it stimulated aerobic biodegradation
as well. This finding generated in-
terest in developing a different soil
aeration technology—called bio-
venting—that would maximize
biodegradation rather than volatili-
zation (1-3). Researchers found
that using lower air flow rates (and
other design differences) accom-
plishes this goal (4, 5). Thus,
bioventing is the process of mov-
ing air through subsurface soils to
provide oxygen to microorganisms
and stimulate aerobic biodegrada-
tion. As Figure 1 shows, the air
movement required for bioventing
can be achieved by blowing air into
the soil (injection bioventing) or by
creating a vacuum to pull air out of
the soil (extraction bioventing).
Although both bioventing and SVE
technology involve moving air
through soil, they differ in design
and objective: biodegradation
versus volatilization.
In 1988, the U.S. Air Force initi-
ated a study at Hill Air Force
Base (AFB) to examine the poten-
tial of bioventing to remediate
JP-4 jet fuel-contaminated soils.
The results were promising,
Low Rate
Air Injection
Figure 1. Schematic diagrams of injection bioventing (A) and extraction bioventing
(B) technology.
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prompting additional studies at Hill
and Tyndall AFBs. Based on suc-
cesses in these warm-weather sites,
the Air Force and the U.S. Environ-
mental Protection Agency's (EPA's)
National Risk Management Re-
search Laboratory (NRMRL) be-
came interested in the possibility
of using bioventing in cold climates.
Microbial degradation occurs
slowly, if at all, however, at low
temperatures. The Air Force and
NRMRL decided to study the use of
soil warming measures to enhance
the effectiveness of bio-venting
in a cold climate. They selected
Eielson AFB in Fairbanks, Alaska, as
the study site. In winter, soil temper-
atures at this site drop to about 0°C.
The field evaluation at Eielson AFB
was undertaken to determine
whether and to what degree soil
warming can enhance the effective-
ness of bioventing jet fuel contami-
nated soil in a cold climate. The
evaluation also aimed to determine
whether soil warming promotes
high-rate, year-round bioremediation
at a lower overall cost than pro-
longed low-rate bioremediation at
ambient temperatures. The results of
the evaluation are summarized below.
They have also been discussed in
other publications (6-8); see those
publications for additional information.
Site History
Eielson AFB is an active base
located in the Alaskan interior,
about 25 miles southeast of Fairbanks
(see Figure 2). The base serves a wide
variety of aircraft and maintains a
high volume of traffic. The climate is
subarctic, with an average annual
temperature near 0°C. Ambient tem-
peratures range from below -30°C in
the winter to above 30°C in the sum-
mer. Permafrost is present in some
areas on Eielson AFB, but not in Site
20, the area selected for this field
Figure 2. Location of Eielson AFB.
evaluation. Site 20 is a 1-acre area
of land centered over two pressur-
ized lines that intersect the site.
The pressurized fuel lines are sus-
pected to be the source of the fuel
release because the area where
the lines intersect is the most highly
and uni-formly contaminated part of
the site.
The Air Force conducted a site
characterization in July 1991,
which revealed that the surface
soil at Site 20 is a mixture of sand
and gravel, with silt concentration
increasing to about 6 ft. The soil
was contaminated with JP-4 jet
fuel from a depth of roughly 2 ft to
the water table at 6 to 7 ft. Total
petroleum hydrocarbon (TPH) levels
ranged from 100 to 3,000 mg/kg,
depending on soil depth and area.
A hydrocarbon sheen was visible in
the ground-water monitoring wells
subsequently installed, and
ground-water samples showed TPH
levels of 15 to 20 mg/L.
In summer 1991, the Air Force and
NRMRL installed and began operat-
ing an in situ soil bioremediation
system: a bioventing system con-
sisting of an air blower plumbed to
air injection/extraction (bioventing)
wells. The system could operate as
an injection or extraction biovent-
ing system; the Air Force and
NRMRL conducted most of the
study in the injection mode, which
is the generally preferred method
of bioventing. Operating the
bioventing system involved using
the blower to inject atmospheric
air into the contaminated subsur-
face at a rate of 25 cubic feet per
minute (ft3/min). Air injection/ex-
traction wells were distributed uni-
formly at 30-ft intervals to provide
relatively uniform aeration. The Air
Force and NRMRL constructed
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four 50-ft square test plots in the
contaminated area:
• A warm water test plot in
which ground water collected
via an extraction well was
pumped through an electric
heater, heated to about 35°C,
then pumped through soaker
hoses buried 2 ft underground
at a rate of 1 gallon per minute
(gpm). Water draining into a re-
turn manifold was returned to
the extraction well for recircula-
tion (see Figure 3). The heated
water was applied below the
ground surface to increase the
temperature of the contami-
nated soil while minimizing
volatilization of contaminants.
Insulation was placed over the
ground surface to retain heat.
• A heat tape test plot i n wh ich
strips of heat tape were bur-
ied at a depth of 3 ft to warm
the soil directly (see Figure 4).
The total heating rate was
about 1 watt per square foot.
Insulation was placed over the
ground surface to retain heat.
• A solar test plot in which insula-
tion was placed over the ground
surface during the winter months,
then replaced with plastic mulch
sheeting during the spring and
summer months to capture solar
heat and passively warm the soil.
• A control test plot, which re-
ceived no soil warming.
All four test plots contained air injec-
tion/extraction wells, thermocouples
for monitoring soil temperature, and
three-level soil gas monitoring points
for monitoring oxygen delivery and
for sampling soil gas during in situ
respiration tests (see Figures 5 and 6).
Additional air injection/extraction
wells, thermocouples, and soil gas
monitoring points were installed at
various points outside the test
plots to permit monitoring across
the contaminated site. The Air
Force and NRMRL monitored natu-
ral background respiration rates
Warm Water Test Plot
50'
Ground Water
From Heater
50'
w
,
<•
y
f
n
k
1
,
f
y
r
A
k
>
L
H..
/
r
Soaker Hoses in
Drainage Tile
To Extraction Well
Figure 3. Circulation of heated ground water in the warm water test plot.
Heat Tape Test Plot
50'
501
Heat Tape Strip #1 Heat Tape Strip #2
Figure 4. Arrangement of heat tape strips in the heat tape test plot.
in an uncontaminated area about
200 ft east of the contaminated
site. This area received air injection
(via one injection/extraction well)
but no soil warming; it also con-
tained two soil gas monitoring
points and one thermocouple.
Ground-water contamination was
monitored via ground-water moni-
toring wells installed at various
points in contaminated and uncon-
taminated areas. These tests were
conducted as part of the field
evaluation, discussed below.
With a couple of exceptions, the
Air Force and NRMRL operated the
bioventing and soil warming sys-
tems for 3 years, from summer
1991 to summer 1994. They termi-
nated warm water circulation after
2 years in order to compare micro-
bial activity in the warm water test
plot with and without active soil
warming, and they operated the heat
tape test plot for only 2 years (from
summer 1992 to summer 1994).
Conducting the
Evaluation
The Air Force and NRMRL, with
support from the Bioremediation
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Solar
S •
sss
o
25'
Scale
50'
O
T O
O
o
o
o
Control
T
O
S S •
3
ST
S
SST
S
• *
Warm Water
o
Water
Warming
System
\
o
TT •
ssss
s
ST
O
Heat Tape
O
o
O
N
@ Ground-water monitoring well
• Air injection/withdrawal well (in use)
O Air injection/withdrawal well (not in use)
S Three-level soil gas probe
T Three-level thermocouple probe
Figure 5. Schematic plan view of the bioventing site showing air
injection/extraction wells, thermocouples, soil gas monitoring points, and
ground-water wells inside and outside the four test plots.
Air Injection/Withdrawal Wells
Plastic Sheeting
— Styrofoam Insulation ^_
r Plywood Three-Level
I Thermocouples
| i— Vinyl Cover :
Three-Level
Soil Gas Probes
Figure 6. Cross section of a test plot showing an air injection/extraction well, a
three-level thermocouple, three three-level soil gas probes, and the surface
covering used in the warm water (year-round), heat tape (year-round), and solar
(summer only) test plots.
Field Initiative, combined their data
to perform a field evaluation of
bioventing under the four test con-
ditions: soil warming via warm
water circulation, soil warming via
heat tape, soil warming via solar
heat, and no soil warming. The
evaluation had three major ele-
ments. The first consisted of a
system performance evaluation,
which involved measuring the ef-
fects of the bioventing and soil
warming on soil gas oxygen levels,
soil temperature, microbial respira-
tion, and contaminant levels. The
second involved several other field
measurements to evaluate the de-
sign and function of the biovent-
ing and soil warming systems. The
third was a cost evaluation to esti-
mate and compare the costs of op-
erating the systems.
System Performance
Soil Gas Sampling
To assess the effectiveness of the
bioventing system in aerating the
soil in the test plots, the Air Force
and NRMRL conducted soil gas
sampling about once a week. Prior
to bioventing, oxygen levels were
low (mostly less than 10 percent),
and carbon dioxide and total hydro-
carbon levels were correspondingly
high (mostly greater than 10 percent
and 5,000 parts per million, re-
spectively). After air injection was
initiated, oxygen levels increased,
while carbon dioxide and total
hydrocarbon levels decreased. Oxy-
gen levels in the warm water test
plot were generally lower than
those in the other test plots, possi-
bly due to the higher moisture con-
tent of the soil and the higher level
of microbial activity (see discussion
of in situ respiration tests below).
Nevertheless, except during in situ
respiration tests (see below), soil
gas oxygen levels almost always ex-
ceeded 8 percent. As a result, oxy-
gen level had no effect on the
performance of the test systems.
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Soil Temperature
Soil temperatures were collected
two to three times a day by means
of an automatic data logger. Warm
water circulation and heat tape
each raised soil temperatures sub-
stantially (see Figure 7). During the
winter, the average soil tempera-
ture in these test plots was about
10°C—several degrees higher
than in the solar and control test
plots. During the third year of op-
eration, when warm water circula-
tion system was terminated, soil
temperatures in the warm water
test plot dropped steadily, falling
to 2 to 3°C below that in the solar
test plot. After the first season of
solar warming, soil temperatures in
the solar test plot were 1 to 8°C
higher than those in the control test
plot, depending on the season.
In Situ Respiration Tests
The Air Force and NRMRL con-
ducted in situ respiration tests at
selected soil gas monitoring points
once a month, and in all soil gas
monitoring points once every 3
months (9, 10). To conduct the tests,
the Air Force and NRMRL monitored
soil gas oxygen and carbon dioxide
levels during air injection, then turned
off air injection and periodically meas-
ured the oxygen and carbon diox-
ide levels over a period of several
days. They used these measurements
to calculate oxygen consumption
and carbon dioxide production
rates, which they in turn used to
estimate biodegradation rates.
Figure 8 shows the average rate
of biodegradation in each test plot
during the study period. The high
moisture content of the soil in the
warm water test plot made soil
gas sampling difficult, especially
in the deeper soil gas monitoring
points, where contamination levels
(and thus respiration/biodegrada-
tion rates) were highest. As a result,
the average biodegradation rates
shown for the warm water test plot
25
20
15
10
5
g 0
v
o
Q.
"\j/-" Background --
A
1991
1992
1993
1994
Figure 7. Average soil temperature in each of the four test plots and in the
background area during the 3 years of bioventing.
CO
•a
CD
oc
c
o
V2
•a
s
D)
CD
•a
o
5 n
• Warm Water Plot
O Solar Plot
v Control Plot
o Heat Tape Plot
OCTIJAN APR AUG MOV JAN MAY JUL MOV JAN APR JUL
1991 1992 1993 1994
Figure 8. Average rate of biodegradation in each test plot during the 3-year study
period, as measured by in situ respirometry.
are probably underestimates. Nev-
ertheless, during warm water circula-
tion, biodegradation rates in the
warm water test plot were higher
than those in the other test plots—
typically, three to four times higher
than those in the solar and con-
trol test plots.
8
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During the winter of 1992 to 1993,
when both were operating, the
warm water and heat tape test
plots showed similar biodegrada-
tion rates, although again the warm
water test plot biodegradation rates
are probably underestimated. After
warm water circulation was termi-
nated, biodegradation rates in the
warm water test plot fell below
those in the solar and control test
plots. The Air Force and NRMRL
speculated that the microorgan-
isms might have adapted to higher
temperatures, causing them to be-
come inactive when exposed to
lower temperatures. After the first
season of solar warming, biodegra-
dation rates in the solar test plot were
slightly higher than those in the con-
trol test plot. Biodegradation rates in
the heat tape test plot could not be
measured in 1994 due to excess
moisture in the test plot caused by
high precipitation and poor drainage.
By plotting logarithm biodegradation
rates in the four test plots against
inverse soil temperatures in these
plots, the Air Force and NRMRL
determined that the biodegradation
rate was temperature dependent, as
expected (see Figure 9). This tem-
perature dependence had a substan-
tial impact on hydrocarbon removal.
The total amount of hydrocarbon re-
moved during the study period was
calculated based on the average bio-
degradation rate per season. Total
hydrocarbon removal was an order
of magnitude higher in the warm
water test plot than in the solar and
control test plots (see Figure 10). To-
tal hydrocarbon removal in the warm
water and heat tape test plots could
not be meaningfully compared be-
cause data for the first year of
operation of the heat tape test
plot were not available.
Contaminant Levels
The Air Force and NRMRL col-
lected soil samples from the test
10
CO
T3
I
O)
E
S3
DC
CO
T3
CO
U>
0)
T3
1-f-
0.1
3,40 3.45 3.50 3.55 3.60
1,000/Temperature f K 1)
3,65
3,70
Figure 9. An Arrhenius plot of the temperature dependence of the biodegradation
rates seen at Eielson AFB.
600
Time (days)
800
1,000
1,200
Figure 10. Calculated cumulative amount of hydrocarbons removed from each of
the four test plots over the 3-year study period.
plots and background area (1) at
the beginning of the study, (2) in
September 1992 (after a little
over a year of operation), and (3)
at the end of the study. The Sep-
tember 1992 sampling was con-
ducted because the initial sampling
did not include samples from the
deeper depths, where much of
the contamination was found. The
Air Force and NRMRL collected
ground-water samples from the
ground-water monitoring wells in
the test plots and background area
at the beginning and end of the
study. They analyzed the soil and
ground-water samples for petro-
leum hydrocarbon contamination
using modified standard EPA
methods for gas chromatography.
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Figures 11 and 12 show initial and
final soil TPH and benzene, toluene,
ethylbenzene, and xylene (BTEX)
levels by soil depth, averaged
across the four test plots. Soil TPH
and BTEX levels dropped dramati-
cally, indicating that bioventing re-
sulted in significant contaminant
removal. Similarly, average TPH and
BTEX levels in ground water dropped
from 6.1 mg/L to 0.65 mg/L and
from 9.4 mg/L to nondetect,
respectively.
Other Field Measurements
Surface Air Emissions Testing
Proposals to use bioventing for soil
remediation have raised concern
that contaminant volatilization
might occur, resulting in transfer of
soil contaminants to the atmos-
phere. To determine if the system
used in this study resulted in signifi-
cant atmospheric loading of volatile
petroleum contaminants, the Air
Force and NRMRL performed two
types of surface air emissions tests:
dynamic surface emissions sam-
pling and helium tracing.
The dynamic surface emissions
sampling method involved enclos-
ing an area of soil under an inert
box, purging the ambient air above
the soil with high-purity air to allow
an equilibrium to be established be-
tween hydrocarbons emitted from
the soil and the organic-free air,
sampling the equilibrated air, quan-
tifying the concentration of BTEX
and TPH in this air by gas chroma-
tography, and calculating emission
rates based on the concentrations
thus measured. Seven such tests
were performed in 1993 and 1994;
most were performed in the control
and background areas, with and
without air injection. The emission
rates reported below represent aver-
ages based on measurements taken
at several locations within a test plot.
1 . 1 1 .'
1
h
11.
• Initial
n Final
iLiln
300
250
200
a! 150
100
50
0.5-1.0 2.5-3.0 4.5-5.0 5.5-6.0 6.5-7.0 7.5-8.0 8.5-9.0 9.5-10.0 10.5-11.0
Depth (ft)
Figure 11. Average TPH concentrations in the soil across the site at the beginning
and end of the bioventing study.
120
100
• Initiul
a Finul
4.S-5.0 S.S-6.0 6.5-7.0 7.5-B.Q B.S-9.0 9.5-10.0 105-11.0
Depth.; ft}
Figure 12. Average BTEX concentrations in the soil across the site at the
beginning and end of the bioventing study.
In general, emissions in the control
area were higher when the biovent-
ing system was on than when it was
off. When extrapolated to assume a
1-acre test area, average benzene
emission rates were 0.00083 Ib/day
with air injection and 0.00021 Ib/day
without air injection in the control
area, and 0.00021 Ib/day without
air injection in the background area
(see Table 1). Thus, bioventing did
increase surface emissions, but
emission levels were not much
higher than background levels, and
they were well below regulatory
limits. Surface emissions were
higher during the warm testing
periods than during the cold testing
periods. This seasonal variation was
less pronounced in 1994 than in
1993, suggesting that soil gas hy-
drocarbon concentrations had
diminished and therefore were less
available for volatilization.
10
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The helium tracer study involved
placing plastic sheeting over the
entire control test plot, pumping air
from underneath the plastic (at a
rate of 2.6 ft3/rnin) while injecting
5.4 percent helium into the soil (at
a rate of 2.5 ft3/min) for about 8
days, and measuring the helium and
TPH concentrations in the effluent
air. One such study was conducted
in September 1993. The TPH con-
centration in the effluent air was
340 ppm, which corresponds to an
emission rate of about 1.5 Ib/acre/day
This is similar to the average TPH emis-
sion rate (3.5 Ib/acre/day) found dur-
ing the same time of year using the
dynamic surface emissions sampling
method. The similarity of these results
suggests that both techniques pro-
vide an accurate means of measuring
surface emissions from bioventing.
Verification of Biodegradation
To provide another confirmation that
bioventing was resulting in biodegra-
dation of petroleum contaminants as
intended, the Air Force and NRMRL
analyzed the ratio of stable carbon
isotopes in the carbon dioxide
(13C02/12C02) in the soil gas samples
collected during the study. Because
the isotopic composition of carbon
dioxide produced by hydrocarbon deg-
radation differs from that of carbon di-
oxide produced by other processes,
analyzing stable carbon isotope ratios
is an effective means of determining
whether biodegradation is occurring
(11, 12). Such tests were performed
six times during 1993 and 1994. Sta-
ble carbon isotope ratios in the con-
taminated areas (-18.40 to-29.16%o)
were consistent with hydrocarbon
degradation, while those in the un-
contaminated background area (-10.12
to -19.12%o) were consistent with
natural organic matter metabolism.
Soil Gas Permeability and
Radius of Influence
For the purpose of this field evalu-
ation, the Air Force and NRMRL
Table 1
Average Benzene and TPH Emission Rates Occurring With and Without Air
Injection
Average Emission Rate
Benzene
Control test plot with air injection
Control test plot without air injection
Background area without air injection
TPH (in control test plot)
Dynamic Surface
Emissions Sampling
0.00083 Ib/day
0.0002 lib/day
0.0002 lib/day
3.5 Ib/day
Helium Tracing
1.5 Ib/day
placed air injection/extraction
wells relatively close together (15 ft
apart) to ensure adequate and uni-
form aeration of the test plots at
Eielson AFB. To determine what
blower size and well placement
configuration would be optimal for
full-scale bioventing operations, the
Air Force and NRMRL measured soil
gas permeability and the radius of in-
fluence of the injection/extraction wells
used at the Eielson AFB site. Soil gas
permeability is the soil's capacity
for gas flow, while radius of influence
is the greatest distance from an
injection/extraction well where
measurable soil gas movement (i.e.,
measurable vacuum or pressure) occurs.
The Air Force and NRMRL meas-
ured the pressure in the various
soil gas monitoring points during
air injection. The pressure values
recorded at a depth of 6 ft are dis-
cussed here because that is the depth
at which most of the contamination
was located. Based on the pressure
measurements, the Air Force and
NRMRL calculated permeability
Table 2
values of 0.56 to 1.0 darcy (see
Table 2), indicating that soil gas per-
meability was relatively uniform
throughout Site 20 and that the soil
warming systems did not signifi-
cantly affect soil gas permeability.
The radius of influence ranged from
40 to 77 ft, with an average of about
61 ft. Taking a conservative approach
and using the smallest radius of
influence measured (40 ft), placing
injection/extraction wells 80 ft apart
should be sufficient to achieve ade-
quate and uniform soil aeration in full-
scale bioventing operations. Nine
wells would treat more than 1 acre
of a contaminated site.
Tests Comparing Air Injection,
Air Extraction, and Air
Extraction With Reinjection
During most of the study period, the
Air Force and NRMRL operated the
bioventing system as an injection
system. Injection bioventing is gener-
ally preferred over extraction biovent-
ing, in part because it is less costly.
Some researchers are concerned,
however, that the injected air could
Permeability of the Soil and Radius of Influence of the Injection/Extraction
Well in Each Test Plot at a Depth of 6 Ft
Test Plot
Warm Water
Heat Tape
Solar
Control
Control
Mode of Bioventing
Injection
Injection
Injection
Injection
Extraction
Permeability
(darcy)
1.0
0.86
0.80
0.56
0.27
Radius of Influence
(ft)
58
77
40
68
36
11
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force contaminated soil vapors to
be emitted. Extraction bioventing
avoids this problem because it cap-
tures contaminated soil vapor (see
Figure 1). It is more costly, however,
and it generates point source emis-
sions that might require permitting
and treatment. Reinjecting the off-
gas might eliminate this problem but
could pose problems in the winter,
when moisture in the extracted gas
could cause the injection/extraction
lines to freeze. To compare the feasi-
bility and efficiency of injection
bioventing, extraction bioventing, and
extraction bioventing with off-gas rein-
jection, the Air Force and NRMRL
operated the bioventing system at
Eielson AFB as an extraction system
in August 1993, and as an extraction
with reinjection system for 5 days in
September 1993.
During the extraction bioventing
test, the Air Force and NRMRL
measured soil gas pressure and flow
rate as well as oxygen and TPH con-
centrations in the soil gas in each
test plot. From these measurements,
they determined that the soil in the
test plots was rapidly aerated. They
also used the measurements to cal-
culate the mass of TPH biodegraded
and volatilized in each test plot. In
total, the biodegradation rate was
about ten times the volatilization
rate during soil vapor extraction
(see Table 3). Biodegradation was
probably even more dominant dur-
ing injection bioventing because
air injection pushes vapors from
contaminated to uncontaminated
areas, creating an expanded
bioreactor and allowing for more
biodegradation.
The Air Force and NRMRL also
found that the positive pressure
created by air injection in the un-
saturated zone resulted in depres-
sion of the water table, while the
partial vacuum created by air ex-
traction resulted in an upwelling of
the water table (see Figure 13). This
Table 3
Rate of Biodegradation and Volatilization in Each Test Plot During Extraction
Bioventing, as Determined by Off- Gas Composition
Test Plot
Warm Water
Heat Tape
Solar
Control
Total
Biodegradation Rate
(Ib/day)
0.078°
0.31
4.4
1.4
6.2
Volatilization Rate
(Ib/day)
0.0028
0.055
0.35
0.19
0.60
a A flow rate of 0.05 ft /min was estimated for this test plot.
was important, because lowering the
water table dewatered the capillary
fringe, exposing more soil to air flow
and allowing this highly contaminated
area to be more effectively treated.
Raising the water table, in contrast,
saturated more contaminated soil,
reducing soil exposure to air flow
(A)
13'
-10'-"
(B)
M- 5'-**
55
•t:b
Figure 13. Depression of the water table during injection bioventing (A) and
upwelling of the water table during extraction bioventing (B) at Eielson AFB. In
Figure ISA, the vertical dimension is exaggerated to more clearly show the water
table depression.
12
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and reducing treatment efficiency.
Not surprisingly, soil permeability
and radius of influence values
were much lower during extraction
bioventing than during injection
bioventing (see Table 2).
Unlike injection bioventing, extrac-
tion bioventing requires use of
explosion-proof blowers with ex-
plosion-proof wiring, a knockout
(air-water separator) to reduce the
moisture content of the extracted
soil gas, technologies to treat the
condensate collected, insulation
and/or heat tape to prevent freezing
of pipes in the winter, and permit-
ting and treatment of point source
emissions. Because injection bio-
venting avoids these costs and af-
fords greater treatment efficiency,
the Air Force and NRMRL con-
cluded that injection bioventing is
generally preferable to extraction
bioventing. They noted, however,
that extraction bioventing might be
preferable at contaminated sites
near possible vapor receptors
(e.g., basements and storm sew-
ers) because soil extraction bio-
venting captures contaminated va-
pors that might otherwise enter
these receptors.
The results of the extraction biovent-
ing with off-gas reinjection test were
generally similar to those of the ex-
traction bioventing test. The Air Force
and NRMRL noted that extraction
bioventing with off-gas reinjection
might be a feasible alternative to
extraction bioventing alone because
reinjection of the off-gas eliminates
point source emissions. Extraction
bioventing with reinjection, however,
might increase the potential of line
freezing in the winter, posing addi-
tional operational problems.
Cost Evaluation and
Comparison
To evaluate the cost-effectiveness
of bioventing with soil warming,
the Air Force and NRMRL estimated
the cost of remediating jet fuel con-
taminated vadose zone soils using
bioventing and soil warming systems
similar to those used at Eielson AFB.
These estimates take into account
the time needed to achieve adequate
remediation based on the biodegra-
dation rate provided by each sys-
tem. They are based on optimal
operating conditions rather than ac-
tual costs because the Eielson AFB
systems were modified and improved
during the course of the study.
The Air Force and NRMRL prepared
cost estimates for two scenarios:
remediation of a 5,000-yd3 site
having an average TPH contamina-
tion level of 8,000 mg/kg (see
Table 4), and remediation of a
5,000-yd3 site having an average
TPH contamination level of
4,000 mg/kg (see Table 5). For a
given level of contamination, the
cost-per-cubic-yard of remediating
soil using the four treatment sys-
tems was about the same. That is,
the costs shown in Table 4 are not
significantly different given the
level of uncertainty associated with
the estimates; similarly, the costs
shown in Table 5 are not signifi-
cantly different. Given the similar
remediation costs, the choice of
bioventing method at a site like
Eielson AFB depends not on total
Table 4
Estimated Cost of Remediating Soil Containing 8,000 mg/kg TPH Using Bioventing0
Task
Site Visit/Planning
Work Plan Preparation
Pilot Testing
Regulatory Approval
Full-Scale Construction
Design
Drilling/Sampling
Installation/Start Up
Remediation Time Required0
Monitoring
Power
Final Soil Sampling
2
Cost per yd
Basic
5,000
6,000
27,000
3,000
7,500
15,000
4,000
18.8 years
61,100
26,320
13,500
$34.28
Warm Water
5,000
6,000
27,000
6,000
7,500
20,000b
26,000
5.6 years
19,600
19,600
13,500
$30.04
Solar Warming
5,000
6,000
27,000
3,000
7,500
15,000
10,500
13.8 years
48,300
48,300
13,500
$31.62
Heat Tape
5,000
6,000
27,000
3,000
7,500
15,000
13,000
6.8 years
22,100
22,100
13,500
$29.82
1 Based on the total time required to remediate a 5,000-yd3 site.
Requires installation and development of one well.
c Estimated based on the average biodegradation rate observed in each of the four test plots at Eielson AFB.
13
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Table 5
Estimated Cost of Remediating Soil Containing 4,000 mg/kg TPH Using Bioventing0
Task
Site Visit/Planning
Work Plan Preparation
Pilot Testing
Regulatory Approval
Full-Scale Construction
Design
Drilling/Sampling
Installation/Start Up
Remediation Time Required0
Monitoring
Power
Final Soil Sampling
o
Cost per yd
Basic
5,000
6,000
27,000
3,000
7,500
15,000
4,000
9.4 years
30,550
13,160
13,500
$25.50
Warm Water
5,000
6,000
27,000
6,000
7,500
20,000b
26,000
2.8 years
9,800
9,800
13,500
$26.12
Solar Warming
5,000
6,000
27,000
3,000
7,500
15,000
10,500
6.9 years
24,150
9,660
13,500
$24.86
Heat Tape
5,000
6,000
27,000
3,000
7,500
15,000
13,000
3.4 years
11,050
17,000
13,500
$24.21
a Based on the total time required to remediate a 5,000-yd3 site.
Requires installation and development of one well.
c Estimated based on the average biodegradation rate observed in each of the four test plots at Eielson AFB.
treatment cost but on the desired
timeframe for remediation: how
quickly the remediation team
wishes to clean up the site versus
how quickly those responsible
wish to pay for the cleanup.
Actual remediation costs will also
depend on site-specific factors,
such as annual temperature pattern,
soil gas permeability, contamination
level, and so on. Regarding con-
tamination level in particular, the
Air Force and NRMRL noted that
the cost of remediation using bio-
venting increases only somewhat
with increasing levels of contamina-
tion. That is, remediating soil con-
taminated with TPH at 8,000 mg/kg
(Table 4) does not cost twice as
much as remediating soil contami-
nated with 4,000 mg/kg (Table 5).
With or without soil warming,
therefore, bioventing offers strong
economies of scale.
Conclusions
With or without soil warming,
bioventing stimulates biodegrada-
tion and results in contaminant
removal, even in a cold climate
such as that at Eielson AFB. Injec-
tion bioventing creates no signifi-
cant air emission problems and is
more efficient and less costly than
extraction bioventing. Although bio-
venting alone stimulates biodegrada-
tion, adding any of the three soil
warming systems tested at Eielson
AFB raises soil temperatures,
microbial respiration rates, and
contaminant biodegradation rates.
Warm water circulation raises
these parameters most, followed
closely by heat tape soil warming
and more distantly by solar heat-
ing. The closeness of the results
achieved with warm water circula-
tion and heat tape might be mis-
leading, since soil moisture
problems associated with warm
water circulation make sampling
at deep monitoring points difficult.
Because contamination levels and
respiration/biodegradation rates are
highest at these points, warm water
circulation might produce even better
results than those reported here.
Nevertheless, heat tape might be the
most efficient means of soil warming
because it enhances biodegradation
without causing the moisture
problems associated with warm
water circulation.
14
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