United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/S-92/002
January 1992
vvEPA Ground Water Issue
TCE Removal from Contaminated Soil and Ground Water
Hugh H. Russell*, John E. Matthews**, and Guy W. Sewell*
Background
The Regional Superfund Ground Water Forum is a group of
EPA professionals, representing EPA's Regional Superfund
Offices, committed to the identification and the resolution of
ground water issues impacting the remediation of Superfund
sites. The Forum is supported by and advises the Superfund
Technical Support Project. Remediation of trichloroethylene-
contaminated soils and ground waters is an issue identified
by the Forum as a concern of Superfund decision-makers.
For further information contact Hugh H. Russell (FTS-743-
2444), John E. Matthews (FTS-743-2408), or Guy W. Sewell
(FTS-743-2232).
Introduction
Trichloroethylene (TCE) is a halogenated aliphatic organic
compound which, due to its unique properties and solvent
effects, has been widely used as an ingredient in industrial
cleaning solutions and as a "universal" degreasing agent.
TCE, perchloroethylene (PCE), and trichloroethane (TCA) are
the most frequently detected volatile organic chemicals
(VOCs) in ground water in the United States (Fischer et al.,
1987). Approximately 20% of 315 wells sampled in a New
Jersey study contained TCE and/or other VOCs above the
1 ppb detection limit (Fusillo et.al., 1985). The presence of
TCE has led to the closure of water supply wells on Long
Island, N.Y. and in Massachusetts (Josephson, 1983).
Detectable levels of at least one of 18 VOCs, including TCE,
were reported in 15.9% of 63 water wells sampled in
Nebraska, a State having a low population density and
industrial base (Goodenkauf and Atkinson, 1986).
Trichloroethylene per se is not carcinogenic; it is thought to
become a human health hazard only after processing in the
human liver (Bartseh et al., 1979). Epoxidation by liver
oxidase enzymes confers a suspected carcinogenic nature
(Apfeldorf and Infante, 1981; Tu et al., 1985). However,
processing in the human liver is not the only way in which
TCE may become a health hazard. Reductive
dehalogenation of TCE through natural or induced
mechanisms may result in production of vinyl chloride (VC)
which, in contrast to TCE, is a known carcinogen [Fed.
Regist. 1984, 49:114, 24334(11)].
Wastewater or municipal water supply treatment systems
which utilize coagulation, sedimentation, precipitative
softening, filtration and chlorination have been found
ineffective for reducing concentrations of TCE to non-
hazardous levels (Robeck and Love, 1983). Other methods
are required for remediation of water contaminated with TCE
if such water is to be used for human consumption. The
purpose of this paper is to: 1) present a synopsis of physico-
chemical properties and reactive mechanisms of TCE, and 2)
delineate and discuss promising remediation technologies
that have been proposed and/or demonstrated for restoring
TCE-contaminated subsurface environmental media.
Physical and Chemical Properties of
Trichloroethylene
Preliminary assessment of remediation technologies feasible
for reclamation of subsurface environmental media
contaminated with TCE must involve consideration of the
compound's physical and chemical properties (i.e., dis-
tribution coefficients, reactivity, solubility, etc.). These
properties are directly responsible for behavior, transport and
fate of the chemical in the subsurface environment.
Knowledge of a compound's physico-chemical tendencies
can be used to alter behavior and fate of that compound in
the environment. Important considerations derived from
physical and chemical properties of TCE presented in Table 1
are:
Density (1.46 g/ml) - Density can be defined as the
concentration of matter, and is measured by the mass per
•Research Microbiologist, "Research Biologist, Robert S.
Kerr Environmental Research Laboratory, U.S. EPA,
Ada, OK.
Superfund Technology Support Center for
Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, Oklahoma ,, „ _ . . _ ,____
U.S. Environmental Protection Agency
Region 5, Library (PL-12J) '
77 West Jackson Boulevard, 12th Flow
Chicago, IL 60604-3590
Office of Soft* W#*te«n<* Enwrswwy
Walter W. Kovallck, Jr., Ph.D.
Printed on Recycled Paper
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unit volume. In relation to liquids, these units are grams per
milliliter. The density of a substance is usually referenced to
pure water which is taken to be 1 gram per milliliter. TCE is
heavier than water; therefore, a spill of sufficient magnitude is
likely to move downward through the subsurface until lower
permeability features impedes its progress. This often results
in formation of a plume or pool(s) of dense nonaqueous
phase liquid (DNAPL) in the aquifer plus a trail of residual
saturation within the downward path. Implicit in this statement
is the fact that residual saturation also will serve as source
areas for contamination and migration of TCE within an
aquifer system. There is an inherent difficulty associated with
location of DNAPL in the subsurface and subsequent removal
of DNAPL pools or plumes using a standard pump and treat
regime. A major reason for this difficulty of removal is that
water coning using conventional extraction wells results in
poor DNAPL/water ratios.
A density difference of about 1% above or below that of
water (1.0 gm/ml) can significantly influence movement of
contaminants in saturated and unsaturated zones
(Josephson, 1983). This does not mean that density
differences of less than 1% do not influence movement of
contaminants. These density differences may even be'
apparent at low solute concentrations (Short, 1991, personal
communication). At a site in New Jersey, it was determined
that although soil and shallow ground water at the source
area were contaminated with benzene, toluene and other
volatile organic compounds with relatively low specific
gravities, the compounds did not migrate downward into
deeper municipal supply wells (Spayd, 1985). This is in
contrast with what could happen if the source area were
contaminated with chlorinated aliphatics, including TCE.
Areas containing insoluble TCE, i.e., DNAPL pools, can serve
as source areas for spreading of contamination. As ground
water moves through and/or around these source areas,
equilibrium concentrations partition into the aqueous phase.
Aqueous phase TCE is then spread through the aquifer by
advection and dispersion. As a result small source areas can
serve to contaminate large portions of an aquifer to levels
exceeding drinking water standards. At a site in Texas, for
example, Freeberg et al. (1987) determined that 8 kg of non-
aqueous phase TCE was responsible for contaminating
12.3 x 10" gallons of water at an average concentration of
176ppb.
Water solubility - Water solubility can be defined as the
maximum concentration of a solute which can be carried in
water under equilibrium conditions and is generally given as
ppm (parts per million) or mg/l (milligrams per liter). The
water solubility limit of TCE is 1000 mg/l, the maximum
concentration of TCE that can be in aqueous solution at
20° C. Water solubility of a compound has a direct relation
on distribution coefficients and biodegradability of a particular
compound. A compound that is relatively insoluble in water
will prefer to partition into another phase; i.e., volatilize into
soil gas or sorb to organic material. Compounds that are
relatively insoluble also are not as readily available for
transport across the bacterial membrane, and thus less
subject to biological action.
K^ value - K^. is defined as the amount of sorption on a unit
carbon basis? The K^ can be predicted from other chemical
properties of compounds such as water solubility and octanol
water coefficient. The low K value of 2.42 for TCE
translates into little retardation by soil or aquifer organic
materials. Since the retardation is so low, pump-and-treat
technologies appear attractive as remediation alternatives.
Mehran et al. (1987) through field and laboratory
investigations determined a retardation factor of
approximately 2.0 for TCE which agrees with the data of
Wilson et al. (1981) and falls within the range of 1-10 given
by Mackay et al. (1985) for sand and gravel aquifers with low
organic content. Measured partition coefficients, however,
may be considerably higher than calculated values, especially
at lower aqueous concentrations. Johnson et al. (1989)
found that at equilibrium concentrations of approximately 1
ppm, the measured partition coefficient of TCE was
significantly higher than calculated values. These authors
also report that other workers who have observed the same
effect theorize that this higher value may be related to clay
interactions.
If the retardation factor is considered to be 2.0, TCE should
migrate at half the speed of water through soil and aquifer
materials low in organic carbon content, and theoretically
somewhat slower in material with a high organic carbon
content.
Henry's Law Constant - Henry's Law states that the amount
of gas that dissolves in a given quantity of liquid at constant
temperature and pressure, is directly proportional to partial
pressure of the gas above the solution. Henry's coefficients,
as a result, describe the relative tendency of a compound to
volatilize from liquid to air. The Henry's Law Constant for
TCE is 0.00892 which is high enough, when combined with
its low solubility in water and high vapor pressure, for efficient
transfer of TCE to the atmosphere. The evaporation half-life
of TCE in water is on the order of 20 minutes at room
temperature in both static and stirred vessels (Dilling, 1975;
Dilling et al., 1975).
Table 1. Physico-chemical Properties of Trtehloroethylene
Density
Water Solubility
Henry's Law Constant
(atm-m3/mol @ 20° C)
Molecular Weight
Boiling Point
Log Octanol-Water Partition
Coefficient
1.46 g/ml
1000mg/L
0.00892
131.4
86.7° C
2.42
The chemical structure of TCE bestows chemical reactivity.
Three chlorine atoms attached to the carbon-carbon double
bond make TCE a highly oxidized compound. Oxidized
molecules readily accept electrons (reduction) under
appropriate conditions, but resist further oxidation. As a
result, chemical reactivity of TCE is greatest under a reducing
atmosphere, conditions that favor transfer of electrons to
TCE.
Due to their size, the three carbon atoms surrounding the
double bond are responsible for stearic hinderance. This
lowers the rate at which large nucleophilic groups can
approach or react with the carbon -carbon double bond.
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Due to its chemical nature, TCE can (under the appropriate
conditions) undergo a number of abiotic transformations, or
interphase transfers. The ease with which any chemical can
undergo such reactions is an important indicator of that
chemical's susceptibility to abiotic remediation processes.
Environmental Distribution of Trichloroethylene
While it has been estimated that about 60% of the total TCE
produced in the United States is lost to the atmosphere, with
negligible discharge into water bodies (Cohen and Ryan,
1985), widespread contamination of terrestrial subsurface
environmental media also has occurred. Given the wide use
of TCE as a degreasing solvent plus its recalcitrance and
chemical properties, this type of distribution pattern is not
surprising.
Trichloroethylene contamination exists in both vadose and
saturated zones of the subsurface environment. This
contamination results from spills, leaking transfer lines,
storage tanks and poor environmental awareness. Because
of its density and low K , TCE will ultimately move downward
in the vadose zone untiFan impermeable barrier is reached.
Such a scenario occurs when a TCE spill is of sufficient
magnitude or deep enough in the vadose zone for
volatilization to be restricted.
Once in the vadose zone, TCE can become associated with
soil pore water, enter the gas phase because of its Henry's
constant, or exist as nonaqueous phase liquid (NAPL). It is
therefore conceivable that upward or downward movement of
TCE can occur in each of these three phases, thereby
increasing area! extent of the original spill in both the vadose
and saturated zones. While movement of large
concentrations of TCE through the vadose zone may be
rapid, surface tension exerted at the capillary fringe may
retard further downward movement of smaller spills.
Nonaqueous phase concentrations of TCE which are large
enough to overcome capillary forces will move downward into
the aquifer. Once the water table is penetrated, lateral flow
may be mediated by the regional ground-water flow. Due to
its high density, the movement of free-phase TCE is still
directed vertically until lower permeability features are
encountered. Once an impermeable layer is encountered,
horizontal movement will occur. Such movement may even
be directed against the natural ground-water flow by the
effects of gravity.
Since permeability is a function of the liquid as well as the
medium, the vertical movement of TCE through an aquifer is
determined by geological properties of the aquifer material;
i.e., granular size of sand or clay lenses. Trichloroethylene
will tend to pool near these impermeable features. Water
passing over and around these pools solubilize TCE that can
be spread throughout the aquifer.
Reaction Mechanisms for Trichloroethylene
Oxidation
Highly oxidized chemicals such as TCE have a high reduction
potential and are thereby resistant to further oxidation. This
is the reason that aerobic biological mediated degradation of
TCE was once thought to be illogical.
It is, however, possible to oxidize TCE using chemicals such
as potassium ferrate. Delucca et al. (1983) determined in
laboratory experiments that 30 ppm potassium ferrate would
completely oxidize 100 ppb TCE in less than fifty minutes
(after an initial 5 to 20 minute delay) at 23 degrees Celsius
and a pH of 8.3. These experiments, however, were
conducted using water that had been double distilled,
deionized and passed through resin columns. These
purification steps effectively removed all organic and
inorganic species that might have inhibited the oxidation of
TCE through a competitive process. The presence of other
solutes with lower oxidation states or more accessible
electrophilic binding sites would effectively lower the
concentration of the radicals by reacting first, thus leaving
fewer radicals to combine with TCE.
In wastewater treatment, ozone has been shown effective at
the removal of organic material. Ozonation, however, is
generally used to remove less oxidized chemicals than
chlorinated aliphatics. The actual mechanism in this case is
either the direct reaction of ozone with the carbon-carbon
double bond or nucleophilic substitution by hydroxyl radicals
which are generated upon the decomposition of ozone.
Destruction of TCE by ozone has been reported by Glaze
and Kang (1988) and Francis (1987), and with some success
by Cheng and Olvey (1972); however, the low concentration
and reactivity with other solutes of hydroxyl radicals is still a
problem. To overcome this problem, investigators have tried
to couple chemical reactions designed to increase production
of hydroxyl radicals. Glaze and Kang (1988) have described
four ways to enhance the oxidation potential of ozone: 1)
variation in pH, 2) addition of hydrogen peroxide, 3) addition
of ultraviolet light and 4) addition of a combination of peroxide
and ultraviolet radiation. Their results indicate that direct
ozonation of TCE in high alkalinity ground water is a slow
process. The addition of hydrogen peroxide at a ratio of 0.5-
0.7 (w/w hydrogen peroxide: ozone) accelerated the oxidation
rate by a factor of two to three. They concluded that
oxidation of TCE under certain conditions is a promising
destruction process.
Reduction
Compounds such as TCE also are susceptible to reduction.
While transition metals may play a role in abiotic reductions,
the major reductive components may be electrons or
reducing equivalents produced from biological reactions or
molecular hydrogen (Barbash and Roberts, 1986). Reduction
of TCE may be possible by any organic compound that has a
sufficiently low oxidation potential for efficient hydrogen
transfer under ambient conditions.
Dehalohydrolysis
Natural dehalohydrolysis of TCE (or other halogenated
VOCs), with the subsequent production of an alcohol, is
possible. Half-lives of such reactions are on the order of
days to centuries (Barbash and Roberts, 1986). Natural
dehalohydrolysis proceeds by either hydrolysis in an aqueous
phase or nucleophilic substitution and elimination reaction at
unsaturated carbons (ethene bond). Removal of a chlorine
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atom from one carbon coincides with removal of a hydrogen
atom from the adjacent carbon. Given the number of soil
and aquifer systems contaminated with TCE, however,
natural dehalohydrolysis is not considered to be a significant
mechanism of degradation.
The rate determining step in nucleophilic attack is hydroxyl
ion (or other nucleophile) reaction with the double bond. The
rates of unimolecular and bimolecular nucleophilic
substitution depend on: 1) structure of the carbon bearing the
leaving group (general order of reactivity in bimolecular
reactions methyl> primary>secondary>tertiary), 2)
concentration and reactivity of the leaving group, 3) nature of
the leaving group (l>Br>CI>F), 4) nature and polarity of the
solvent (concentration of ions such as bicarbonate) and, 5) in
the case of Sn2 type reactions, the concentration of the
radical. It is quite evident then, should nucleophilic
substitution occur, the daughter product or products
(dichloroethylene or vinyl chloride) would be increasingly
resistant to nucleophilic displacement, yet still be of
environmental consequence. This fact, coupled with the low
rates of natural dehydrohalogenation of TCE point to the
need for some type of catalyst or enhancement of reactivity.
In regards to catalysts, a number of chemical compounds or
processes have been developed, or borrowed from the
wastewater treatment industry. Ultraviolet radiation has been
suggested as one method to enhance reaction rates through
the formation of reactive hydroxyl radicals. Ultraviolet light
may be an important mechanism for the degradation of
recalcitrant chemicals in surface waters and the photolytic
zone (top 1-2 mm) of soil (Miller et al., 1987). In a laboratory
setting with a six-fold molar excess of dissolved oxygen,
under an oxygen atmosphere, the photooxidizable half-life of
TCE has been determined to be 10.7 months (Dilling et al.,
1975). Given the interphase transfer potential of TCE,
photodegradation should not occur to any extent before
transfer to the atmosphere.
Surface Treatment Technologies
Air Stripping
Air stripping is an applicable technology for removal of TCE
from contaminated water. A constant stream of air is used to
drive TCE from solution, taking advantage of the low Henry's
Law constant and water solubility. Since TCE is fairly
recalcitrant to other remediation efforts, air stripping seems to
be the current method of choice to return water to potability.
A number of pilot-scale demonstration and full-scale case
studies have shown this alternative to be effective for
removal of TCE from contaminated water. This process,
however, only shifts the compound to another medium. New
restrictions on venting of volatile organic compounds to the
atmosphere may preclude the use of such technologies or
require treatment of the air-stripped off-gases by carbon
adsorption.
The typical air stripper is designed to allow for percolation of
large volumes of air through contaminated water. This has
the effect of changing conditions to favor volatilization of
TCE. Application limitations usually occur as the
concentration of TCE falls below a threshold level where
volumes of air larger than logistically possible are required to
continue the stripping process.
In 1982, 10-12 United States utilities were using some form
of aeration technique to strip volatile organic chemicals such
as TCE from water supplies. The air strippers used were of
three principal types: redwood slat aerators, packed towers
and spray towers, all of which are designed to allow
maximum contact of water, air and TCE (Robeck and Love,
1983). Since that time many sites and municipal supply
systems have utilized air strippers (generally packed towers)
as a method for removing VOCs from contaminated ground
water.
Wurtsmith AFB - At Wurtsmith AFB, Oscoda, Michigan, TCE
was detected in drinking water at a concentration of 6,000
ppb, with a concentration of 10,000 ppb in the centerline of
the plume (Gross and Termath, 1985). The remedial design
selected was for two packed towers that could be run parallel
or in series. The towers were run constantly for one year,
consistently obtaining effluent concentrations below 1.5 ppb.
The one problem encountered was bacterial growth which
plugged the towers. This had the effect of lowering the
surface area available for transfer of TCE into the gas phase.
Constant chlorination was found to retard bacterial growth,
while not affecting operational efficiency of the tower.
Savannah River. Georgia - TCE was found in ground water at
the Savannah River Plant in 1981 near an abandoned settling
basin which had been used for storage of process waters
(Boone et al., 1986). The horizontal area of the plume
contour at 100 ppb total concentration chlorocarbons was
estimated to be 360 acres. The total mass of chlorocarbons
was thought to be 360,000 pounds. In 1983, air stripping
was thought to be the best available technology for
remediation; therefore, two pilot air stripping units of 20 and
50 gpm capacity were designed. The two pilot strippers,
when operated properly, reduced the concentration of total
chlorinated hydrocarbons in ground water from 120,000 ppb
to less than detection limits of 1 ppb. Data from these pilot
units were used to design a production tower to process
contaminated water at a rate of 400 gpm.
In 1985, the production tower (400 gpm column) was placed
on line and fed by a network of 11 recovery wells. As of
1986, some 65,000 pounds of chlorinated solvents had been
recovered from the aquifer through the combined operation
of pilot and production towers. In 13 months, the production
tower alone, which was reported to operate at 90% efficiency,
had removed some 33,500 pounds of chlorocarbons from
115 million gallons of water.
Refrigerator Manufacturing Facility - In the late 1960's, a
refrigerator manufacturing facility was granted permission for
a waste disposal area in the upper of two peninsulas formed
by the "S" meander of a river. This practice resulted in the
contamination of the underlying aquifer with chlorinated
aliphatics. TCE concentrations were approximately 35 ppm.
The treatment system consisted of installation of 14 wells for
extraction of ground water which was processed by air-
stripping. Effluent from the "stripper" was then discharged
through a recharge basin that was capped with 1.5 feet of
pea gravel. Recharge through the pea gravel was thought to
serve two purposes: 1) additional removal of TCE by
percolation through the gravel, and 2) flushing residual TCE
contamination from the vadose zone.
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It was reported that the system operated at an influent flow
rate of 210 gpm (4000 ppb TCE) and a TCE removal
efficiency of 78% (Thomson et al., 1989). Additional removal
attributed to spraying of process water over and percolation
through the gravel raised the TCE removal efficiency to
greater than 95%. Full scale operation began in the summer
of 1987. From July 1987 to October 1987, approximately
24.5 million gallons of ground water were treated, and 775
pounds of TCE were removed.
Combined Air Stripping and Carbon Adsorption
A currently popular method for remediating water
contaminated with TCE is to combine the technologies of air
stripping and granular activated carbon (GAG) adsorption.
This treatment train is attractive because it ameliorates
shortcomings of both technologies.
In the case of air stripping, the residual concentration of TCE
in treated effluent may be above local or regional drinking
water standards, thereby necessitating a "polishing" step
prior to use or discharge. Limitations associated with GAG
adsorption are:
1) a given sorbent has a finite capacity for sorbtion of a
given contaminant; once this limit is reached, contaminant
breakthrough occurs. This breakthrough results from
competition between contaminants and normal solutes for
unbound sites; less tightly bound solutes are displaced by
those solutes having a greater affinity. When loss of binding
efficiency becomes great enough, breakthrough of
contaminants occurs. Once breakthrough occurs, the
sorbent must be changed or cleaned. Adsorption of TCE to
GAG, based on equilibrium concentrations of 1 ppm at
neutral pH and 20°C, is 28 mg/g (Amy et al., 1987).
2) high dissolved organic carbon (DOC) and other
contaminants can compete with TCE for binding sites
available on the sorbent, thus increasing the likelihood of
breakthrough. A concentration of 10 ppm natural organic
matter in river water has been shown to reduce TCE
adsorption by up to 70% (Amy et al., 1987).
Recognizing the limitations of sorption (the first may only be
economically limiting), use of this technology as a polishing
step following air stripping can be a viable link in a surface
treatment process for TCE. Air stripping can be used to
remove the majority of TCE, followed by adsorption which is
used to polish the stripper effluent. This approach will lower
construction costs of the air stripper and increase life
expectancy of the adsorbent.
Rockaway Township - In 1979, ground-water contamination
by TCE and lesser amounts of diisopropyl ether and methyl
tertiary butyl ether was discovered in three of the water
supply wells serving Rockaway, New Jersey. The first
technology used to treat the ground water was sorption
utilizing GAG. Quick breakthrough of ether contaminants
resulted in rapid loss of GAC sorptive effectiveness. This led
to consideration and ultimate use of air stripping as a
primary treatment technology (McKinnon and Dykson, 1984).
A counter current packed tower with a water flow rate of 1400
gpm and an air flow rate of 37,500 cfm was installed and
placed on line Feb. 4, 1982. By July 1983, the GAC system
was taken off line because of the excellent performance of
the packed tower in addition to reduced influent levels of the
ether compounds.
Subsurface Remediation Technologies
So/7 Venting
Soil venting is an in situ air stripping technique used to
remove volatile contaminants from the vadose zone. Air is
forced into the soil subsurface through a series of air inlets,
then vented or extracted under vacuum through extraction
pipes. Air laden with organic vapors moves along an induced
flow path toward withdrawal wells where it is removed from
the unsaturated zone and treated and/or released to the
atmosphere (Baeher et a)., 1988). Success of the method
depends on rate of contaminant mass transfer from
immiscible and water phases to the air phase, and on ability
to establish an air-flow field that intersects the distributed
contaminant. In many cases, treatment of off-gases may be
required due to air quality standards.
Trichloroethylene, due to its high potential for interphase
transfer to the gaseous phase, should be an excellent
candidate for soil venting technologies. Gary et al. (1989)
suggested that by forcing air into the water table below the
contamination and maintaining sufficient air entry pressure
throughout a significant volume of soil, TCE should be
trapped at the soil water interface and released to the gas
phase. This release to the gaseous phase would favor
transfer to the soil surface.
Field results reported by Mehran et al. (1987) link the TCE
concentration in soil gas with current levels in ground water;
therefore, the theory suggested by Gary et al. would appear
to be correct for contamination at or near the soil-water
interface. This also may point to the possibility that
movement of gaseous TCE from the saturated zone into the
soil gas phase may act to further spread contamination. Soil
venting technology for removal of TCE has not been fully
tested in the field or, at least, has not been reported in
refereed literature; however, results from two pilot-scale tests
demonstrate the potential applicability of the technology for
removal of TCE from the vadose zone (Coia et al., 1985 and
Dankoetal., 1989).
The first test site was an area where open burning of solvents
such as TCE had occurred for over thirty years (Coia et al.,
1985). The site consisted of sandy glacial soils with a TCE
concentration of 5,000-7,000 mg/kg. A closed, forced
ventilation system was installed. The system consisted of
perforated PVC pipes extended vertically into the vadose
zone, and separate extraction pipes. For 14 weeks, two pilot
systems were run, one designated the high-contaminated
zone (50-5,000 ppm TCE) and the second the low (5-50 ppm
TCE) zone. In the low-contaminated zone, extraction pipes
were installed on 20 foot spacing and air was forced into the
soil at 50 cfm. In contrast, in the high-contaminated zone,
extraction pipes were installed on 50 foot spacing and air was
forced into the soil at 50-225 cfm. At the end of the study, 1
kg of TCE was removed from the low-contaminated zone and
730 kg (10-20%) from the high-contaminated zone.
The second test site was the Verona Superfund site in Battle
Creek, Michigan (Danko et al., 1989). A number of private
and city wells in the Verona Field were discovered to be
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contaminated with VOCs, in August 1981. The predominant
contaminants based on total mass were perchloroethylene
(PCE), cis/trans-dichloroethylene (DCE), TCE, 1,1.1-
trichloroethane (TCA) and toluene. A Record of Decision
(ROD) was issued in 1985 specifying corrective action which
would include a network of ground-water extraction wells,
followed by air stripping plus a soil vapor extraction system
for vadose zone contamination.
A soil vapor extraction system was installed to specifically
remove VOCs from the most contaminated vadose zone
source area. The system design consisted of a network of 4*
PVC wells screened from approximately 5 ft. below grade to
3 ft. below the water table. A surface collection manifold
connected to a centrifugal air/water separator was attached
to a carbon adsorption system. The outlet end of the carbon
adsorption system was connected to a vacuum system which
pulls air from the subsurface into the extraction wells. The
pilot system began operation in November 1987. By August
1989, total mass of VOCs removed was reported to be
40,000 Ibs.
In-Well Aeration
Another remediation method based on interphase transfer
potential of TCE is in-well aeration. In-well aeration can be
accomplished using either an air lift or electric submersible
pump and sparger. The air lift pump may or may not be used
with a sparger.
In-well aeration has been tested at sites near Collegeville, PA
and at Glen Cove, NY (Coyle et al., 1988). At the
Pennsylvania she, the air lift pump run used with a sparger
removed 78% of the TCE; the electric pump run concurrently
with a sparger removed 82%. At the New York site, the air
lift pump without a sparger removed 65% of the TCE; with
sparging, the same pump removed approximately 73%.
Technologies based strictly on volatilization of TCE, however
effective at subsurface remediation, do not satisfy the intent
of Section 121 of CERCLA. This Section states that
"remedial actions in which treatment permanently and
significantly reduces the volume, toxicrty or mobility of the
hazardous substances, pollutants, and contaminants, in a
principal element, are to be preferred over remedial actions
not involving such treatment." Air stripping simply transfers
TCE from one medium (soil or water) to another
(atmosphere) without any significant reduction in volume or
toxicity.
Bioremediation
Biological remediation is one methodology which has the
potential to satisfy the intent of Section 121 of CERCLA.
Although bioremediation of environments contaminated with
TCE can best be described as in its infancy, this technology
remains attractive since the possibility exists for complete
mineralization of TCE to CO2, water, and chlorine instead of
simply a transfer from one medium to another. In the case of
biological degradation, bacteria may produce necessary
enzymes and cofactors which act as catalysts for some of the
chemical processes already described.
Anaerobic degradation - Because of the oxidized state of
TCE, ecological conditions under which degradation is most
likely to occur is a reducing environment. Biological
degradation (and possibly mineralization) of TCE under
anaerobic conditions has been studied for a number of years
(Bouwer and McCarty, 1983; Bouwer et al., 1981). The first
reported biological attenuation of TCE was that of the
obligate anaerobic methanogenic bacteria through a process
known as reductive dehalogenation. Under anaerobic
conditions, oxidized TCE can function as an electron sink and
is readily reduced by electrons (or reducing equivalents)
formed as a result of metabolism (oxidation) of organic
electron donors by members of methanogenic consortia.
Volatile fatty acids and toluene may serve as oxidizable
substrates (electron donors) which can be coupled to
reduction of haloorganic molecules such as TCE with the
resultant removal of a chlorine atom (Sewell et al., 1990).
Freedman and Gossett (1989) reported conversion of PCE
and TCE to ethylene without significant conversion to carbon
dioxide. The conversion occurred in an anaerobic system
stimulated with electron donors such as hydrogen, methanol,
formate and acetate, with added yeast extract. Presumably,
the electron donors provided the electrons or reducing
equivalents necessary for complete chlorine removal by
reductive dehalogenation. PCE mineralization has been
reported by other authors (Vogel and McCarty, 1985),
although it seems that in the absence of sufficient oxidizable
organic compounds a buildup of DCE(s) or vinyl chloride also
will occur (Bouwer and McCarty, 1983; Bouwer et al., 1981).
Theoretically, under anaerobic conditions, with sufficient
quantities of other readily oxidizable substrates and the
necessary auxiliary nutrients, methanogenic consortia may
be capable of converting TCE to harmless end-products.
More research is needed, however, to determine how
effective this remediation may be, and what actual
requirements are needed to drive the process. Advantages
of anaerobic processes are that there appears to be no
apparent lower concentration limit to activity nor is there a
need to perfuse the subsurface with copious amounts of
oxygen.
Aerobic Degradation - It has long been thought that TCE is
resistant to degradation under aerobic conditions due to its
already oxidized state. Recently, a number of
monooxygenases produced under aerobic conditions have
been shown to degrade TCE (Nelson et al., 1987; Marker and
Young, 1990). Under these conditions, there is no buildup of
vinyl chloride, and complete mineralization is possible. An
aromatic compound such as toluene or phenol is required,
however, for induction of the enzymes responsible. As a
result any application of this system would require the
presence of a suitable aromatic compound or other inducer.
The inducer requirement might be alleviated by manipulation
of the proper genetic sequence for monooxygenase
production, but other carbon and\or energy sources would
probably be required for growth.
Methylotrophic degradation - The enzyme methane
monooxygenase (MMO) produced by methylotrophic bacteria
growing in the presence of oxygen at the expense of
methane has a wide range of growth substrates and
pseudosubstrates; one of which is TCE. This enzyme
epoxidates TCE. The resulting chemical complex is unstable
and quickly hydrolyzes to various products dependent on the
pH of the menstruum. TCE epoxide in phosphate buffer at
pH 7.7 has a half-life of 12 seconds (Miller and Guengerich,
1982). If TCE epoxidation by MMO follows enzyme kinetics
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similar to that of a true growth substrate and inhibitor rather
than some enzyme modification due to its reaction with
MMO, the question arises as to whether or not methane
necessary for production of MMO will competitively inhibit
TCE epoxidation (or more correctly overcome the TCE
inhibition). If such is the case, only a certain percentage of
TCE may be epoxidated before it is transported away from
the bacteria in a flow situation. Since this process is co-
metabolic, methane is a strict requirement. Removal of the
true substrate will result in rapid loss of production of MMO,
thereby reducing the ability to epoxidate TCE. Methods to
enhance this ability are genetic in nature. Genetic recom-
bination resulting in constitutive expression of the MMO
genes is required in order that methane will no longer be
required for induction. Even if constitutive expression is
achieved, small amounts of methane still may be needed as
a carbon and energy source.
Mixed Consortia Degradation
Recently, it has been shown that a mixed consortia of
bacteria can effectively mineralize TCE (Henson et al., 1988;
Wilson and Wilson, 1985). This involves co-metabolism of
TCE (epoxidation) by bacteria that oxidize gaseous
hydrocarbons such as methane, propane and butane,
followed by hydrolysis of the TCE epoxide. The hydrolysis
products are then utilized by other naturally occurring
bacteria. Wackett et al. (1989) surveyed a number of
propane oxidizing bacteria for their ability to degrade TCE.
While TCE oxidation was not common among the bacteria
surveyed, unique members could oxidize TCE. High
concentrations (>15% v/v) of the gaseous hydrocarbons was
found to inhibit co-metabolism of TCE. Oxygen
concentrations also could be limiting in aqueous treatment
systems since oxygen is required by the gaseous, alkane-
utilizing and heterotrophic population.
Bioremediation of Extracted Ground Waters/
Subsurface Air Streams
Surface bioreactors could be used to remediate TCE-laden
ground water or subsurface air streams extracted using any
type of pump-and-treat system. Water treatment technology
would consist of passing contaminated water along with
suitable concentrations of methane or other gases such as
propane through the reactor. The methylotrophic or gaseous
hydrocarbon-utilizing portion of the mixed population would
epoxidate the TCE. This would spontaneously hydrolyze the
TCE to glyoxylate, formate and carbon monoxide which
would then be utilized by the heterotrophic population.
Laboratory work at the Robert S. Kerr Environmental
Research Laboratory in Ada, Oklahoma (RSKERL-Ada) has
shown that TCE can be removed from water utilizing surface
bioreactors (Wilson and Pogue, 1987; Wilson and White,
1986). A trailer-mounted pilot-scale system has been
designed and constructed for use in field trials (Miller and
Callaway, 1991).
Treatment of extracted air streams would consist of passing
off-gases stripped from pumped ground waters or collected
from soil venting systems through surface bioreactors. A
suitable concentration of hydrocarbon gases would be added
to the feed stream to initiate the cometabolic process.
Systems developed for treatment of contaminated air
streams would not be limited by the oxygen concentration.
Treatment of contaminated air streams such as those
resulting from air stripping operations or soil venting
processes would have the additional advantage of eliminating
any role water chemistry may play on the treatment process.
Variations in water chemistry from site to site could be
virtually ignored if contaminants are first transferred into an
air stream prior to treatment.
In Situ Bioremediation
In situ bioremediation for removal or reduction of TCE
contamination in the saturated zone is still in the research
mode at this time. The RSKERL-Ada has an active research
program directed toward this promising technology.
Anaerobic processes without support of secondary organic
compounds often lead to the production of vinyl chloride as
the final end product. Aerobic processes, on the other hand,
require presence of an inducer compound which may not be
available. Since stimulation of populations of mixed consortia
and methylotrophs will usually require injection of methane,
oxygen and other nutrients, aerobic processes also are
subject to mass transport limitations. For example, given a
retardation factor of 2.0 for TCE as the standard, it is
possible that injection of required stimulants may result in
transport of TCE partitioned into the aqueous phase away
from appropriate bacteria before such bacteria are stimulated
and begin actively producing the enzyme MMO. If injection
does not result in complete transport of aqueous-phase TCE,
its concentration may be diluted such that the TCE:methane
or TCErsubstrate ratio is too low for effective treatment. In
situ processes utilizing methanotrophs or mixed consortia
require modification of the currently conceived design of in
situ bioremediation in the saturated zone to account for the
possibility of aqueous-phase TCE transport away from the
biologically activated portion of the aquifer.
Coupling of anaerobic in situ processes to surface based
aerobic reactors is a promising variation of this technology
also under consideration. In such a system, in situ anaerobic
bacteria would be stimulated to reductively remove chlorine
atoms from TCE. Resultant daughter products more
susceptible to aerobic degradation could then be pumped to
the surface and effectively treated in a bioreactor.
Moffet Field Study - A form of in situ aquifer bioreclamation
has been studied at a test site on the Moffet Field Air Station
in California (Roberts et al., 1989; Semprini et al., 1987). The
test aquifer was shallow, confined, and composed of coarse-
grained alluvial sands. To create the test zone, an extraction
well and injection well 6 meters apart with 3 intermediate
monitoring wells were installed. The 3 monitoring wells were
used to gather data for tracer and degradation studies.
Parameters including methane, oxygen and halogenated
solvent concentrations were continuously monitored.
Test zone microbiota were stimulated by injecting ground
water containing methane and oxygen in alternating pulses.
Within a few weeks, complete methane utilization was
observed. This confirmed the presence of indigenous
methanotrophic bacteria. Although methanotrophic bacteria
utilize methane as a carbon source, atmospheric oxygen also
is required for growth.
During the initial phase of the field test, TCE was injected at
an average concentration of 100 g/l; concentrations were
-------
then lowered to 60 g/l to determine the effect of sorption on
observed concentration losses. Early breakthrough results at
the first monitoring well, after normalizing concentrations to
account for adsorption, indicated that microbial degradation
could have been as high as 30%.
A second phase of this study involved injection of
approximately 1 ppm normalized concentrations of vinyl
chloride, trans-DCE, cis-DCE and TCE. At 2-4 meters from
the injection well, transformation rates were found to be 95%,
90%, 45% and 29%, respectively. The recalcitrance of TCE
was attributed to a number of factors, including degree of
chlorination and low solubility of methane and oxygen.
Results from the Moffet Field study do not provide a
wholesale license for initiation of in situ bioremediation of
aquifers contaminated with TCE. Injection of methane,
oxygen and other nutrients into an aquifer to stimulate a
methylotrophic population would, in aquifers having little TCE
sorption potential, push aqueous-phase TCE from the
microbially stimulated portion of the aquifer before the
appropriate bacteria could adapt or be stimulated. This
technological limitation may be overcome in the future by
redesigning the injection protocols currently utilized for in situ
biological remediation. Such scenarios have been developed
by Roberts et al. (1989). However, given the low rates of TCE
degradation in initial field tests, this scenario awaits
documentation.
Summary
Due to its chemical structure and unreactive nature, TCE is
not easily transformed to environmentally safe compounds.
The most efficient and cost effective method for removal of
TCE from polluted ground water at this time appears to be air
stripping, especially for aquifers which are utilized as potable
water sources. Aquifers subjected to bioremediation would
require further treatment of the ground water before human
consumption, thus increasing the overall cost.
The primary alternative now being studied for removal of TCE
from contaminated soil in the vadose zone is soil venting. Soil
venting appears to be both efficient and cost-effective for
VOC removal from the vadose zone in many instances.
Limitations will occur in soil contaminated with a mixture of
waste (TCE mixed with low volatility waste) as well as in
highly impermeable soils. These limitations highlight the
need for continued research into soil bioventing, including
anaerobic and aerobic techniques based on co-metabolism of
TCE driven by the less volatile constituents.
With new restricted air emissions standards, the use of air
stripping and soil venting technologies may require that off-
gases from them be treated in some manner before
discharge to the atmosphere. Use of biological reactors to
treat air or water streams from air strippers or soil venting
operations may be a cost effective alternative to GAG. The
movement toward clean air will require that research and
demonstration studies be conducted pertaining to
development of efficient cost-effective technologies.
Biological remediation, though unproven at this point, could
destroy TCE completely, thus meeting the intent of Section
121 of CERCLA. More research is required, however, before
full-scale implementation of such technology. Anaerobic
treatment of aquifers contaminated with TCE may be a viable
option. Extensive monitoring would be necessary to assure
that the final product of this remediation is not vinyl chloride.
Extensive protocols also may be developed for utilization of
native methylotrophs in aquifers through stimulation with
methane and oxygen before contact with TCE contaminated
ground water. Additionally, biological methods may be
designed to effectively treat off-gases from air strippers or
soil vacuum extraction systems.
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