Environmental Health Perspectives Volume 103, Supplement 5,
June 1995
In Situ Bioremediation of Chlorinated Solvents
Lewis Semprini
Department of Civil Engineering, Oregon State University,
Corvallis, Oregon
Abstract
Chlorinated solvents and their natural transformation
products are the most frequently observed groundwater
contaminants in the United States. In situ
bioremediation using anaerobic or aerobic co-metabolic
processes is a promising means of cleaning up contaminated
aquifers. Studies show that under natural conditions
trichloroethylene can be anaerobically degraded to
dichloroethylene, vinyl chloride, and ethylene. Pilot scale
field studies of in situ aerobic co-metabolic
transformations have shown that indigenous microbes grown on
phenol are more effective at degrading trichloroethylene and
cis-1,2-dichloroethylene than microbes grown on
methane. Modeling studies support field observations and
indicate that the removal of trichloroethylene and
cis-dichloroethylene results from the biostimulation of
an indigenous microbial population. Field tests and modeling
studies indicate that, at high TCE concentration, degradation
becomes stoichiometrically limited. -- Environ Health
Perspect 103(Suppl 5):00-00 (1995)
Key words: trichloroethylene, dichloroethylene,
co-metabolism, transformation, anaerobic, aerobic, modeling,
field studies, phenol, methane
This paper was presented at the Conference on
Biodegradation: Its Role in Reducing Toxicity and Exposure
to Environmental Contaminants held 26-28 April 1993 in
Research Triangle Park, North Carolina.
This research was jointly funded by the U.S. Department
of Energy and the U.S. Environmental Protection
Agency-sponsored Western Region Hazardous Substance Research
Center under agreement R-815738. The article has not been
subjected to review by these agencies, and no official
endorsement should be inferred.
Address correspondence to Dr. Lewis Semprini, Department
of Civil Engineering, Oregon State University, Corvallis, OR
97330. Telephone (503)737-6895. Fax (503) 737-3099.
Introduction
Chlorinated solvents and their natural transformation
products are the most frequently observed groundwater
contaminants in the United States. These solvents, consisting
primarily of chlorinated aliphatic hydrocarbons (CAHs), have
been widely used as degreasing agents for industrial purposes
and were improperly disposed of at waste and refuse sites or
have leaked from underground storage tanks. They are
relatively recalcitrant to degradation, fairly mobile, and
denser than water when present as a separate phase. This has
resulted in the generation of contaminant plumes that are
often large in aerial extent. Methods commonly used for
remediation of groundwater, such as pump-and-treat, have been
somewhat effective, but these methods are costly to operate,
they have resulted in the transfer of contaminants to other
environmental compartments, or they require surface treatment.
They often do not achieve final treatment levels. Thus,
innovative treatment methods are being explored that may prove
to be more effective and less costly. This paper will discuss
the potential for in situ bioremediation of CAHs via
the process of co-metabolism. Presented will be the results
from field studies in which both natural (intrinsic) and
enhanced transformation have been observed.
The major solvents that are frequently observed as
groundwater contaminants include carbon tetrachloride (CT),
tetrachloroethylene (PCE), and trichloroethylene (TCE). They
can be transformed by chemical (abiotic) and biological
(biotic) processes in the subsurface to a range of products,
including chloroform (CF), methylene chloride (MC),
cis-1,2-dichloroethylene (c-DCE),
trans-1,2-dichloroethylene (t-DCE),
1,1-dichloroethylene (1,1-DCE), vinyl chloride (VC),
1,1-dichloroethane, and chloroethane. Many of the chemicals
can be degraded biologically; however, microorganisms
generally cannot obtain energy for growth from the
transformation (1). For both aerobic and anaerobic
transformations, the presence of a cosubstrate as a carbon and
energy source is needed. Thus, transformations can be brought
about by co-metabolism or through interactions of the CAHs
with enzymes and cofactors produced by microorganisms for
other purposes. Much of the effort of in situ
bioremediation of CAHs is centered on promoting
co-metabolism. Promoting CAH co-metabolism in the subsurface
may entail adding the appropriate growth substrate and
electron donor, such as oxygen, to simulate the microbial
population, while effectively contacting target contaminants
with the stimulated population. To date, there has not been a
documented full-scale application of an in situ
co-metabolic process to guide the design and application
of this technology.
The potential for CAH biotransformation through primary
substrate utilization or through co-metabolism is presented in
Table 1. Most of the CAHs are degraded via co-metabolism. The
potential for aerobic co-metabolism of completely substituted
CAHs, such as CT and PCE, is essentially zero, while it is
very high for less saturated compounds, such as MC and VC. The
general trend for aerobic co-metabolism indicates a better
potential the lower the degree of chloride substitution. The
rates of aerobic co-metabolism are very compound specific. For
example, rates for the dichloroethylene (DCE) isomers vary
greatly. Aerobic transformations can result in complete
degradation of CAHs to carbon dioxide, water, and chloride.
Anaerobic conditions in general show an opposite trend,
with a greater transformation potential the greater the degree
of substitution. A potential problem with the anaerobic
processes is that CAHs are reduced to less substituted
intermediates that are often transformed at slower rates and
thus may accumulate. TCE can be reduced anaerobically to DCE,
which can be transformed into VC (3). Recently,
however, it has been demonstrated that PCE and TCE can be
anaerobically degraded to ethylene (4-6), which is a
nontoxic end product. This finding has sparked new interest in
in situ CAH bioremediation via anaerobic processes.
Results
Anaerobic Transformations of TCE in Saint Joseph,
Michigan
A detailed study of the distribution of TCE and its
anaerobic transformation products was performed in a sand
aquifer near the town of St. Joseph, Michigan (7,8).
The study was undertaken to improve the understanding of the
distribution of the CAHs years after the contamination
occurred and to obtain information on natural mixing and
transformation processes under anaerobic conditions. The
industrial site, which is on the National Priority List (NPL),
is approximately 750 m east of Lake Michigan. The sampling
consisted of obtaining 155 groundwater samples at 5-ft
intervals between the groundwater table, located within 35 to
40 ft from the ground surface, and a clay layer, located 65 to
90 ft below the ground surface.
The concentrations of the CAHs varied significantly with
depth. Relatively high concentrations (several milligrams per
liter of TCE, c-DCE, and VC) existed at all locations
within 20 m of the center of the plume. The dominant DCE
isomer present was c-DCE, with maximum concentrations
of c-DCE, t-DCE, and 1,1-DCE of 133, 3.9, and
5.3 mg/l, respectively, occurring at the same locations.
Methane and ethylene were also observed, with maximum
concentrations of 12.3 and 6.6 mg/l, respectively. The high
methane indicates methanogenic conditions existed, and the
presence of ethylene indicates that some of the TCE had been
completely dechlorinated.
Detailed contour analyses of these data are presented by
Semprini et al. (8). The contaminant distributions
showed consistent spatial relationships. Methane was not
observed in areas of high TCE concentration. c-DCE was
observed in a transition zone between high and decreasing TCE
concentrations. TCE concentrations decreased with depth, and
methane concentrations increased with depth. Vinyl chloride
and ethylene were found to be associated with high methane
concentrations. Sulfate was absent from areas of high methane
concentration and present in zones where TCE concentration was
high and methane concentration was low. The contamination also
showed definite trends with depth. A typical distribution of
CAHs and methane in a wellbore is shown in Figure 1. Sulfate
concentrations decreased with depth. The results showed
sequential dechlorination with depth, consistent with the
development of more highly anaerobic conditions of
methanogenesis. It is not known whether the presence of high
TCE concentrations inhibited the development of methanogenic
conditions at shallower depths, whether the presence of
sulfate inhibited CAH transformations, or whether there were
higher concentrations of a substrate contaminant at the depth
to drive the aquifer to become more anaerobic.
Figure 1. CAH, methane, and sulfate
profile versus depth in wellbore T1-6 from the St. Joseph
National Priority List site.
Detailed mass flux estimates based on the contour analysis
indicated that ethylene represented 10 to 20% of the CAH flux.
Analysis of the substrate(s) driving the anaerobic conditions
was not performed. The CAH results are consistent with the COD
distribution reported by McCarty and Wilson (7), which
shows high upgradient COD levels and a reduction in COD in the
region where methanogenic conditions exist. The results
indicate that significant intrinsic anaerobic transformation
of TCE has been and probably still is occurring at the site.
Studies of Enhanced Aerobic Transformations of CAHs at
the Moffett Naval Air Station
In situ aerobic co-metabolism of CAHs has been
studied in a shallow aquifer at a pilot test facility at the
Moffett Naval Air Station, California. In situ
biotransformation of TCE, c-DCE, t-DCE, and
VC was observed by a microbial population grown on methane and
oxygen (9,10). At the same site, but along a different
experimental leg, the biotransformation of TCE, c-DCE,
and t-DCE was recently observed by a microbial
population grown on phenol and oxygen (11,12).
These pilot scale experiments were performed under induced
gradient conditions of injection and extraction. Groundwater
was extracted at a rate of 10 l/min and amended at the surface
with the chemicals of interest, including the growth
substrates methane and dissolved oxygen (DO) or phenol and DO
and the target CAH contaminants, and then injected into the
test zone at a rate of 1.5 l/min. The injection and extraction
wells were 6 m apart. TCE and c-DCE, the two CAHs
common to both studies, were not present in the groundwater
and were continuously added in a controlled manner. The
concentration response of methane, phenol, DO, TCE, and
c-DCE was monitored at the downgradient monitoring
wells located 1, 2.2, and 3.8 m from the injection well. The
growth substrates and DO were added in alternating pulses to
limit biofouling near the injection well and to distribute the
microbial growth. Methane was added at a time-averaged methane
concentration of 6 mg/l. In the initial phenol study
(11), phenol was added at a time-averaged concentration
of 6.25 mg/l, which after 500 hr of addition was increased to
12.5 mg/l. The CAHs were added in a concentration range of 40
to 120 mg/l. In the subsequent study (12), phenol was
injected at concentrations ranging from 12.5 to 25 mg/l, and
TCE concentrations were varied from 62 to 1000 µg/l.
Comparisons of test results and model simulations for the
methane studies are presented by Semprini and McCarty
(13,14). Good agreement was obtained between model
simulations and field results using a porous media transport
model that included nonsteady-state processes of microbial
growth, electron donor and acceptor utilization, and the
co-metabolic transformation of contaminants using competitive
inhibition kinetics. Transport processes of advection,
dispersion, and rate-limited sorption and desorption were also
included. The results showed that VC and t-DCE were
transformed at rates similar to the methane growth substrate,
while c-DCE and TCE were transformed at rates
approximately a factor of 10 and 100 times lower than methane,
respectively. In the 2-m biostimulated zone, the following
degrees of transformation were achieved: TCE, 20%;
c-DCE, 50%; t-DCE, 90%; and VC, 95% (9).
Simulations of the initial phenol experiment have also been
performed using the same model (15). Figure 2 presents
the field observations and model simulations for c-DCE
and TCE response at monitoring well SSE1 (1 m from the
injection well) due to biostimulation through phenol addition.
The model simulations agree with the field observations and
indicate that decreases in CAH concentrations with time
resulted from the biostimulation of the phenol-utilizing
population. The further decrease in concentration after 24
days resulted from the increase in the phenol-utilizing
population due to the increase in the phenol injection
concentration. The oscillation in the concentration of TCE and
c-DCE results from the pulsed injection of phenol,
where phenol competitively inhibits the transformation of the
TCE and c-DCE. With prolonged phenol injection, over
95% of the c-DCE and 90% of the TCE were degraded in
the 2-m biostimulated zone.
Figure 2. TCE and c-DCE response
at monitoring well SSE1 due to biostimulation with phenol and
oxygen at Moffett Field and comparison model simulations.
Comparison of the methane and phenol experiments indicates
that phenol was a much better co-metabolic growth substrate
for the degradation of TCE and c-DCE at the Moffett
test site. Methane was a better growth substrate for degrading
t-DCE. Model simulations of both experiments indicate
that phenol was a better substrate for several reasons. Phenol
addition resulted in a greater stimulated biomass. More phenol
could be added since it required less DO consumption. It also
had a higher yield coefficient than methane. The rate
coefficients for the co-metabolic transformation for TCE and
c-DCE were also a factor of two to three greater for
the phenol-users compared to the methane-users.
The results of the subsequent phenol experiments in which
TCE concentration was gradually increased from 62.5 to 1000
µg/l are shown in Figure 3. TCE removals at the furthest
monitoring well SSE3 were essentially the same at about 87%
for TCE concentrations up to 500 µg/l. At 1000 µg/l the
removal decreased to 75%. Hopkins et al. (12) indicated
that the lower removal at the higher concentration may have
resulted from the TCE concentration being closer to the
half-saturation coefficient (Ks) value, resulting
in a deviation from first order kinetics; TCE transformation
product toxicity began to have a measurable effect; or there
was insufficient reducing power to carry out the
transformation. The tests indicate that TCE concentration is
an important factor to consider when evaluating in situ
co-metabolic treatment.
Figure 3. Normalized TCE concentrations
with time at monitoring locations during phenol injection at a
time-averaged concentration of 12.5 mg/l, during which TCE
injection concentration was raised in steps from 62.5 to 1000
µg/l. Adapted from Hopkins et al. (12).
Figure 4 presents model simulations of the experiments
presented in Figure 3. In these simulations, the model
presented by Semprini and McCarty (13,14) was adapted
to include a transformation capacity model for TCE degradation
of the form proposed by Alvarez-Cohen and McCarty (16)
and Anderson and McCarty (17). The simulations agree
reasonably well with the field observations. The simulations
indicate that the inclusion of more complicated kinetic
expressions is required to model TCE transformation over a
broad range of concentrations.
Figure 4. Model simulations of the TCE
response shown in Figure 3, using a transformation capacity
model.
Summary
At the St. Joseph site, complete transformation of TCE to
ethylene occurred, with ethylene representing up to 20% of the
contaminant mass flux. VC and ethylene were associated with
high methane and low sulfate concentrations, which indicates
that their production was associated with methanogenic
conditions in the aquifer. The results are encouraging and
indicate that it may be possible to enhance transformations at
such sites through the controlled addition of an anaerobic
growth substrate and possibly minor nutrients. Sites that
should be considered are those that are actively degrading the
CAHs anaerobically and that pose no risk of exposure to vinyl
chloride if it is produced. Another consideration is that it
may be possible to promote aerobic co-metabolism as a final
polishing step to degrade DCE and VC that is formed
anaerobically.
Aerobic studies indicate that for the geochemical and
microbial conditions present at the Moffett site, phenol was a
much better growth substrate than methane for degrading TCE
and c-DCE. This is encouraging since TCE is present at
so many sites, and c-DCE is often a major intermediate
of the anaerobic transformations of PCE and TCE. The studies
also indicate that TCE concentration is an important factor to
consider. Studies at other sites are needed to determine if
consistent results are obtained.
Model simulations provide a useful tool in analyzing the
results from field studies in which complicated biological,
physical, and transport processes are occurring. The
simulations indicate that complex kinetic models are needed to
describe the co-metabolic transformation of TCE over a broad
range of concentrations. These models will also be of value in
the design of in situ remediation systems.
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Last Update: September 23, 1998
