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/A newsletter about soil, sediment, and ground-water characterization and remediation technologies
Issue 34
This issue o/Technology News and Trends highlights strategies for remediating sites with
inorganic contaminants and radionuclides. Enhanced research has led to increased use of
bioremediation as a viable technology for removing or transforming inorganic contami-
nants. Due to the length of time needed to address radionuclide contamination, research also
focuses on the potential for monitored natural attenuation (MNA) to complement aggressive
cleanup technologies.
Hanford Demonstrates Bioimmobilization of Hexavalent
Chromium in Ground Water
January 2008
The U.S. Department of Energy (DOE) is
evaluating long-term efficacy of lactate-
stimulated bioreduction to treat ground water
contaminated with hexavalent chromium
[Cr(VI)] atHanford's "Site 100H" along the
Columbia River in Washington. The study
includes identification of critical microbial
community-structure changes and stressors
helping to control and predict
biogeochemical processes causing Cr(VI)
bioimmobilization. Polylactate in the form of
Hydrogen Release Compound® (HRC) was
injected into the ground water in 2004.
Cr(VI) concentrations now are below the
drinking water standard of 10 ppb due to
transformation of Cr(VI) into insoluble Cr
(III) complexes, which is largely affected
by bioimmobilization stressors. Common
stressors identified during the study include
oxygen, nitrate, salt, and sulfate.
Chromium contamination at Site 1OOH likely
resulted from a release of sodium dichromate
once used to control corrosion at Hanford's
former plutonium reactor systems, and to
decontaminate shut-down reactor
complexes. Ground-water analysis in 2004
at Site 100H showed a Cr(VI) concentration
of approximately 100 mg/L, a level
unchanged over the previous 20 years. Sand
and gravel extend approximately 50 feet
below ground surface (bgs) at the site. The
sand and gravel are underlain by clay and
silt layers that in turn overlay basalt. The
water table is at 42 feet bgs.
Bench-scale studies on Site 100H ground-
water and sediment samples showed that
introduction of various forms of lactate
stimulated an increase in bacterial population
exceeding 108 cells/g. This increase generated
reducing conditions leading to nearly
complete Cr(VI) removal from the pore
solution after only three weeks of incubation.
The most viable remediation alternative
involved use of a diothionite reducing
permeable reactive barrier (PRB), but onsite
studies suggested difficulty in long-term PRB
maintenance sufficient to prevent Cr(VI)
breakthrough.
Field testing in 2004 involved a single injection
of 40 pounds of HRC labeled with stable
isotope carbon (13C) in a well extending 50
feet bgs. A multi-screened extraction well
approximately 15 feet from the injection
location was pumped for 27 days to obtain
water samples from the sand/gravel, clay,
and silt layers.
Post-injection analysis of ground water
indicated an increase in the 813C (ratio of
13C to other carbon isotopes) of dissolved
inorganic carbon from 15%o (parts per
thousand) to over 50%o, exceeding the
HRC's proportion and indicating that CO2
was created as a byproduct of lactate
metabolism. Depletion of competing terminal
electron acceptors (oxygen, nitrate, and
sulfate) occurred sequentially. Evidence of
subsurface lactic acid buildup further
indicated that the injection stimulated
bioreduction of Cr(VI) to Cr(III) through
precipitation. Naturally occurring microbial
reducers of the sulfate and iron (Fe)
[continued on page 2]
Contents
Hanford Demonstrates
Bioimmobilization of
Hexavalent Chromium
in Ground Water page 1
SRNL Evaluates
Sustainable
Remediation Strategies
for Metals and
Radionuclides page 2
Bioremediation
Evaluated for Long-Term
Immobilization of
Uranium page 3
Research Shows
Growing Potential of
Bioremediation for
Arsenic and Selenium page 5
EPA Releases
Technical Resource for
MNA of Inorganics page 6
CLU-IN Resources
CLU-IN's Contaminant Focus
area provides background
information on a range of issues
concerning soil or ground water
with arsenic or chromium (VI)
contamination. Topics include
contaminant chemistry and
behavior, toxicology, and
regulatory guidance. Visit these
topics at http://cluin.org/
contaminantfocus/.
Recycled/Recyclable
Printed with Soy/Canola Ink on paper that
contains at least 50% recycted fiber
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[continued from page 1]
apparently maintain the presence of
hydrogen sulfide and ferrous iron [Fe(II)],
subsequently maintaining Cr(VI) below 5
ppb in the injection well.
DOE's "16s rDNA microarray" method
was used to detect the composition and
diversity of microbes in ground-water
samples (Figure 1). The method is
capable of identify ing the 9,900 microbial
species of 16S rDNA from up to 550,000
probes in a 1.28 cm2 array.
Evidence of bioreduction was supported
by a decrease in reduction/oxidation
from 240 to -130 mV and a decrease in
oxygen content from 9 mg/L to nearly
zero. Cr(VI) concentrations in the
monitoring well decreased to less than
5 ppb as a result of the polylactate
injection, and have remained lower than
upgradient (background) concentrations.
Study findings suggest that the
prevailing mechanisms for Cr(VI)
reduction are direct enzymatic
chromate reduction and/or abiotic
geochemical processes involving
formation of insoluble complexes of
Cr(III) with Fe(II) or sulfide (S2').
•3- 3000
Desulfovibrio hatophilus
Geobacter metellineducens
Dechloromonas agitatus
Pseudomonas put/da
11 17
Days Since HRC Injection
Overall study results demonstrate
significant potential for using naturally
occurring microorganisms to enhance
in-situ Cr(VI) immobilization.
Ongoing monitoring indicates minimal
chemical rebound. DOE will further
examine the lateral extent and potential for
rebound as well as the impact of Site 100H
Cr treatment on regional ground water.
Future onsite investigations also will
determine the optimal number of injection
wells needed for Crbioimmobilization and
the appropriate frequency of lactate
reinjection. Based on the favorable Site 100
results, DOE anticipates using this remedy
to control Cr(VI) concentrations in
ground water at other sites such as the
Idaho National Laboratory, Savannah
River Site, and Pantex Plant. Updated
project details are available from DOE's
Office of Environmental Management at
http://esd.lbl.gov/ERT/hanfordlOOh/.
Contributed by Terry Hazen, DOE
ftchazen&lbl.gov or 510-486-6223)
SRNL Evaluates Sustainable Remediation Strategies for Metals and Radionuclides
DOE's Savannah River National
Laboratory (SRNL) recently initiated
studies under the Department's Office
of Environmental Management (EM) to
identify methods for increasing
sustainability of remediation addressing
metal- and radionuclide-contaminated
ground water. Sustainable strategies will
help meet site-specific cleanup objectives,
including long-term risk reduction, while
minimizing maintenance, cost, and
collateral environmental damage
associated with remediation. Current
SRNL work focuses on estimating the
duration of aggressive remediation
strategies before natural processes can
be relied upon to return the site to pre-
contamination conditions. Opportunities
for using follow-on natural processes
are critical to sites with contaminants
known to exist in the environment for
a long time.
One SRNL study area at the Savannah
River Site (SRS), SC, is a 1-km2 metals/
radionuclides waste site known as the "F-
Area Seepage Basins," where a modified
funnel-and-gate barrier system has
operated since 2005 to treat ground water
containing strontium (Sr)-90, uranium
(U) isotopes, iodine-129, technetium-99,
and tritium. The ground water is acidic
(pH 3.2-4.0), aprimary factor facilitating
mobility of certain contaminants and
associated risk drivers. In the current
treatment strategy, alkaline solutions of
pH 10 are injected periodically into the
gates to neutralize ground water and
reduce mobility of some contaminants.
Injection frequency is determined by
monitoring pH in wells downgradient
from the injection wells; when a trigger
of pH 5.5 is reached, alkaline solution is
reinjected. In the three years of operation,
injections were required approximately
each 12 months at one gate and 18
months at the second gate.
The treatment strategy is more
sustainable than the previous, inefficient,
pump-and-treat (P&T) system. P&T
operations cost approximately $1
million per month and produced a
significant quantity of solid radioactive
waste requiring disposal. The alkaline-
enhanced funnel-and-gate system,
however, treats all contaminants by
mixing the stratified plume at the
barrier wall as well as pH-sensitive
contaminants such as 90Sr and uranium
isotopes at the gates. Early analytical
data from downgradient wells indicate
the system effectively reduces
concentrations of 90Sr, uranium
isotopes, and tritium to below drinking
water standards.
[continued on page 3]
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[continued from page 2]
Chemical effects of the alkaline injection
were demonstrated in a small-scale field
test conducted over five months in 2002-
2003, prior to installing the full-scale
injection system. Alkaline solutions were
injected below the water table at closely
spaced points upgradient of the extraction
well. The extraction well was pumped
continually to draw injectate toward the
well. Breakthrough of the injectate at the
extraction well was indicated by decrease
in specific conductance. The pH did not
increase until four weeks after
breakthrough due to the need for the
injectate to first neutralize acidified
surfaces of the aquifer minerals (Figure 2).
Adjustments to the injection system in
late December 2002 caused temporary
decreases in pH and specific
conductance four to five weeks later.
This series of chemical changes controls
the time needed to return ground water
to a near-natural pH of 6. When acid flux
from the vadose zone becomes
insignificant, uncontaminated ground
water with pH near 6 migrates into the
plume zone. The resulting pH front will
migrate more slowly than ground water
as a result of the buffering effect of
acidified mineral surfaces. Duration of
the injection system will depend on the
migration rate of the trailing pH gradient
through the treatment zone, the point
at which contaminants will not
remobilize. By avoiding remobilization
of sequestered metals and radionculides
and reducing treatment duration,
sustainability of this remediation
strategy is improved.
Findings from this and other waste sites
suggest the rate of biogeochemical gradient
migration through a treatment zone is
controlled by hydrogeology and mineralogy
as well as relative biogeochemical
conditions of the treatment zone, plume
zone, and uncontaminated zone. At sites
employing treatment technologies that
establish reducing zones, for example,
mineralogy affects remediation
sustainability. The reduced iron and
manganese minerals formed during
treatment will act as redox buffers and
decrease migration rates of dissolved
oxygen gradients, consequently
remobilizing contaminants.
Understanding factors that control the rate
at which F-Area Seepage Basins ground
water returns to near natural conditions
is a primary goal of this study, driven by
SRS long-term remediation. EM's broader
goal is to address these issues for other
wastes and at other sites using different
remediation strategies.
Contributed by Miles Denham, SRNL
(miles.denham(q)srnl.doe.gov or
803-725-5521)
7001
I 600-1
CO
^500-
400-
O 300-
8 200-
M
100
Injection Begins
Probable Breakthrough
-•-Specific Conductance
—pH
Figure 2. Weekly
measurements of
extraction well
ground-water
parameters at the
SRS F-Area
Seepage Basins
indicate a delay in
pH response after
breakthrough of
the alkaline
injectate.
Bioremediation Evaluated for long-Term Immobilization of Uranium
Oak Ridge National Laboratory
(ORNL) and Stanford University
researchers are developing a strategy
for bioimmobilizing uranium in the
highly contaminated subsurface of the
former "S3 Ponds" site at DOE's Oak
Ridge Field Research Center in Oak
Ridge, TN. The approach employs a
treatment train involving flow-field
hydraulic control, subsurface
preconditioning, and delivery of
ethanol as an electron donor for in-
situ U(VI) reduction/immobilization.
Results of pilot-scale field tests will
be integrated into DOE decision-
making in 2015 regarding S3 Ponds
cleanup. Additional EM assistance in
evaluating site-specific environmental
engineering, hydrogeology, microbiology,
geo-chemistry, and physics is provided by
the University of Oklahoma, Argonne
National Laboratory, Michigan State
University, Montana State University, and
Georgia Institute of Technology.
Storage of atomic weapons production
wastes in the unlined S3 Ponds from 1951
until 1984 caused extensive subsurface
contaminant plumes migrating in three
separate pathways, including one that
discharges to a nearby creek. Remediation
planning required detailed analysis of
geophysical and geochemical conditions.
The plume depths range from 30 to 100
ft bgs within subsurface media containing
fracture densities as high as 100-200
fractures per meter. The fractures
account for less than 5-10% of matrix
porosity but carry more than 95% of
ground-water flow. The surrounding
highly porous soil and sediment have a
low permeability and serve as a sink (and
continuing source) of contamination.
Sample analyses indicated the highest
contaminant concentrations existed in
ground water at a depth of 30-50 feet
bgs. To date, maximum concentrations
measured for metals are 40 mg/L of
depleted uranium, 540 mg/L aluminum
(Al), 930 mg/L calcium (Ca), and 11-
14 mg/L nickel. Perchloroethene and
c/5-dichloroethene in concentrations of
[continued on page 4]
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[continued from page 3]
2-3 mg/L and 1 mg/L, respectively, are
co-contaminants. Concentrations of
8,000 g/L nitrate and 1 g/L sulfate
resulted from disposal of nitric and
sulfuric acids that lowered the pH of the
ground water to 3.4-3.6, hampering in-
situ uranium bioremediation.
Solid-phase uranium in hot spots with
concentrations of 200-700 mg/kg serve
as a long-term source of U(VI)
contamination, and aqueous-phase
uranium concentrations currently
exceed the federal drinking water
standard by over 1,000-fold. Most of
the uranium is associated with the solid
phase. Laboratory and field tests
showed that uranium sorption points
(0.1 g soil per 15 mL solution) and
desorption points (13.5 g/15 mL
solution) are strongly pH dependent;
high uranium adsorption was observed
at pH around 6.0.
Bioremediation at the S3 Ponds relies
on converting soluble U(VI) into
sparingly soluble U(IV), which is more
resistant to dissolution and desorption
and posing less potential for ground-
water migration. A range of
microorganisms, including certain
sulfate-reducing (SRB) and iron(III)-
reducing bacteria (FeRB), can mediate
this conversion. Reduced compounds
such as sulfide and green rusts
generated by microorganisms also can
convert U(VI) to U(IV) under certain
reduced conditions.
Prior to startup of the pilot-scale
bioimmobilization system, a nested
circulation well system containing an
inner loop and an outer loop was
installed to improve hydraulic control in
the treatment area. Injection of clean
water into the outer loop protected the
inner loop from invasion of
contaminated ground water. Extracted
ground water was then treated through
an aboveground system employing
vacuum stripping to remove volatile
organic compounds (VOCs),
precipitation of Al and Ca sludge in a
settling tank, and nitrate removal through
a biological granular-activated-carbon
fluidized bed reactor. Treated water was
injected back into the outer loop.
In November 2002, a bromide tracer study
was conducted to characterize the test area's
hydrology. The treatment-area subsurface
was flushed with clean water (tap water
and nitrate-free water from an aboveground
treatment facility) during the following fall
to achieve a pH of 4.0-4.5 and remove
clogging agents and inhibitors such as Al,
Ca, nitrate, and VOCs. A second clean-
water flushing was performed in November
2003 to increase pH to 6.0-6.3, facilitating
maximum uranium sorption capability and
enhancing conditions for bioremediation.
Residual nitrate in ground water was
removed further by in-situ denitrification.
After an additional year of ethanol injections,
uranium concentrations decreased below
the maximum contaminant level (0.03 mg/
L) within fast-flowing zones of the
subsurface (3-8 m/day hydraulic
conductivity) demonstrating that they were
hydrologically connected to the injection
well. Intermittent ethanol injections
sequentially stimulated in-situ denitrification
followed by sulfate and Fe(III) reduction
and U(VI) reduction. Treatment-area
sediment samples changed color from
yellow-brown to dark green or black,
providing further evidence of reduction
and expansion within the zone of
reduction. Reduction of U(VI) to U(IV)
was confirmed by X-ray absorption near-
edge structure spectroscopy. Before
biostimulation, no U(IV) was observed in
sediment samples. After biostimulation, up
to 80% of the uranium sorbed to soil was
reduced to U(IV).
Microbial community analysis indicated
that SRB and FeRB populations are
stimulated by delivery of electron donor.
Prior to biostimulation, only denitrifiers
were found in ground water, at an
extremely low level (3 cells/mL). The
amount of bacterial DNA also was too
low to extract from sediment samples.
Most probable estimates for denitrifiers,
SRB, and FeRB in sediments (cells/g dry
weight) after biostimulation increased to
107-108. Post-treatment tests indicate that
microorganisms capable of reducing
U(VI) to U(IV) (including SRB
Desulfovibrio, Desulfoporosinus, and
Desulfotomaculum spp. and FeRB
Geobacter wAAnaeromyxobacter spp.)
were present in both ground water and
sediment. These results confirm that
biostimulation promoted biotic and
secondary abiotic reductions of U(VI) in
ground water.
The pilot system continues to operate to
evaluate stability of reduced uranium.
Bioreduced U(IV) was stable following
suspension of ethanol delivery for a 50-
day period when anaerobic conditions
were maintained. Two-year microcosm
tests also confirmed long-term stability of
the reduced uranium. Other field tests
involving injections of dissolved oxygen
(DO) or nitrate to the reduced-zone
subsurface demonstrated reoxidizationof
U(IV) and remobilization of U(VI).
Subsurface delivery of ethanol as an
electron donor, however, effectively
restored reducing conditions and
decreased uranium concentrations to the
previously low levels. Ongoing research
involves additional characterization of
U(VI) reduction by delivery of multiple
or slowly-degrading electron donor
sources and maintenance of long-term
stability of immobilized uranium.
Monitoring results to date indicate that
very low aqueous-phase concentrations
of uranium can be achieved despite high
solid-phase concentrations due to the
low solubility of U(IV) and low rates of
desorption/dissolution relative to the rate
of reduction. Findings suggest that long-
term bioremediation at S3 Ponds will
need strategies for DO and nitrate
control or methods to increase
resistance of the immobilized uranium
to remobilization by reoxidation.
Contributed by Craig Criddle, Ph.D
fcriddle&stanford.edu or
650-723-9032) and Wei-Min Wu, Ph.D
(wei-min.wu(q)stanford.edu or
650-724-5310), Stanford University,
and Philip Jardine, Ph.D
(jardinepm&ornl.gov or
865-574-8058) and David Watson
(watsondb (q).ornl. gov or
865-241-4749), ORNL
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Research Shows Growing Potential of Bioremediation for Arsenic and Selenium
Collaborative research among Duquesne
University, the University of Pittsburgh,
and the U.S. Geological Survey (USGS)
is underway to identify bacteria and plant
species with potential to improve
bioremediation efficacy for arsenic (As)
and selenium (Se). Bioremediation of
these contaminants is an alternative to
abiotic methods based on activated
alumina, coagulation/filtration, lime
softening, and reverse osmosis. Abiotic
methods generally cost more, remove
As(III) less effectively, require
manipulation of pH, and produce more
waste. Integrated laboratory and
greenhouse test results show significant
potential for arsenite-oxidizing, arsenate-
reducing, or sulfidogenic bacteria as
well as hyperaccumulating yeast
(Saccharomyces) and brake fern (Pteris).
Many Pteris species such as P. vittata
(ladder brake) and P. cretica (Cretan
brake) have proven to hyperaccumulate
As in the stems and leaves. Arsenic
typically enters the plant as As(V) through
phosphate channels in the roots, where
it is reduced to As(III) and stored in other
plant tissues. When accumulation of
As(III) is complete, harvested plant
tissues are disposed as hazardous waste.
In Duquesne/University of Pittsburgh
greenhouse studies, P. cretica exposed
to water containing 300 [ig/L As(III)
resulted in 100% oxidization to As(V)
and subsequent plant uptake of As(V)
within 100 hours (Figure 3). Rates of
As uptake showed similar trends across
Pteris spp. Arsenic uptake in control
experiments with non As-accumulating
Boston fern (Nephorlepsis exaltata) also
showed rapid As(III) oxidation, but no
uptake of As.
Researchers also are evaluating various
changes in the rhizosphere of P. cretica.
Addition of an antibiotic mixture to the
water was found to significantly reduce
microbial populations, reducing the
As(III) oxidation rate by approximately
50% within 48 hours and consequently
delaying As uptake. These results
suggested that the rhizosphere microbiota
is one of the controlling factors in As
uptake. Testing now focuses on the use
of specific nutrient amendments to
enhance rhizosphere bacterial activity.
Selenium (Se) commonly accumulates as
a result of erosion and industrial or
agricultural runoff folio wed by evaporation
in areas with extensive hydrocarbon
content. It is frequently present in
wastewater discharged by petroleum-
processing facilities. Phytoremediation
employing plant species that
hyperaccumulate (selenium weed Neptunia
amplexicaulis, and Astragalus spp. such
as loco weed and milk vetch) or volatilize
(Indian mustard Brassica juncea) has
shown limited success with Se.
In contrast, more than 20 bacterial species
have demonstrated capability to respire
selenate, converting Se(VI) to Se(IV) and
Se(0). Field tests show effective stimulation
of selenate-respiring bacteria under
anaerobic conditions, although co-
contaminants such as nitrate commonly
inhibit selenium reduction. Various
methods for avoiding co-contaminant
inhibition have been integrated in pilot-scale
treatment systems. In the Panoche
Drainage District of northern California,
for example, subsurface drainage is dosed
with algae and injected into settling ponds.
This strategy successfully removes 80%
of the Se through microbial precipitation
but results in biomass accumulation.
The USGS/Duquesne study group
evaluated a range of bacteria with
potential for avoiding biomass
accumulation during Se removal. Test
results indicated that the soil bacterium
Sulfuro spirillum barnesii simultaneously
reduces Se(VI) and respires nitrate.
Washed suspensions of nitrate-grown
cells removed more than 98% of the 50-
|jM Se(VI) as nanospheres of Se(0). The
cells exhibited high affinity for both
nitrate (0.7 |jM) and selenate (21 \\M)
and two separate enzyme pathways.
Results from these studies will be applied
in upcoming USGS field applications.
Site-specific information about other
projects addressing As and Se
contamination are available at http://
water.us gs.gov/nrp/proj.bib/
oremland.html. http://toxics.usgs.gov/
topics/rem_act/saco.html and http://
pubs.usgs.gov/fs/fs-031-03/.
Contributed by John Stolz, Ph.D.,
Duquesne University fstolz&.duq.edu or
412-396-6333), Radisav Vidic, Ph.D.,
University of Pittsburgh
(vidic&.engr.pitt.edu or 412-624-1307),
and Ronald S. Oremland, USGS
(650-329-4482)
—•-- As (Total)
-•--As (III)
--A-- As(V)
0
20
40 60
Time (hours)
80
Figure 3.
Tests using
Pteris cretica
showed that
As(III) must
be oxidized
before it can
by the plant.
100
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Solid Waste and
Emergency Response
(5203P)
EPA 542-N-08-001
January 2008
Issue No. 34
United States
Environmental Protection Agency
National Service Center for Environmental Publications
P.O. Box 42419
Cincinnati, OH 45242
Presorted Standard
Postage and Fees Paid
EPA "
Permit No. G-35
Official Business
Penalty for Private Use $300
EPA Releases Technical Resource for MNA of Inorganics
The U.S. EPA Office of Research and Development, in cooperation with the
Office of Superfund Remediation and Technology Innovation and Office of
Radiation and Indoor Air, recently published a two-volume technical resource
for selection of MNA as a site-specific remedy component for inorganic
contaminants in ground water. Volume 1, Technical Basis for Assessment,
provides an overview of the technical basis for MNA of inorganic contaminants
(EPA/600/R-07/139). Volume 2, Assessment for Non-Radionuclides, addresses
technical aspects of attenuation mechanisms and data collection for arsenic,
cadmium, chromium, copper, lead, nickel, nitrate, perchlorate, and selenium
(EPA/600/R-07/140).
Together, the documents describe a tiered analysis applicable to MNA site
screening through an iterative collection of site-specific data that progressively
reduces MNA uncertainty. This analysis helps cleanup managers develop detailed
information about site hydrogeology, mechanisms and rates of contaminant
attenuation, aquifer capacity to sustain attenuation of contaminant mass, and
long-term stability of immobilized contaminants. The documents also describe
methods for determining attenuation mechanisms by measuring key ground-
water chemical and physical parameters (including reduction/oxidation
characteristics), identifying chemical speciation of contaminants and key
reactants in ground water, and evaluating reactions between contaminants and
solid components within the aquifer. Both volumes of Monitored Natural
Attenuation of Inorganic Contaminants in Ground Water may be downloaded
on CLU-IN at http://www.cluin.org.
Contact Us
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Office of Superfund Remediation
and Technology Innovation
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Phone:703-603-7198
Fax:703-603-9135
EPA is publishing this newsletter as a means of disseminating useful information regarding innovative and alternative treatment techniques and
technologies. The Agency does not endorse specific technology vendors.
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