United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(5102G)
EPA 542-N-98-008
September 1998
Issue No. 29
vvEPA Ground Water Currents
CONTENTS
Natural Attenuation of
Chlorinated VOCs in
Wetlands Pg. 1
Enhancing Aquifer
Reclamation through
Selective Colloid
Mobilization Pg. 2
Vadose Partitioning
Interwell Tracer Test
forDNAPL
Investigation Pg. 3
EPA Solicitation for
Small Business
Innovation Research Pg. 4
About this Issue
This issue features the
results of a five-year study
on natural attenuation in
wetlands at Aberdeen
Proving Ground, a new
method for enhancing pump-
and-treat remediation, and
improved techniques for
measuring vadose zone
DNAPL contamination.
Natural Attenuation of
Chlorinated UOCs in
Wetlands
by Lisa D. Olsen and Michelle
M. Lor ah, U.S. Geological
Survey, Baltimore, Maryland
The U.S. Geological Survey (USGS)
investigated the natural attenuation of
chlorinated volatile organic compounds
(VOCs) at Aberdeen Proving Ground, MD.
Investigations focused on a contaminant
plume that discharges from a sand aquifer,
through organic-rich wetland sediments,
to a freshwater tidal creek. Over a five-
year period, the hydrogeology and
geochemistry along wetland flow paths
and the rates of biodegradation and
sorption were studied. Several major
findings of the study indicate that natural
attenuation is occurring at the West
Branch Canal Creek site. Based on the
results of this study, natural attenuation
has been proposed as a clean-up remedy
in an interim record of decision for the
West Branch Canal Creek site.
Between World War I and the late 1970s,
the area along the West Branch Canal
Creek was used to develop, test, and
manufacture military-related chemicals.
Wastes from these processes
were discharged into the creek and
surrounding marsh. As a result, contami-
nants migrated to the underlying aquifer.
Compounds identified in the plume
include trichloroethylene (TCE), 1,1,2,2-
tetrachloroethane (PCA), carbon
tetrachloride, and chloroform. The
relatively thin layers of wetland sediments
were shown to reduce contaminant
concentrations in the ground water before
it discharges to the wetland surface and
adjacent tidal creek.
In the aquifer, concentrations of the parent
compounds TCE and PCA ranged from
about 100 to 2,000 parts per billion,
whereas concentrations of daughter
products were low or undetectable. In
contrast, parent compounds commonly
were not detected in the wetland sediment
porewater, but the daughter compounds
1,2-dichloroethylene (1,2-DCE), vinyl
chloride (VC), 1,1,2-trichloroethane, and
1,2-dichloroethane were observed. Thus,
the presence of daughter products in the
naturally anaerobic wetland sediments
indicate that TCE and PCA have been
degraded by reductive dechlorination
reactions. A conceptual model of the
natural attenuation processes occurring in
this area is shown in Figure 1.
Biodegradation processes were evaluated
through field evidence of the occurrence
of parent and daughter compounds. In
addition, the distribution of redox-
sensitive constituents, such as methane,
sulfate and sulfide, ferric and ferrous iron,
and dissolved oxygen, were studied.
Biodegradation processes were studied
further in anaerobic and aerobic micro-
cosms.
In anaerobic microcosms, maximum
potential first-order rate constants for
biodegradation of TCE and PCA in the
wetland sediments ranged from 0.10 to
0.31 per day under methanogenic condi-
tions, corresponding to half-lives of 2-7
days. The rate constant for TCE biodegra-
dation under sulfate-reducing conditions
was 0.045 per day (half-life of 15 days).
These estimated rate constants are 2-3
orders of magnitude higher than those
reported in the literature for TCE biodegra-
dation in microcosms constructed from
sandy aquifer sediments. Reported
differences may be attributed to the high
percentage (18%) of organic material in
the wetland sediments.
Recycled/Recyclable
Printed with Soy/Canola Ink on paper that
contains at least 50% recycled fiber
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In aerobic microcosms, biodegradation rates
forcis-l,2-DCE, trans-l,2-DCE, andVC
were in the same range as those measured
for TCE and PCA under anaerobic condi-
tions. This finding indicates that the
production of these daughter products by
anaerobic biodegradation of TCE and PCA
could be balanced by their consumption
where oxygen is available in the wetland
sediment, such as near roots and land
surface. Researchers also found that, under
aerobic conditions, biodegradation of cis-
1,2-DCE, trans-l,2-DCE, and VC occurred
only in the presence of methane consump-
tion. This indicates that methanotrophs are
involved in the biodegradation process.
Aerobic biodegradation was fastest for VC
and slowest for TCE, showing that faster
degradation by methane-utilizing cultures
occurs when the compounds are less
halogenated.
Analysis of equilibrium sorption iso-
therms indicated that advective water
flow would cause the movement of
contaminants in the wetland sediments to
be 6-10 times faster than contaminant
sorption alone. Currently, the USGS is
conducting small-scale tracer tests to
better quantify ground-water flow rates,
dispersion, and volatilization in the
wetland sediments.
Detailed information on this study is
provided in the report Natural Attenua-
tion of Chlorinated Volatile Organic
Compounds in a Freshwater Tidal
Wetland, Aberdeen Proving Ground,
Maryland (USGS Water-Resources
Investigations Report 97-4171), which
may be obtained from the USGS at 303-
202-4700. The USGS is continuing to
monitor natural attenuation rates at the
West Branch Canal Creek site. For
additional information, contact Michelle
Lorah(USGS) at410-238-4301 ore-mail
mmlorah@usgs.gov.
Figure 1: Conceptual Model for the West Branch Wetland Site.
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Enhancing Aquifer
Reclamation through
Selective Colloid
Mobilization
by John C. Seaman, Ph.D., and
PaulM. Bertsch, Ph.D., Univer-
sity of Georgia
Research at the Savannah River Ecology
Laboratory (SREL) of the University of
Georgia has resulted in the development of
an in situ means for controlling the
mobilization and subsequent transport of
native colloidal iron oxides to enhance the
efficiency of pump-and-treat remediation.
This technology, known as Selective
Colloid Mobilization™ (SCM), was
developed in partnership with the U.S.
Department of Energy (DOE) in an effort to
reduce the costs, shorten project durations,
and increase the effectiveness of conven-
tional ground-water remediation methods.
The SCM process employs non-hazardous
chemical compounds such as calcium or
cationic surfactants to mobilize iron oxide
colloids, which commonly serve as a
primary resident phase for many contami-
nants in highly weathered, low-carbon
systems. Ground water containing the
mobilized colloids and associated contami-
nants is pumped from a contaminated
aquifer to ground surface, where the pH of
the ground water is adjusted to promote
coagulation of the colloids. Following
removal of the coagulated colloids, the
ground water is returned to the aquifer and
the SCM process is repeated until clean-up
goals are met.
In a series of intermediate-scale column
experiments, coarse-textured, oxide-coated
Atlantic Coastal Plain sediments were
leached with solutions containing calcium
salts or cationic surfactants at various ionic
strengths and pH levels. Colloid dispersion
resulted from mild changes in solution
chemistry. The introduction of dilute
calcium chloride solutions resulted in
colloid mobilization and in a decrease in
effluent pH attributed to aluminum cation
exchange and hydrolysis reactions, as well
as specific cation sorption reactions on
hydrous oxide surfaces that can impart
greater net-positive charge. The use of
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cationic surfactants was even more effective
at mobilizing iron oxides without inducing
column plugging, a major cause of forma-
tion damage that often plagues pump and
treat reclamation systems.
In collaboration with Westinghouse-
Savannah River, SREL researchers recently
completed an extensive series of under-
ground injection experiments that
evaluated the potential for formation
damage adjacent to injection wells
resulting from the long-term application of
reclaimed ground water. Although these
studies did not evaluate the patented SCM
technology directly, they confirmed the
basic geochemical processes on which the
technology was based.
In these injection well experiments,
approximately 40,000 gallons of treated
water from a pilot-scale treatment plant
were used. Ground-water samples were
collected and either filtered or digested
using EPA Method #3015 to discriminate
between soluble contaminant metals and
metals associated with suspended colloids,
respectively. Sample turbidity was deter-
mined to evaluate the potential for colloid
mobilization, and a correlation coefficiency
for sample turbidity compared to sample
metal concentration was estimated. Higher
correlations, and accordingly higher metal
concentrations, were identified in most of
the digested samples (containing metals
associated with suspended colloids) than in
the filtered samples (containing metals in a
soluble state) (Figure 2). These results
confirmed that many of the metals of
regulatory importance were associated with
colloids (primarily iron oxides) that could
be mobilized in response to changes in
solution chemistry.
SCM application is limited to highly
weathered, oxide-coated sediments similar
to those found on the Atlantic Coastal Plain
(a geographic region extending from
Mississippi to New Jersey) and regions of
Asia and the Far East. The process is
effective in the removal of most radionu-
clides, metals, hydrophobic organics, and
oxyanions such as arsenate.
During 1998-1999, SCM field application
will be conducted at a selected DOE site.
Contact Dr. Paul Bertsch (University of
Georgia) at 803-725-5637 or e-mail
Figure 2. Correlation Coefficients for
Sample Turbidity (NTU) Compared to
Metal Concentrations in Filtered and
Digested Ground-Water Samples.
Variable
NTU
Al
Ba
Ca
Cd
Cs
Fe
K
Na
Pb
Rb
Sr
U
Filtered
1.000
0.147
-0.018
-0.025
-0.076
0.239
0.115
-0.243
0.240
0.028
0.213
-0.067
0.161
Digested
1.000
0.247
0.880
0.315
-0.060
0.803
0.942
0.957
0.183
0.964
0.977
0.855
0.904
bertsch@srel.edu, or Dr. John Seaman
(University of Georgia) at 803-725-0977 or
e-mail seaman@srel.edu for more informa-
tion.
Vadose Partitioning
Interwell Tracer Test for
DNAPL Investigation
by Gary A. Pope, Ph.D., Univer-
sity of Texas, and Paul Mariner,
Duke Engineering & Services
The University of Texas at Austin, TX,
conducted possibly the first field gaseous
partitioning interwell tracer test (PITT) to
measure contamination from dense non-
aqueous phase liquid (DNAPL) in the
vadose zone. This testing, which was
conducted in December 1995 in coopera-
tion with Duke Engineering & Services,
provided important data for future DNAPL
remediation.
The PITT was applied in unsaturated
alluvium beneath two side-by-side organics
disposal trenches at the Chemical Waste
Landfill, Sandia National Laboratories,
Albuquerque, NM. The water table at this
site is approximately 500 feet below the
landfill. Residual DNAPL consisted of a
mixture of trichloroethene (TCE) and other
chlorinated solvents, high-molecular
weight hydrocarbons, and polychlorinated
biphenyl (PCB) oils.
One injection well and one extraction well
were installed 55 feet apart on opposite
sides of the target area. Each well was
screened at intervals of 10-35 feet, 40-60
feet, and 65-80 feet below ground surface.
After steady-state air injection and extrac-
tion rates were established, a slug of non-
partitioning tracers (methane and sulfur
hexafluoride [SFJ) and TCE DNAPL-
partitioning gaseous tracers was added to
the injection stream. Octafluorocyclo-
butane, dodecafluorodimethylcyclobutane,
perfluoro-l,3-dimethylcyclonexane, and
perfluoro-l,3,5-trimethylcyclohexane
(C9F18) served as the TCE DNAPL-partition-
ing tracers. The respective TCE partition
coefficients (mg/L in TCE per mg/L air) for
these partitioning tracers were measured in
the laboratory to be 9, 16, 72, and 162.
In addition, a water-partitioning tracer,
difluoromethane (with a water-air partition
coefficient of approximately 1.7 and a
TCE-air partition coefficient of approxi-
mately 2.0), was included to measure water
saturations in the targeted zones. Though
the water-partitioning tracer also partitions
into TCE, the effect of TCE on overall
retardation is negligible when TCE
saturation is less than 5 percent of water
saturation, as it was in this case. Tracer
concentration breakthrough curves at the
extraction well and monitor locations were
obtained using on-line chromatographs.
SF6 and C9F18 provided the most reliable
non-partitioning and TCE DNAPL-
partitioning tracer data for this test. Initial
analysis indicated that TCE DNAPL
penetrated non-uniformly to a depth of 20-
30 feet below ground surface. Multi-level
monitor well data showed that TCE
decreased in volume with increasing depth.
The average TCE saturation in the shallow
zone between 10 and 35 feet below ground
surface was measured to be 0.11 percent,
based on a calculated C9F18 retardation
factor of 1.2. One monitor point at 10 feet
below ground surface indicated a TCE
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saturation of 0.25 percent, corresponding to
a retardation factor of approximately 1.4
(Figure 3). No TCE DNAPL was detected in
the intermediate or deep zones. Average
water saturations in the shallow,
intermediate, and deep zones were
measured to be 23 percent, 13 percent, and
10 percent, respectively.
The PITT results provided a basis for
deciding the voluntary corrective action
strategy at the site. In addition to allaying
fears that DNAPL had penetrated to the
water table, the results show that a combina-
tion of excavation and vapor extraction
would satisfy clean-up objectives.
For more information on the PITT (patent
pending), contact Dr. Gary Pope (University
of Texas) at 512-471-3235 or e-mail
gary_pope@brazos.pe.utexas.edu, or Paul
Mariner (Duke Engineering & Services) at
970-256-0535 or e-mail
pemarine@dukeengineering.com.
EPA Solicitation for Small
Business Innovation
Research
A solicitation for research proposals from
science and technology-based firms will
open on September 17, 1998, and close on
November 19,1998. "Phase I" contracts of
up to $70,000 will be awarded to small
businesses for investigation of the scientific
merit and technical feasibility of proposed
technologies or products. Recipients of
Phase I contracts will be eligible to compete
for "Phase II" contracts of up to $295,000 to
complete the research and development
required for technology or product commer-
cialization.
The solicitation will be posted on the
National Center for Environmental Re-
search and Quality Assurance Web site at:
http://www.epa.gov/ncerqa; users may click
on "Small Business" to access the current
(1999) solicitation and "Archive" to view
last year's solicitation. For additional
assistance, call the EPA Small Business
Innovation Research Helpline at 1-800-
490-9194.
Figure 3. SF6 and Cfls Tracer Response at Shallow Monitor Point.
CT
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