EPA/600/A-96/093
Design and Interpretation of Microcosm Studies for Chlorinated Compounds
Barbara H. Wilson and John T. Wilson
National Risk Management Research Laboratory,
U.S. Environmental Protection Agency, Ada, Oklahoma
^
Darryl Luce
Region 1
U.S. Environmental Protection Agency, Boston, MA
Introduction
'There are three lines of evidence used to support natural attenuation as a remedy for chlorinated
solvent contamination in ground water. They are 1) documented loss of contaminant at field
scale, 2) geochemical analytical data, and 3) direct microbiological evidence. The first line of
evidence (documented loss) involves using statistically significant historical trends in contaminant
concentration in conjunction with aquifer hydrogeological parameters,such as seepage velocity
and dilution to show that a reduction in the total mass of contaminants is occurring at the site.
The second line of evidence (geochemical data) involves the use of chemical analytical data in
mass balance calculations to show that decreases in contaminant concentrations can be directly
correlated to increases in metabolic by-product concentrations. This evidence can be used to
show that concentrations of electron donors or acceptors in ground water are sufficient to
facilitate degradation of the dissolved contaminants (i.e., there is sufficient capacity). Solute fate
and transport models can be used to aid the mass balance calculations, and to collate information
on degradation.
Microcosm studies are often used to provide a third line of evidence. The potential for
biodegradation of the contaminants of interest can be confirmed by the use of microcosms,
through comparison of removals in the living treatments with removals in the controls.
Microcosm studies also permit an absolute mass balance determination based on biodegradation
of the contaminants of interest. Further, the appearance of daughter products in the microcosms
can be used to confirm biodegradation of the parent compound.
1
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When to Use Microcosms,
There are two fundamentally different applications of microcosms. They are frequently used in
a qualitative way to illustrate the important processes that control the fate of organic
contaminants. They are also used to estimate rate constants for bjotransformation of
contaminants that can be used in a site-specific transport and fate model of a plume of
contaminated ground water. This paper only discusses microcosms for the second application.
Microcosms should be used when there is no other way to obtain a rate constant for attenuation
of contaminants, in particular, when it is impossible to estimate the rate of attenuation from data
from monitoring wells in the plume of concern. There are situations where it is impossible to
compare concentrations in monitoring wells along a flow path due to legal or physical
impediments. In many landscapes, the direction of ground-water flow (and water table elevations
in monitoring wells) can vary over short periods of time due to tidal influences or changes in
barometric pressure. The direction of ground-water flow may also be affected by changes in the
stage of a nearby river or pumping wells in the vicinity. These changes in ground-water flow
direction do not allow simple snap-shot comparisons of concentrations in monitoring wells
because of uncertainties in identifying the flow path. Rate constants from microcosms can be
used with average flow conditions to estimate attenuation at some point of discharge or point of
compliance.
Application of Microcosms.
The primary objective of microcosm studies is to obtain rate constants applicable to average flow
conditions. These average condition can be determined by continuous monitoring of water table
elevations in the aquifer being evaluated. The product of the microcosm study and the
continuous monitoring of water table elevations will be a yearly or seasonal estimate of the extent
of attenuation along average flow paths. Removals seen at field scale can be attributed to
biological activity. If removals in the microcosms duplicate removal at field scale, the rate
constant can be used for risk assessment purposes.
2
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Selecting Material for Study
Prior to choosing material for microcosm studies, the location of major conduits of ground-water
flow should be identified and the geochemical regions along the flow path should be determined.
The important geochemical regions for natural attenuation of chlorinated aliphatic hydrocarbons
are regions that are actively methanogenic; regions that exhibit sulfate reduction and iron
reduction concomitantly; and regions that exhibit iron reduction alone. The pattern of
biodegradation of chlorinated solvents varies in different regions. Vinyl chloride tends to
accumulate during reductive dechlorination of trichloroethylene (TCE) or tetrachloroethylene
(PCE) in methanogenic regions (1,2); it does not accumulate to the same extent in regions
exhibiting iron reduction and sulfate reduction (3). In regions showing iron reduction alone, vinyl
chloride is consumed but dechlorination of PCE, TCE, or dichloroethylene (DCE) may not occur
(4). Core material from each geochemical region in major flow paths represented by the plume
must be acquired, and the hydraulic conductivity of each depth at which core material is acquired
must be measured. If possible, the microcosms should be constructed with the most transmissive
material in the flow path.
Several characteristics of ground water from the same interval used to collect the core material
should be determined. These characteristics include temperature, redox potential, pH, and
concentrations of oxygen, sulfate, sulfide, nitrate, ferrous iron, chloride, methane, ethane, ethene,
total organic carbon, and alkalinity. The concentrations of compounds of regulatory concern and
any breakdown products for each site must be determined. The ground water should be analyzed
for methane to determine if methanogenic conditions exist and for ethane and ethene as daughter
products. A comparison of the ground-water chemistry from the interval where the cores were
acquired to that in neighboring monitoring wells will demonstrate if the collected cores are
representative of that section of the contaminant plume.
Reductive dechlorination of chlorinated solvents requires an electron donor to allow the process
to proceed. The electron donor could be soil organic matter, low molecular weight organic
compounds (lactate, acetate, methanol, glucose, etc.), H2, or a co-contaminant such as landfill
3
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leachate or petroleum compounds (5,6,7). In many instances, the actual electron donor(s) may
not be identified.
Several characteristics of the core material should also be evaluated. The initial concentration
of the contaminated material (^g/kg) should be identified prior to construction-of the microcosms.
Also, it is necessary to know if the contamination is present as a nonaqueous phase liquid
(NAPL) or in solution. A total petroleum hydrocarbon (TPH) analysis wijl determine if any
hydrocarbon-based oily materials are present. The water-filled porosity is a parameter generally
used to extrapolate rates to the field. It can be calculated by comparing wet and dry weights of
the aquifer material.
To insure sample integrity and stability during acquisition, it is important to quickly transfer the
aquifer material into a jar, exclude air by adding ground water, and seal the jar without
headspace. The material should be cooled during transportation to the laboratory. Incubate the
core material at the ambient ground-water temperature in the dark before the construction of
microcosms.
At least one microcosm study per geochemical region should be completed. If the plume is over
one kilometer in length, several microcosm studies per geochemical region may need to be
constructed.
Geochemical Characterization of the Site
The geochemistry of the subsurface affects behavior of organic and inorganic contaminants,
inorganic minerals, and microbial populations. Major geochemical parameters that characterize
the subsurface encompasses (1) pH; (2) redox potential, Eh; (3) alkalinity; (4) physical and
chemical characterization of the solids; (5) temperature; (6) dissolved constituents, including
electron acceptors; and (7) microbial processes. The most important of these in relation to
biological processes are redox potential, alkalinity, concentration of electron acceptor, and
chemical nature of the solids.
-------
Alkalinity Field indications of biologically active portions of a plume may be identified by
increased alkalinity, compared to background wells, from carbon dioxide due to biodegradation
of the pollutants. Increases in both alkalinity and pH have been measured in portions of an
aquifer contaminated by gasoline undergoing active utilization of the gasoline components (8).
Alkalinity can be one of the parameters used when identifying where to collect biologically active
core material.
pH Bacteria generally prefer a neutral or slightly alkaline pH level with an optimum pH range
for most microorganisms between 6.0 and 8.0; however, many microorganisms can tolerate a pH
range of 5.0 to 9.0. Most ground waters in uncontaminated aquifers are within these ranges.
Natural pH values may be as low as 4.0 or 5.0 in aquifers with active oxidation of sulfides, and
pH values as high as 9.0 may be found in carbonate-buffered systems (9), However, pH values
as low as 3.0 have been measured for ground waters contaminated with municipal waste leachates
which often contain elevated concentrations of organic acids (10). In ground waters contaminated
with sludges from cement manufacturing, pH values as high as 11.0 have been measured (9).
Redox The oxidation/reduction (redox) potential (Eh) of ground water is a measure of electron
activity that indicates the relative ability of a solution to accept or transfer electrons. Most redox
reactions in the subsurface are microbially catalyzed during metabolism of native organic matter
or contaminants. The only elements that are predominant participants in aquatic redox processes
are carbon, nitrogen, oxygen, sulfur, iron, and manganese (11). The principal oxidizing agents
in ground water are oxygen, nitrate, sulfate, manganese (IV), and iron (III). Biological reactions
in the subsurface both influence and are affected by the redox potential and the available electron
acceptors. The redox potential changes with the predominant electron acceptor, with reducing
conditions increasing through the sequence oxygen, nitrate, iron, sulfate, and carbonate. The
redox potential decreases in each sequence, with methanogenic (carbonate as the electron
acceptor) conditions being most reducing. The interpretation of redox potentials in ground waters
is difficult (12). The potential obtained in ground waters is a mixed potential that reflects the
potential of many reactions and cannot be used for quantitative interpretation (11). The
5
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approximate location of the contaminant plume can be identified in the field by measurement of
the redox potential of the ground water.
To overcome the limitations imposed by traditional redox measurements, recent work has focused
on the measurement of molecular hydrogen to accurately describe the predominant in situ redox
reactions (13, 14, 15). The evidence suggests that concentrations of H2 in ground water can be
correlated with specific microbial processes, and these concentrations can be used to identify
zones of methanogenesis, sulfate reduction, and iron reduction in the subsurface (3).
Electron acceptors Measurement of the available electron acceptors is critical in identifying the
predominant microbial and geochemical processes occurring in situ at the time of sample
collection. Nitrate and sulfate are found naturally in most ground waters and will subsequently
be used as electron acceptors once oxygen is consumed. Oxidized forms of iron and manganese
can be used as electron acceptors before sulfate reduction commences. Iron and manganese
minerals solubilize coincidently with sulfate reduction, and their reduced forms scavenge oxygen
to the extent that strict anaerobes (some sulfate reducers and all methanogens) can develop.
Sulfate is found in many depositional environments, and sulfate reduction may be very common
in many contaminated ground waters. In environments where sulfate is depleted, carbonate
becomes the electron acceptor with methane gas produced as an end product.
Temperature The temperature at all monitoring wells should be measured to determine when the
pumped water has stabilized and is ready for collection. Below approximately 30 feet, the
temperature in the subsurface is fairly consistent on an annual basis. Microcosms should be
stored at the average in situ temperature. Biological growth can occur over a wide range of
temperatures, although most microorganisms are active primarily between 10° and 35°C (50° to
95°F).
Chloride Reductive dechlorination results in the accumulation of inorganic chloride. In aquifers
with a low background of inorganic chloride, the concentration of inorganic chloride should
increase as the chlorinated solvents are degraded. The sum of the inorganic chloride plus the
6
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contaminant being degraded should remain relatively consistent along the ground-water flow path.
The following tables (Table 1, Table 2) list the geochemical parameters, contaminants, and
daughter products that should be measured during site characterization for natural attenuation.
The tables include the analyses that should be performed, the optimum range for natural
attenuation of chlorinated solvents, and the interpretation of the value in relation to biological
processes.
Microcosm Construction
During construction of the microcosms, it is best if all manipulations take place in an anaerobic
glovebox. These gloveboxes exclude oxygen and provide an environment where the integrity of
the core material may be maintained, since many strict anaerobic bacteria are sensitive to oxygen.
Stringent aseptic precautions not necessary for microcosm construction. It is more important to
maintain anaerobic conditions of the aquifer material and solutions added to the microcosm
bottles.
The microcosms should have approximately the same ratio of solids to water as the in situ aquifer
material, with a minimum or negligible headspace. Most bacteria in the subsurface are attached
to the aquifer solids. If a microcosm has an excess of water, and the contaminant is primarily
in the dissolved phase, the bacteria must consume or transform a great deal more contaminant
to produce the same relative change in the contaminant concentration. As a result, the kinetics
of removal at field scale will be underestimated in the microcosms.
A minimum of three replicate microcosms for both living and control treatments should be
constructed for each sampling event. Microcosms sacrificed at each sampling interval are
preferable to microcosms that are repetitively sampled. The compounds of regulatory interest
should be added at concentrations representative of the higher concentrations found in the
geochemical region of the plume being evaluated. The compounds should be added as a
concentrated aqueous solution. If an aqueous solution is not feasible, dioxane or acetonitrile may
7
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be used as solvents. Avoid carriers that can be metabolized anaerobically, particularly alcohols.
If possible, use ground water from the site to prepare dosing solutions and to restore water lost
from the core barrel during sample collection.
For long term microcosm studies, autoclaving is the preferred method for sterilization. Nothing
available to sterilize core samples works perfectly. Mercuric chloride is excellent for short term
studies (weeks or months). However, mercuric chloride complexes to clays, and control may
be lost as it is sorbed over time. Sodium azide is effective in repressing metabolism of bacteria
that have cytochromes, but is not effective on strict anaerobes.
The microcosms should be incubated in the dark at the ambient temperature of the~aquifer. It
is preferable that the microcosms be incubated inverted in an anaerobic glovebox. Anaerobic jars
are also available that maintain an oxygen-free environment for the microcosms. Dry redox
indicator strips can be placed in the jars to assure that anoxic conditions are maintained. If no
anaerobic storage is available, the inverted microcosms can be immersed in approximately two
inches of water during incubation. Teflon-lined butyl rubber septa are excellent for excluding
oxygen and should be used if the microcosms must be stored outside an anaerobic environment.
The studies should last from one year to eighteen months. The residence time of a plume may
be several years to tens of years at field scale. Rates of transformation that are slow in terms of
laboratory experimentation may have a considerable environmental significance. A microcosm
study lasting only a few weeks to months may not have the resolution to detect slow changes that
are of environmental significance. Additionally, microcosm studies often distinguish a pattern
of sequential biodegradation of the contaminants of interest and their daughter products.
Microcosm Interpretation
As a practical matter, batch microcosms with an optimal solids/water ratio, that are sampled every
two months in triplicate, for up to eighteen months, can resolve biodegradation from abiotic
losses with a detection limit of 0.001 to 0.0005 per day. Rates determined from replicated batch
8
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microcosms are found to more accurately duplicate field rates of natural attenuation than column
studies. Many plumes show significant attenuation of contamination at field calibrated rates that
are slower than the detection limit of microcosms constructed with that aquifer material.
Although rate constants for modeling purposes are more appropriately acquired from field-scale
studies, it is reassuring when the rates in the field and the rates in the laboratory agree.
The rates measured in the microcosm study may be faster than the estimated field rate. This may
not be due to an error in the laboratory study, particularly if estimation of the field-scale rate of
attenuation did not account for regions of preferential flow in the aquifer. The regions of
preferential flow may be determined by use of a down-hole flow rneter or by use of a geoprobe
method for determining hydraulic conductivity in one to two feet sections of the aquifer.
Statistical comparisons can determine if removals of contaminants of concern in the living
treatments are significantly different from zero or significantly different from any sorption that
is occurring. Comparisons are made on the first-order rate of removal, that is, the slope of a
linear regression of the natural logarithm of the concentration remaining against time of
incubation for both the living and control microcosm. These slopes (removal rates) are compared
to determine if they are different, and if so, extent of difference that can be detected at a given
level of confidence
The Tibbetts Road Case Study
The Tibbetts Road Superfund Site in Barrington, N.H.. a former private home, was used to store
drums of various chemicals from 1944 to 1984, The primary ground-water contaminants in the
overburden and bedrock aquifers were benzene and TCE with respective concentrations of 7,800
jig/L and 1,100 ng/L. High concentrations of arsenic, chromium, nickel, and lead were also
found.
Material collected at the site was used to construct a microcosm study evaluating the removal of
benzene, toluene, and TCE. This material was acquired from the most contaminated source at
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the site, the waste pile near the origin of Segment A (Figure 4). Microcosms were incubated for
nine months. The aquifer material was added to 20-mL headspace vials, dosed with one mL of
spiking solution, capped with a Teflon-lined, gray butyl rubber septa, and sealed with an
aluminum crimp cap. Controls were prepared by autoclaving the material used to construct the
microcosms overnight. Initial concentrations for benzene, toluene, and TCE were, respectively,
380 [ig/L, 450 [ig/L, and 330 [ig/L. The microcosms were thoroughly mixed by vortexing, then
stored inverted in the dark at the ambient temperature of 10°C.
The results (Figures 1, 2, and 3; Table 3) show that significant biodegradation of both petroleum
aromatic hydrocarbons and the chlorinated solvent had occurred. Significant removal in the
control microcosms also occurred for all compounds. The data exhibited more variability in the
living microcosms than in the control treatment, which is a pattern that has been observed in
other microcosm studies. The removals observed in the controls are probably due to sorption;
however, this study exhibited more sorption than typically seen.
The rate constants determined from the microcosm study for the three compounds are shown in
Table 4. The appropriate rate constant to be used in a model or a risk assessment would be the
first-order removal in the living treatment minus the first-order removal in the control, in other
words the removal that is in excess of the removal in the controls.
The first-order removal in the living and control microcosms was estimated as the linear
regression of the natural logarithm of concentration remaining in each microcosm in each
treatment against time of incubation. Student's t distribution with n-2 degrees of freedom was
used to estimate the 95% confidence interval. The standard error of the difference of the rates
of removal in living and control microcosms was estimated as the square root of the sum of the
squares of the standard errors of the living and control microcosms, with n-4 degrees of freedom
(16).
Table 5 presents the concentrations of organic compounds and their metabolic products in
monitoring wells used to define line segments in the aquifer for estimation of field-scale rate
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constants. Wells in this aquifer showed little accumulation of trans-DCE; 1,1-DCE; vinyl
chloride; or ethene, although removals of TCE and cis-DCE were extensive. This can be
explained by the observation (4) that iron-reducing bacteria can rapidly oxidize vinyl chloride to
carbon dioxide. Filterable iron accumulated in ground water in this aquifer.
^
The extent of attenuation from well to well listed in Table 5, and the travel time between wells
in a segment (Figure 4) were used to calculate first-order rate constants for each segment (Table
6). Travel time between monitoring wells was calculated from site-specific estimates of hydraulic
conductivity and from the hydraulic gradient. In the area sampled for the microcosm study, the
estimated Darcy flow was 2.0 feet per year. With an estimated porosity in this particular glacial
till of 0.1, this corresponds to a plume velocity of 20 feet per year.
SUMMARY
Table 7 compares the first-order rate constants estimated from the microcosm studies to the rate
constants estimated at field scale. The agreement between the independent estimates of rate is
good; indicating that the rates can appropriately be used in a risk assessment. The rates of
biodegradation documented in the microcosm study could easily account for the disappearance
of trichloroethylene, benzene, and toluene observed at field scale. The rates estimated from the
microcosm study are several-fold higher than the rates estimated at field scale. This may reflect
an underestimation of the true rate in the field. The estimates of plume velocity assumed that
the aquifer was homogeneous. No attempt was made in this study to correct the estimate of
plume velocity for the influence of preferential flow paths. Preferential flow paths with a higher
hydraulic conductivity than average would result in a faster velocity of the plume, thus a lower
residence time and faster rate of removal at field scale.
References
1. Weaver, J.W., J.T. Wilson, D.H. Kampbell. 1995. EPA Project Summary. EPA/600/SV-
95/001. U.S. EPA. Washington, D.C.
2. Wilson, J.T., D. Kampbell, J. Weaver, B. Wilson, T. Imbrigiotta, and T. Ehlke. 1995.
11
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Symposium on Bioremediation of Hazardous Wastes: Research, Development, and Field
Evaluations. U.S. EPA. Rye Brook, N.Y.
3. Chapelle, F.H. 1996. Identifying redox conditions that favor the natural attenuation of
chlorinated ethenes in contaminated ground-water systems. Proceedings of the Symposium
on Natural Attenuation of Chlorinated Organics in Ground Water. September 11-13, 1996.
Dallas, TX.
4. Bradley, P.M. and F.H. Chapelle. 1996. Anaerobic mineralization of vinyl chloride in
Fe(III)-reducing aquifer sediments. Environmental Science and Technology. In Press.
5. Bouwer, E.J. 1994. Bioremediation of chlorinated solvents using alternate electron
acceptors. In: Handbook of Bioremediation. Lewis Publishers. Boca Raton, FL.
6. Sewell, G.W. and S.A. Gibson. 1991. Stimulation of the reductive dechlorination of
tetrachloroethylene in anaerobic aquifer microcosms by the addition of toluene.
Environmental Science and Technology. 25(5):982-984.
7. Klecka, G.M., J.T. Wilson, E. Lutz, N. Klier, R. West, J. Davis, J. Weaver, D. Kampbell,
and B. Wilson. 1996. Intrinsic remediation of chlorinated solvents in ground water.
Proceedings of the IBC/CELTIC Conference on Intrinsic Bioremediation. March 18-19,
1996. London, U.K.
8. Cozzarelli, I.M., J.S. Herman, and M.J. Baedecker. 1995. Fate of microbial metabolites of
hydrocarbons in a coastal plain aquifer: the role of electron acceptors. Environmental
Science and Technology. 29(2):458-469.
9. Chapelle, F.H. Ground-water Microbiology and Geochemistry. John Wiley & Sons, Inc.
New York, New York.
10. Baedecker, M.J., and W. Back, 1979. Hydrogeological processes and chemical reactions at
a landfill. Ground Water. 17(5):429-437.
11. Stumm, W., and J.J. Morgan. 1970. Aquatic Chemistry. Wiley Intcrscience. New York,
New York.
12. Snoeyink, V.L. and D. Jenkins. 1980. Water Chemistry. John Wiley & Sons. New York,
NY.
13. Chapelle, F.H., P.B. McMahon, N.M. Dubrovsky, R.F. Fugii, E.T. Oaksford, and D.A.
Vroblesky. 1995. Deducing the distribution of terminal electron-accepting processes in
hydrologically diverse groundwater systems. Water Resources Research. 31:359-371.
14. Lovley, D.R., F.H. Chapelle, and J.C. Woodward. 1994. Use of dissolved H2 concentrations
12
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to determine distribution of microbially catalyzed redox reactions in anoxic groundwater.
Environmental Science and Technology. 28:1255-1210.
15. Lovley, D.R. and S. Goodwin. 1988. Hydrogen concentrations as an indicator of the
predominant terminal electron-accepting reactions in aquatic sediments. Geochemica et
Cosmochimica Acta. 52:2993-3003.
*s
16. Glantz, S.A. 1992. Primer of Biostatistics. McGraw-Hill, Inc. New York, NY.
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Table 1. Geochemical Parameters
Analysis
Redox Potential
Sulfate
Nitrate
Oxygen
Oxygen
Iron II
Sulfide
Hydrogen
Hydrogen
pH
Range
<50 millivolt
against Ag/AgCl
<20 mg/liter
<1 mg/liter
<0.5 mg/liter
>1 mg/liter
>1 mg/liter
>1 mg/liter
>1 nMolar
<1 nMolar
5 < pH< 9
Interpretation
Reductive pathway possible
Competes at higher concentrations with
pathway
Competes at higher concentrations with
pathway
Tolerated, toxic to reductive pathway at
concentrations
Vinyl chloride oxidized
Reductive pathway possible
Reductive pathway possible
reductive
reductive
higher
Reductive pathway possible, vinyl chloride may
accumulate
Vinyl chloride oxidized
Tolerated range
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Table 2. Contaminants and Daughter Products
Analysis
Interpretation
PCE
TCE
1,1,1 -Trichloroethane
cis-DCE
trans-DCE
Vinyl Chloride
Ethene
Ethane
Methane
Chloride
Carbon Dioxide
Alkalinity
Material spilled
Material spilled or daughter product of perchlqroethylene
Material spilled
Daughter product of triehloroethylene
Daughter product of triehloroethylene
Daughter product of dichloroelhylenes
Daughter product of vinyl chloride
Daughter product of ethene
Ultimate reductive daughter product
Daughter product of organic chlorine
Ultimate oxidative daughter product
Results from interaction of carbon dioxide with aquifer
minerals
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Table 3. Concentrations of TCE, Benzene, and Toluene in the Tibbetts Road Microcosms
Compound
TCE
Mean ± Standard
Deviation
Benzene
Mean ± Standard
Deviation
Toluene
Mean ± Standard
Deviation
Time Zero
Microcosms
328
261
309
299 ± 34.5
366
280
340
329 ±44.1
443
342
411
399 ± 51.6
Time Zero
Controls
337
394
367
366 ± 28.5
396
462
433
430 ± 33.1
460
557
502
506 ± 48.6
Week 23
Microcosms
1
12.5
8.46
7.32 ± 5.83
201
276
22.8
167 ± 130
228
304
19.9
184± 147
Week 23
Controls
180
116
99.9
132 ±42.4
236
180
152
189 ± 42.8
254
185
157
199 ± 49.9
Week 42
Microcosms
2
-2
2
2.0 ± 0.0
11.1
20.5
11.6
14.4 ± 5.29
2
2.5
16.6
7.03 ± 8.29
Week 42
Controls
36.3
54.5
42.3
44.4 ± 9.27
146
105
139
130 ±21.9
136
92
115
114 ± 22.0
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Table 4, First-order Rate Constants for Removal TCE, Benzene, and Toluene in the
Tibbetts Road Microcosms.
Parameter
TCE
95% Confidence Interval
Minimum Rate Significant at 95% Confidence
Benzene
95% Confidence Interval
Minimum Rate Significant at 95% Confidence
Toluene
95% Confidence Interval
Minimum Rate Significant at 95% Confidence
Living
Microcosms
Autoclaved
Controls
Removal
Above
Controls
First-order Rate of Removal (per year)
6.31
±2.50
3.87
± 1.96
5.49
±2,87
2.62
± 0.50
1.51
± 0.44
1.86
±0.45
3.69
±2.31
1.38
2.36
± 1.83
0.53
3.63
±2.64
0.99
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Table 5. Concentration of Contaminants and Metabolic By-products in Monitoring Wells
along Segments in the Plume used to Estimate Field-scale Rate Constants.
Parameter
Monitoring well
TCE
dj-DCE
trans-DCE
1,1-DCE
Vinyl Chloride
Ethene
Benzene
Toluene
o-Xylene
m-Xylene
p-Xylene
Ethyl benzene
Methane
Iron
Segment A
SOS
Up
Gradient
200
740
0.41
0.99
<1
<4
510
10000
1400
2500
1400
1300
353
79S
Down
Gradient
Segment B
70S
Up
Gradient
52S
Down
Gradient
Segment C
70S
tJp
Gradient
......... . -^ug/mcrj ----- .
13.7
10.9
<1
<1
<1
<4
2.5
<1
8.4
<1
22
0,7
77
710
220
0.8
<1
<1
7
493
3850
240
360
1100
760
8
67
270
0.3
1.6
<1
<4
420
900
71
59
320
310
3
710
220
0.8
<1
<1
7
493
3850
240
360
1100
760
8
53S
Down
Gradient
3.1
2.9
<1
, <1
<1
<4
<1
<1
<\
<1
<1
<1
<2
27000
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Table 6. First-order Rate Constants in Segments of the Tibbetts Road Plume.
Flow Path Segments in Length and Time of Ground-water Travel
Compound
TCE
cis-DCE
Benzene
Toluene
o-Xylene
/n-Xylene
p-Xylene
Ethylbenzene
Segment A
130 feet = 6.5 years
Segment B
80 feet = 4.0 years
Segment C
200 feet .= 1 0 years
First-order Rate Constants in Segments ( per year)
0.41
0.65
0.82
>1.42
0.79
>1.20
0.64
1.16
0.59
produced
0.04
0.36
0.30
0.45
0.31
0.22
0.54
0.43
>0.62
>0.83
>0.55
>0.59
>0.70
>0.66
19
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Table 7. Comparison of First-order Rate Constants in a Microcosm Study, and in the Field at the
Tibbetts Road NPL Site.
Parameter
Trichloroethylene
Benzene
Toluene
Microcosms Corrected for
Controls
Average
Rate
Minimum Rate
Significant at 95%
Confidence
Field Scale
Segment A
*s
Segment B
3.69
2.36
3.63
1.38
0.53
0.99
0.41
0.82
>1.42
0.59
0.04
0.36
Segment C
0.54
>0.62
>0.83
20
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Figure Titles
Figure 1. TCE Concentrations in the Tibbetts Road Microcosm Study.
Figure 2. Benzene Concentrations in the Tibbetts Road Microcosm Study.^
Figure 3. Toluene Concentrations in the Tibbetts Road Microcosm Study.
Figure 4. Location of Waste Piles and Flow Path Segments at the Tibbetts Road Superfund Site.
21
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DISCLAIMER
The U.S. Environmental Protection Agency through it's
Office of Research and Development partially funded and
collaborated in the research described here. It has been
subjected to the Agency's peer review and has been approved for
publication in an EPA document.
/
L
-------
1000
100
10 15 20 25 30 35 40 45
o TCE Microcosm
TCE Control
Figure 1. TCE Concentrations in the Tibbetts Road Microcosm Study.
-------
1000
100
O)
3
10 15 20 25 30
Time (Weeks)
35
40
a Benzene Microcosm
Benzene Control
45
Figure 2. Benzene Concentrations in the Tibbetts Road Microcosm Study.
-------
1000
1 \
100
O)
3
0
10 15 20 25 30
Time (Weeks)
35
a Toluene Microcosm
Toluene Control
40
45
Figure 3. Toluene Concentrations in the Tibbetts Road Microcosm Study.
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Waste Pile
Ground Water
Flow Segment
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TECHNICAL REPORT DATA
(Please read Imtrucitons en the reverse before cemplciingf
1. REPORT NO.
EPA/600/A-96/093
2.
4..TIT,LI
DESIGN AND INTERPRETATION OF MICROCOSM STUDIES FOR
CHLORINATED COMPOUNDS
5. REPORT DATE
5 PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
1)BARBARA H. WILSON AND JOHN T. WILSON, (2) DARRYL LUCE
3. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
DU.S. EPA, NRMRL, SPRD (2)REGION 1
P.O. BOX 1198 U.S. EPA
ADA, OKLAHOMA 74820 BOSTON, MASSACHUSETTS
IQ. PROGRAM ELEMENT NO,
1 I. CONTRACT/GRANT NO.
IN-HOUSE RPJW9
12. SPONSORING AGENCY NAME ANO ADDRESS
U.S. EPA, NRMRL, SPRD
P.O. BOX 1198
ADA, OKLAHOMA 74820
13. TYPE Of REPORT ANO PERIOD COVERED
BOOK CHAPTER
14. SPONSORING AGENCY CODE
EPA/600A
15. SUPPLEMENTARY NQT6S
16. ABSTRACT
Table 7 compares the first-order rate constants estimated from the microcosm studies to the rate
constants estimated at field scale. The agreement between the independent estimates of rate is
good; indicating that the rates can appropriately be used in a risk assessment. The rates of
biodegradation documented in the microcosm study could easily account for the disappearance
of trichloroethylene, benzene, and toluene observed at field scale. The rates estimated from the
microcosm study are several-fold higher than the rates estimated at field scale. This may reflect
an underestimation of the true rate in the field. The estimates of plume velocity assumed that
the aquifer was homogeneous. No attempt was made in this study to correct the estimate of
plume velocity for the influence of preferential flow paths. Preferential flow paths with a higher
hydraulic conductivity than average would result in a faster velocity of the plume, thus a lower
residence time and faster rate of removal at field scale.
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22. <"*iCS
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