United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(5102G)
EPA542-N-01-008""'
December 2001
Issue No. 42
Ground Water Currents
^6ter Treatment
CONTENTS
DUS Expedites Ground-
Water Cleanup at
Savannah River Site Pg. 1
Sodium Dithionite
Injections Used for
Chromium Reduction Pg. 2
Cross-Hole Radar
Method Monitors
PRB Installation Pg. 3
About this Issue
This issue highlights a range
of methods for field-testing,
implementing, and monitoring
innovative technologies for
removing organic compounds
and metals from ground
water.
i.S. Enviionmenta! Protection Agency
i«*ion 5. Library (PL-12J)
West Jackson Boulevard, 12th
K, 606C4-3590
DUS Expedites Ground-Water
Cleanup at Savannah River
Site
by Michelle Ewart, U.S.
Department of Energy, and James
Kupar, Bechtel Savannah River,
Inc.
In September 2000, the U.S. Department
of Energy (DOE) began operating a
dynamic underground stripping (DUS)
system to remove volatile organic
compounds (VOCs) at DOE's Savannah
River Site (SRS). At its peak operation,
dense non-aqueous phase liquid
(DNAPL) was removed at a rate of 1,200
pounds each day. It is estimated that
DUS extracted material 15 times faster
than the original soil vapor extraction
methods and 75 times faster than the
pump and treat configuration already
operating within the SRS "M-Area."
The M-Area includes an accumulation of
several unknown releases over the past
40 years from above-ground tanks used
to store solvents. Within this 10,000
square-foot area. DNAPL containing
tetrachloroethylene (PCE) and trichloro-
ethylene (TCE) extends to a depth of 165
feet. Typical of the Atlantic coastal plain,
the geology in this area consists of clay-
rich confining intervals within more
transmissive, sandier intervals. The local
aquifer is located at a depth of 130 feet.
The DUS process used at the M-Area
involved a series of steam injections
followed by a combined air/steam
injection, i.e., hydrous pyrolysis oxida-
tion (HPO). Using electrical resistance
tomography in combination with thermo-
couples, the system's heating fronts were
imaged to monitor progress and schedule
the frequencies of injections. DUS does
not require the precise identification of
contaminant location.
The 100- by 100-foot treatment zone at
the M-Area extended from a depth of 20
feet below ground surface downward
through 110 feet of vadose zone into the
water table until reaching the top of the
"green clay" confining layer at a depth
of 165 feet. Continuous injections
applied steam at a temperature of 230 C
and maximum rate of 20,000 pounds per
hour. Injection wellhead pressures
ranged from 40 to 70 pounds per square
inch, depending on the lithology of
screen intervals, depths, and operational
specifications of the system. Two
months after initiation of the steaming
and extraction process, steam break-
through occurred at the central
extraction well. The HPO process began
one month later, and the applied boiling
point (90 C) was reached throughout the
soil block after another three months. At
that point, a total of 45 x 10'' British
thermal units of steam had been injected.
Over the 12 months of active DUS
operation, a total of approximately
70,000 pounds of PCE and TCE was
removed (Figure 1).
At the peak of operation, vapors from the
system's extraction wells reached a level
of 6,600 parts per million volume (ppmv).
A total of 3 billion standard cubic feet of
air at temperatures approaching 200 F
was extracted and cooled over the course
of operations. The resulting condensatc
was directed at a rate of 20 gallons per
minute to a surface-level separator where
the DNAPL (totaling less than five
gallons) was removed from water and
dispositioned at an onsite waste facility.
Almost 160 million standard cubic feet of
PCE- and TCE-bearing air was extracted
as noncondensable flow and discharged
through the attached soil vacuum
extraction unit.
[continued on page 2]
Recycled/Recyclable
Printed with Soy'Canola Ink on paper that
contains at least 50% recycled fiber
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Figure 1. Total VOC Mass Removal
S 40000
Sep Oct
2000 2OOO
Nov Dec
2000 2000
Jan
2001
Feb Mar
2001 2001
Date
Apr
2001
May
2001
Jun
2001
Jul Aug
2001 2001
Sep
2001
[continuedfrom page 1]
Factors found to affect the success of
DUS application at the M-Area included
the: (1) different injection methods
required for varying lithologies, (2) site-
specific criteria used to determine
optimal shut-down points and processes,
and (3) inherent difficulties associated
with precise measurement of HPO
efficiency. Most significantly, product
removal rates were found to be limited by
the maximum temperature that could be
achieved with this closed DUS system.
Following a cool-down period of six
months, the system will be further
examined to measure the completeness of
remediation in the target zone. Total
costs for this project aie estimated at S4.8
million.
DUS operations at the M-Area were
designed and conducted by integrated
Water Resources. Inc. (based in Santa
Barbara, CA) in cooperation with IT
Corporation. In addition, technical
assistance for the project was provided by
Lawrence Livermore National Laboratory
through the DOE's Subsurface Contami-
nants Focus Area (SCFA) Lead Laboratory
Support Program.
A final project report will be available in
early 2002 on the SCFA Web site,
www.envnet.org/scfa. For more informa-
tion, contact Michelle Ewart (DOE) at
803-725-1115 or michelle.ewart^
srs.gov. or Jim Kupar (Bechtel Savannah
River, Inc.) at 803-952-6525 or
james.kupar@srs gov.
Sodium Dithionite Injections
Used for Chromium
Reduction
by Cynthia J. Paul, U.S. EPA/
Office of Research and
Development/National Risk
Management Research Laboratory
A field-scale pilot study was conducted
in 1999 at the U.S. Coast Guard Support
Center in Elizabeth City, NC, to evaluate
the effectiveness of injecting sodium
dithionite into the upper aquifer and
lower vadose zone to create a permeable
reactive barrier (PRB) system utilizing
naturally occurring iron for hexavalent
chromium (Cr) (VI) reduction. Within
three days of chemical injection, Cr(VI)
concentrations dropped to below the
target cleanup levels. Based on these
results, a full-scale treatment system
employing sodium dithionite injections
was implemented earlier this year.
Past operations ol a chrome-plating shop
at the Support Center created a contami-
nant plume with mixed organic and
metal compounds, particularly Cr(VI)
Due to indications that chromium in the
soils would remain immobile and present
no potential risk, initial cleanup efforts
focused on treating the diffuse chromium
plume. A PRB using zero-valent iron was
installed at the site in 1996 and has since
proved effective for remediating the
mobile chromium plume.
A water-main break in the immediate
vicinity of the plating shop during 1994,
however, caused an artificial rise in the
water table directly beneath the shop. As a
result, soluble Cr(VI) in concentrations
reaching 28.0 mg/L had migrated into the
ground water. Although concentrations
decreased to approximately 4.4 mg/L by
1997, additional treatment was required to
meet the maximum contaminant level of
0.1 mg/L for ground water. The U.S. Coast
Guard estimated that additional contami-
nant release into the mobile plume being
treated by the PRB would result in the
potential need for replacement of the PRB
in approximately 10 years. Based on these
findings, immediate treatment of the
contaminant source was found more cost
effective than eventual replacement of
the PRB.
Prior to the source control pilot study, steps
were taken to re-characterize the site
through the collection of soil cores from 32
locations within the plating shop. Prelimi-
nary screening was conducted using x-ray
fluorescence and inductively coupled
plasma spectroscopy to determine total
chromium levels. Laboratory batch
extractions were conducted to selectively
remove Cr(VI) from contaminated soils in
order to delineate the extent of the soluble,
easily mobilized Cr(VI). Results indicated
that the contaminant plume had migrated
vertically through a series of sand and silt/
clay loam layers into the ground water at 2
meters below ground surface and horizon-
tally toward the Pasquotank River, which is
located approximately 60 meters from the
source area. These conditions provided a
continual release of Cr(VI) into the mobile
contaminant plume treated by the PRB.
Additional laboratory studies were con-
ducted to better understand the potential
for in situ reduction/oxidation (redox)
manipulation (ISRM) and to evaluate
sodium dithionite (Na,S,O4), /-ascorbic-
acid (C^Hj.C^). and free hydroxylamme (FH-
5()'M) as potential reductants. Results
showed sodium dithionite to be the most
effective al reducing chiomium with the
least adverse side effects. Injecting sodium
dithionite into areas with high levels of
[continued on page 3]
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[continuedfrom page 2]
naturally occurring iron (Fe) was found to
create a spatially-fixed reducing zone
analagous to a chemical PRB. Through
such injection, Fe(III) is reduced to Fe(ll)
and redox-sensitive contaminants such as
Cr(VI) are immobilized or precipitated as
they migrate through the reducing zone.
In the field pilot, a solution of sodium
dithionite (17 kg), potassium bicarbonate
(19 kg), and bromide tracer (5.63 kg) in
1,874 liters of distilled and deionized water
was injected into ground water at depths
ranging from 1.2 to 2.4 meters below ground
surface (Figure 2). The reductant was
injected continuously for 38 hours at 1 liter/
minute. To evaluate the use of ISRM in the
capillary fringe areas, test data were
collected from the 1-meter treatment area
immediately surrounding the injection well.
Field results indicated that Cr(VI) concen-
trations dropped from pre-treatment levels
of 2.2-5.0 mg/L to non-detect levels within
60 hours of injection and remained at
virtually the same level throughout the
remaining course of the pilot project's 48-
week monitoring period. Data showed that
byproducts of the redox process (sulfatc
and dissolved iron) significantly increased
during the initial 60-hour period but
decreased to original levels approximately
32 weeks after the injection phase. Simi-
larly, other process indicators such as pH
and specific conductance initially rose but
returned to or near original levels within
approximately 48 weeks, which is typical
of the redox process. Pumping and
treatment of the injected material was not
required because sodium dithionite is
known to produce no adverse side effects
or byproducts.
Costs for the pilot project totaled approxi-
mately $750,000, including expenses for
soil excavation in the upper area of the
treatment zone. Comparative analysis
estimated that the addition of chemical
injections for source control at the
Support Center site helped to reduce the
overall project (including mobile plume
treatment through the PRB) costs by 40
percent and the cleanup time by 28 years.
In comparison to conventional pump and
treat methods, analysis estimated that this
combined approach for source control and
plume treatment achieved nearly an 80
percent reduction in costs. More informa-
tion on the pilot study is available on the
Figure 2. Sodium Dithionite Injection Field Pilot Schematic
Internet at www.epa.gov/ORD/NRMRL or
from Cynthia Paul (National Risk
Management Research Laboratory) at
580-436-8556 or e-mail
paul.cindy@epa.gov.
Cross-Hole Radar Method
Monitors PRB Installation
by Jennifer G. Savoie, John W.
Lane, Jr., and Peter K. Joes ten,
U.S. Geological Survey
The U.S. Geological Survey (USGS). in
cooperation with the U.S. Air Force Center
foi Environmental Excellence (AFCEE),
conducted a study using cross-hole,
common-depth radai scanning to deter-
mine the lateral and vertical extent of
permeable reactive barriers (PRBs)
installed near the "Chemical Spill-10"
(CS-10) source area at the Massachusetts
Military Reservation (MMR) on Cape
Cod, MA The study also enabled
researchers to determine whether any large
holes in the barriers existed that could
allow contaminated ground water to pass
untreated through the remediation zone.
Based on the survey lesulls, researchers
were able to determine that the PRBs were
installed in the planned locations.
Ground water beneath and downszradicnt
from the CS-10 source area has been
affected by contaminants such as chlori-
nated solvents during decades of past
military operations. These contaminants
entered the ground-water system and
continue to move with the ground-water
flow in the underlying sand and gravel
aquifer toward nearby streams, ponds, and
coastal bays. Depth to ground water at
the CS-10 source area is about 80 feet
below ground surface (bgs). and the
ground-water How rate is estimated at 0.5-
1 foot per day. The contaminant of
greatest concern is tetraehloroethene,
with concentrations at the source area of
approximately 250 micrograms per liter
The University of Waterloo in Ontario.
Canada, conducted a field trial invoking
the installation of two demonslratiou-
width, lull depth, granular-iron walls in
the path of the giound-waler plume. The
depth of the plume (80 120 feet bgs)
exceeded practical limits tor conven-
tional emplacement methods such as
trenching To address this limitation,
hydraulic fracturing and injection
methods \\ere used to install the walls.
In the field, radai surveys were conducted
before and after the iron wall installa-
tions Cross-hole radar scans were
conducted in 20 boreholes on opposite
sides of the walls using a radar system
containing electric-dipole antennas with
center frequencies of 100 and 250
/continued on page 4\
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[continued from page 3]
Megahertz. Radar-scan measurements of
the regions between pairs of boreholes
were acquired by moving a radar transmit-
ter and receiver in unison at
10-centimeter increments from the tops of
casings to the bottoms of the boreholes.
As anticipated, the amplitudes of radar
waves crossing the iron zones decreased
significantly between the pre- and
post-installation surveys, indicating the
presence of iron.
The results of two-dimensional,
finite-difference/time-domain numerical
modeling based on laboratory-scale
physical models were used to interpret
changes in radar amplitude observed in
the field. The numerical and physical
models simulated a wall of perfectly
conductive material embedded in
saturated sand that was representative of
the MMR field conditions. Modeling
results indicated that the amplitude of a
radar pulse transmitted across the edge of
the conductive wall was about 43 percent
of one transmitted through background
material.
Figure 3 shows the transverse sections for
the locations of the two iron walls based
on radar scanning results. These data
indicate that the "A-Wall" contains two
zones: the upper zone is about 33 feet
wide, extending 82-102 feet bgs; the
lower zone is about 26 feel wide, extend-
ing 103-113 feet bgs. The "B-Wall" is
interpreted as a 30 foot-wide continuous
zone extending about 89-113 feet bgs. It
was determined that no holes existed
within the boundaries of either iron wall.
The USGS has since employed this radar
method at other sites, including the USGS
Fractured Rock Hydrology Test Site at
Mirror Lake, NH, to locate and character-
ize bedrock fractures and lithologic
changes and to monitor tracer tests. For
more information on the use of borehole
radar technology at the MMR site, visit
the USGS Web site at http://
water.usgs.gov/ogw/bgas/publications/
wri004145/index.html or contact John W.
Lane, Jr, (USGS) atjwlane@usgs.gov.
Information on the MMR PRB field trial
is available from Rose Forbes (AFCEE) at
rose.forbes@ mmr.brooks.af.mil or David
W. Hubble (University of Waterloo) at
dwhubble® uwaterloo.ca.
Figure 3. Transverse Sections of
A-Wall and B-Wall
West
A-Wall
^w^pj^Ww^p.
B-Wali
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December 2001
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77 y ..i,'>,?rjN BUD Fl 9
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