EPA/600/A-97/002
"PREPRINT EXTENDED ABSTRACT"
Presented Before the Division of Environmental Chemistry
American Chemical Society
San Francisco, CA April 13-17, 1997
REMEDIATION OF CHROMATE-CONTAMINATED GROUND WATER USING AN
IN-SITU PERMEABLE REACTIVE MIXTURE: FIELD PILOT TEST.
ELIZABETH CITY, NORTH CAROLINA
R.W. Puls, C.J. Paul, and P.J. Clark*
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
P.O. Box 1198, Ada, Oklahoma
'National Risk Management Research Laboratory
U.S. Environmental Protection Agency
26 Martin Luther King Drive
Cincinnati, OH 45268
Introduction
A pilot-scale field test was conducted near an old chrome plating facility on the U.S. Coast
Guard (USCG) Support Center near Elizabeth City, North Carolina to evaluate the in situ
remediation of ground water contaminated by hexavalent chromium using a permeable reactive
barrier technology. The reactive media consisted of two types of zero-valent iron mixed with
natural aquifer material and coarse sand. The remedial effectiveness of this innovative in situ
technology was monitored over a twenty month period following emplacement in September
1994. In addition to monitoring the removal of contaminants from the aqueous phase,
performance monitoring included attempts at mass balance calculations, changes in aqueous and
solid phase geochemistry and changes in permeability. The success of this small-scale test
prompted a full-scale implementation of the technology in late June, 1996.
The field test had multiple objectives; these were: (a) to evaluate whether zero-valent iron
could remediate, in situ, ground water contaminated with chromate using a permeable reactive
barrier system; (b) determine if the electrochemical corrosion mechanism hypothesis formulated
in prior laboratory studies was consistent with field observations and results; © evaluate which
geochemical parameters might best indicate effective system performance; and (d) identify
"new" mineral phases formed as a result of barrier emplacement which might affect system
performance and longevity.
Field Site
The field site is located at the USCG Support Center near Elizabeth City, North Carolina,
about 100 km south of Norfolk, Virginia and 60 km inland from the Outer Banks region of North
Carolina. The base is located on the southern bank of the Pasquotank River, about 5 km
southeast of Elizabeth City. Hangar 79, which is only 60 m south of the river, contains a chrome
plating shop which had been in use for more than 30 years and discharged acidic chromium
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wastes and associated organic solvents through a hole in the concrete floor. These wastes
infiltrated the soils and the underlying aquifer immediately below the shop's foundation resulting
in percent levels of chromium (Puls et al, 1994a) in the surface soils.
The site geology has been described in detail elsewhere (Puls et al. 1994a,b), but essentially
consists of typical Atlantic coastal plain sediments, characterized by complex and variable
sequences of surficial sands, silts and clays. Ground-water flow velocity is extremely variable
with depth, with a highly conductive layer at roughly 4.5 to 6.5 meters below ground surface,
This layer coincides with the highest aqueous concentrations of chromate and chlorinated organic
compounds. The ground-water table ranges from 1.5 to 2,0 m below ground surface.
Details of the installation and initial monitoring results were reported at the Spring, 1995
ACS Meeting in Anaheim, California (Puls et al. 1995) and elsewhere (Puls et al. 1996). In this
paper we present performance monitoring results for more than one year following installation.
Performance Monitoring
Hydrologic data in support of the small-scale field test was collected beyond that data
generated from traditional slug testing during initial site characterization research. Hydraulic
conductivity data was collected from laboratory constant head permeameter tests using sediments
collected immediately adjacent to the "fence" area. Hydraulic conductivity was also estimated
"in situ" using pore dissipation techniques with a Hydrocone® water sampler and cone
penetrometer. This device is able to track the filling rate of the water sampler which is
hydraulically pushed to the desired depth to estimate the sediment's horizontal permeability at
that location. The time rate of filling, head pressures, and argon back pressure data permits a
computer to apply these data points to standard rate of rise slug test models to determine
permeability. These measurements were made over 30 cm vertical intervals. Tracer tests to
evaluate ground water flow velocity through the test area were performed before (June 1994) and
following (April 1995) "fence" installation. Bromide was used as a conservative tracer.
The test area was cored and sediment samples collected using 2.5 and 5 cm i.d. core
barrels before iron emplacement, and 5, 8, and 20 months following emplacement, to examine
the iron and aquifer sediment surfaces. Sediment grain samples were also isolated from water
pumped from selected monitoring wells downgradient of the iron mixture. Electron microprobe
(JEOL 733) and scanning electron microscopy with energy dispersive X-ray (Hitachi S-570)
were used for surface analyses of sediment samples.
Site mapping of contaminant distributions in three dimensions was accomplished with
cone penetrometer, Geoprobe®, Hydropunch®, and traditional monitoring wells resulting in
more than 300 discrete sampling points. Seven multi-level well clusters, and ten single-point
monitoring wells were used to assess contaminant removal within a 5.5 m treatment area to a
depth of 8 m.
Changes in geochemistry were also periodically monitored. With the disappearance of
chromate there is a concomitant appearance of ferrous iron, a decrease in Eh, decrease in
dissolved oxygen and increase in pH. These geochemieal changes are identical to prior
laboratory observations by Powell et al. (1995a) and are consistent with the following reactions:
Fe° + 2H20 * Fe2 + H2 + 2OH'
Fe° + CrO42'+ 4H2O v(Fes,Cr,.x)(OH)3+ 5OH-
A range of decreasing redox zones were reflected in the data from within the iron mixture
to locations downgradient of the iron mixture. Much of this data was collected using temporary
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sampling points in addition to the permanent wells. Wells located within or immediately
downgradient of the iron mixture show reduction of chromate to below detection limits. In these
treated zones, ferrous iron is present in excess of 2 mg/L, Eh is reduced to below 200 mv, pH
increased to more than 7.5, dissolved oxygen is consumed, and sulfides are detected. The most
sensitive geochemical indicators for corrosion-promoted reduction of chromate were dissolved
oxygen (decreased to < 0.1 mg/L), ferrous iron production (to > 2 mg/L), and Eh decreases (<
200 mv). System pH changes lagged considerably due to the aquifer sediment buffering
capacity. Twenty-five percent by volume of the iron mixture was natural aquifer sediments.
Laboratory experiments indicated that the aquifer sediment was capable of pH-buffering through
dissolution of aluminosilicate matrix minerals which served to maintain the corrosion process
and enhance chromium removal efficiency (Powell et al. 1995a,b).
Iron sulfides have been detected primarily as coatings on mineral surfaces in samples
from several wells and on recovered cores from these same locations. The formation of an iron
hydroxide cement around native mineral grains is evident in cores recovered after 20 months
following iron emplacement. The formation of these cementatious coatings raises concerns
about system longevity with regards to the maintenance of sufficient permeability within the iron
reaction zone. Using SEM-EDS analysis, chromium has been detected on surfaces of iron
obtained from Ada Iron & Metal (AI&M). Peak heights correspond roughly with the estimated
mass of chromate which has moved through the reaction zones over the period of testing (20
months). No chromium has been detected on iron obtained from Master Builders Supply (MBS)
using these same techniques. Passive sampling techniques are also being employed to evaluate
the potential mobilization of colloidal constituents downgradient of the iron mixture.
Appreciable quantities of colloidal particles were observed during the March, May, and June,
1995 sampling trips. The most ubiquitous particles detected include silica and kaolinite and
range in size from 0.1 to several microns.
Summary
An in situ field test using elemental iron as a reactive substrate mixed with coarse sand
and native aquifer materials was installed by researchers of the National Risk Management
Research Laboratory (NRMRL) to monitor, evaluate and test the efficacy of this passive in situ
treatment technology to remediate ground water contaminated with chromium. The field test
began in September 1994 and continued to April 1996. Results indicate that complete treatment
of chromium in the ground water at this site is possible using this technology. Chromium
concentrations have been reduced to less than 0.01 mg/L, much less than drinking water limits.
Significant reductions in TCE were also achieved, although the test was not designed to
remediate the chlorinated organic compounds present at the site. The chromate is reduced via
corrosion of the elemental iron to the nontoxic and insoluble chromic ion (Cr3t) which
presumably forms an insoluble mixed chromium-iron hydroxide phase. Most iron
oxyhydroxides appear as coatings on mineral grains. As a result of this field test by NRMRL
researchers, a full-scale demonstration of the technology was begun in June, 1996. This
demonstration consists of a 50 m long, 8 m deep and 0.6 m wide trench of elemental iron
installed to intercept the entire mixed waste plume (chromate, TCE, cis-dichloroethylene, vinyl
chloride) downgradient of the old chrome plating shop at the site. This full-scale demonstration
is a cooperative effort among researchers at NRMRL, the University of Waterloo, and the U.S.
Coast Guard.
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References
Powell, R. M., Puls, R. W., Hightower, S. K., and D. A. Sabatini. 1995a. "Coupled Iron
Corrosion and Chromate Reduction: Mechanisms for Subsurface Remediation". Environmental
Science and Technology, 29(8):1913-1922.
Powell, R.M., R.W. Puls, S.K. Hightower, and D.A. Clark. 1995b. Corrosive and Geochemical
Mechanisms Influencing In-Situ Reduction of Chromate by Metallic Iron. American Chemical
Society, Division of Environmental Chemistry, Anaheim, CA, April 2-7, 1995.
Puls, R.W., D.A. Clark, C.J. Paul and J. Vardy. 1994a. Transport and Transformation of
Hexavalent Chromium Through Soils and Into Ground Water. J. Soil Conlam. 3(2):203-224.
Puls, R.W., C.J. Paul, D. Clark, J. Vardy, and C. Carlson. 1994b. Characterization of
Chromium-Contaminated Soils Using Field-Portable X-Ray Fluorescence. Ground Water
Monitoring and Remediation, 14(3): 111-115.
Puls, R.W., R.M. Powell, and C.J. Paul. 1995. In-Situ Remediation of Ground Water Contaminated
with Chromate and Chlorinated Solvents Using Zero-Valent Iron: A Field Study. American
Chemical Society, Division of Environmental Chemistry, Anaheim, CA, April 2-7, 1995.
Puls, R.W., C.J. Paul, and R.M. Powell. 1996. In Situ Immobilization and Detoxification of
Chromate-Contaminated Ground Water Using Zero-Valent Iron: Field Experiments at the USCG
Support Center, Elizabeth City, North Carolina. Fourth Annual Great Lakes Geotechnical
and Geoenvirommental Conference In-Situ Remediation of Contaminated Sites, University
of Illinois at Chicago, May 17, 1996.
Disclaimer
This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.
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TECHNICAL REPORT DATA
1. REPORT NO.
EPA/600/A-97/OQ2
3.
4. TITLE AND SUBTITLE
Remediation of Chromate-Contaminated Ground Water using an In-Situ
Permeable Reactive Mixture: Field Pilot Test. Elizabeth City, North
Carolina
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
R. W. Puls1, C. J. Paul1, P. J. Clark2
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
'USEPA 2USEPA
NRMRL/SPRD-ADA NRMRL
P.O. Box 1198 26 Martin Luther King Drive
Ada, OK 74820 Cincinnati OH 45268
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
In-House RSRP3
12. SPONSORING AGENCY NAME AND ADDRESS
USEPA
National Risk Management Research Laboratory
Subsurface Protection and Remediation Division
P. 0. Box 1198
Ada, OK 74820
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/15
15. SUPPLEMENTARY NOTES
April 13-17, 1997.
To be Published in proceedings at American Chemical Society, San Francisco, CA.
16. ABSTRACT A pilot-scale field test was conducted near an old chrome plating facility on the U. S. Coast
Guard (USCG) Support Center near Elizabeth City, North Carolina to evaluate the in-situ remediation of ground
water contaminated by hexavalent chromium using permeable reactive barrier technology. The reactive media
consisted of two types of zero-valent iron mixed with natural aquifer material and coarse sand. The remedial
effectiveness of this innovative in situ technology was monitored over a twenty month period following
emplacement in September 1994. In addition to monitoring the removal of contaminants from the aqueous phase,
performance monitoring included attempts at mass balance circulations, changes in aqueous and solid phase
geochemistry and changes in permeability. The success of this small-scale test prompted a full-scale
implementation of the technology in late June, 1996.
17.
KEY WORDS AND DOCUMENT ANALYSIS
A.
DESCRIPTORS
B. IDENTIFIERS/OPEN ENDED TERMS
C. COSATI FIELD, GROUP
Permeable Reactive Barriers
Chromate
Zero-Valent Iron
Ground Water
Performance Monitoring
18. DISTRIBUTION STATEMENT
To be Released to the Public
19. SECURITY CLASS(THIS REPORT)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS(THIS PAGE)
Unclassified
22. PRICE
EPA FORM 2220-1 (REV.4-77)
PREVIOUS EDITION IS OBSOLETE
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