United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Ada, OK 74820
EPA/600/F-97/008
July 1997
Permeable Reactive Subsurface Barriers for the Interception
and Remediation of Chlorinated Hydrocarbon and
Chromium(VI) Plumes in Ground Water
Office of Research and Development
U.S. EPA REMEDIAL TECHNOLOGY FACT SHEET
Scope of this fact sheet:
This document concerns the use of permeable reactive subsurface barriers for the remediation of plumes of
chlorinated hydrocarbons and Cr(VI) species in ground water, using zero-valent iron (Fe°) as the reactive substrate. Such
systems have undergone thorough laboratory research, pi lot-testing, and are now being installed as full-scale remedial
technologies at field sites. Although research is progressing for other contaminants and different reactive substrates,
these technologies are not as mature and will not be considered in this document.
Chemistry of TCE and chromate remediation by
Fe°:
Chlorinated hydrocarbons such as trichloroethylene (TCE)
have been widely used as commercial solvents and are
commonly found as ground water contaminants. As chlorinated
hydrocarbons contact iron metal, they react at the iron surface.
Figure 1 illustrates the reductive dechlorination of TCE to
ethene and ethane which are easily biodegraded. Electrons are
provided by the corrosion (or oxidation) of the iron metal. Two
competing pathways, sequential hydrogenolysis (A) and
reductive p-elimination (B), each lead to ethene and ethane as
final products. A reactive subsurface barrier is designed to
provide sufficient contaminant residence time for intermediate
products, such as cis-1,2-DCE and vinyl chloride (VC), to fully
degrade to ethene and ethane.
Chromium is also a very common contaminant, typically
having been released to the environment as a result of plating
and other industrial operations. It occurs in the subsurface in
eitherthe Cr(VI) or Cr(lll) valence states. The higher oxidation-
state Cr(VI) forms, e.g. chromate (Figure 2), are far more toxic,
carcinogenic, and mobile in the ground water than the reduced
Cr(l II) species. At typical ground water pH of 6 to 9, Cr(lll)tends
to precipitate from the ground water as chromium hydroxide,
Cr(OH)3 (Figure 2). When iron is present, the Cr(lll) can
precipitate as a mixed chromium-iron hydroxide solid solution,
which has a lower solution equilibrium activity than either pure
solid-phase hydroxide (2). Hence both the toxicity and mobility
of chromium are greatly decreased when it is reduced from
Cr(VI)toCr(lll).
Both TCE and chromate (as well as many other chemicals)
have been shown to be reduced by Fe°, resulting in the
dechlorination of the TCE and the precipitation and immobilization
of Cr. The Fe° donates the electrons necessary to reduce the
contaminants and becomes oxidized to Fe2+ or Fe3+. A variety of
Fe precipitates and otherchemical species can occur, dependant
upon the system geochemistry.
Concept and definition of reactive barriers:
Environmental scientists are generally familiarwith the concept
of barriers for restricting the movement of contaminant plumes
in ground water. Such barriers are typically constructed of
highly impermeable emplacements of materials such as grouts,
slurries, or sheet pilings to form a subsurface wall. The goal of
such constructions is to eliminate the possibility that a
contaminant plume can move toward and endanger sensitive
receptors such as drinking water wells or discharge into surface
waters. Permeable reactive barrier walls reverse this concept of
subsurface barriers. Rather than serving to constrain plume
migration, permeable reactive barriers are designed as
preferential conduits for the contaminated ground water flow.
When the contaminated water passes through the reactive
zone of the barrier, (for example, a zone comprised of granular
iron), the contaminants are either immobilized or chemically
transformed to a more desirable (e.g., less toxic, more readily
biodegradable, etc.) state. A permeable reactive subsurface
barrier can be defined as:
an emplacement of reactive materials in the subsurface
designed to intercept a contaminant plume, provide a
preferential flow path through the reactive media, and
transform the contaminant(s) into environmentally
acceptable forms to attain remediation concentration goals
at points of compliance.
Barrier configurations:
Currently, two basic designs are being used in full-scale
implementations of reactive barriers: (1) the funnel and gate
and (2) the continuous trench. Other designs are being
researched and evaluated.
The design of a funnel and gate system is shown in Figure 3a.
Basically, an impermeable funnel, typically consisting of
interlocking sheet pilings or slurry walls, is emplaced to enclose
and direct the flow of contaminated water to a gate or gates
containing the permeablezone of reactive Fe metal. The design
must prevent the contaminant plume from flowing around the
barrier. Due to directing large amounts of waterthrough a much
smaller cross-sectional area of the aquifer, ground water
velocities within the barrier will be higher than those resulting
from the natural gradient. The continuous trench (Figure 3b) is
simply a trench that has been excavated and simultaneously
backfilled with reactive Fe, allowing the water to pass through
the barrier under its natural gradient.
Both configurations require that information on contaminant
concentration, contaminant degradation rate in the presence of
the reactive substrate, and ground water flow rate through the
barrier be known. This allows determination of the required
residence time in the zone needed to achieve remedial goals,
hence allowing calculation of the required thickness of the
reactive zone.
1
-------
H
x-
Cl
cis-l,2-DCE
H\ /'
c^=c
c,/ Xc,
TCE
2e- B
-2 Cl- 1
H-C=C—C, 2*-+g+ H-
-Cl-
chloroacetylene acetylene
Figure 1. Reductive dechlorination of TCE to ethene and ethane.
Site characterization:
A complete site characterization is of critical importance for
the successful installation of a reactive barrier. The entire plume
must be directed through and remediated within the reactive
zone of the barrier. The plume must not be able to pass over,
under, or around the barrier and the reactive zone must be
capable of reducing the contaminant to concentration goals
without rapidly plugging with precipitates or losing its reactivity.
To achieve the required performance requires knowledge of:
• plume location
• plume direction
• contaminant concentrations
• hydrologic changes with time
• concentration attenuation overtime and distance
• stratigraphic variations in permeability
• confining layers
• fracturing, and
• aqueous geochemistry
Thebarrierdesign, location, emplacement methodology, and
estimated life expectancy are based on the site characterization
information, therefore faulty information could jeopardize the
entire remedial scenario. A complete discussion of site
characterization is beyond the scope of this document, but
guidance documents are in preparation thatwill contain extensive
discussions of characterization requirements.
Compliance and performance monitoring:
Monitoring for regulatory compliance and treatment
performance are both necessary when using reactive barrier
technology. When locating the wells, selecting the screen
lengths, and designing other aspects of the monitoring well
system, the sampling program objectives and site conditions
should be carefully considered.
Compliance monitoring determines whether regulatory
contaminant concentration requirements a re being met. Typically
the compliance monitoring criteria will be set bythe State where
the site is located. Normal compliance monitoring parameters
include:
• the contaminants of interest
• potential contaminant daughter (degradation) products
• general water quality parameters
In general, several monitoring wells should be installed to
determine:
• are regulatory goals being achieved?
• does contaminant breakthrough occur (immediately or
overtime)?
• is the contaminant flowing around the wall?
Typical well locations would include:
• upgradient of the wall
• within the reactive zone of the wall
• immediately downgradient of the reactive zone discharge
• at each end of the wall
• below the wall, and
• above the reactive zone (if possible)
In addition to the contaminants, their products, and the routine
water quality parameters listed above, performance monitoring
of permeable reactive barriers should include:
• hydrologic parameters (baseline and changes overtime)
• precipitates on the iron surfaces (and rate of buildup)
• Eh
• dissolved oxygen, and
• ferrous iron
HO
OH
' S
+ 2
Iron
metal
Figure 2.
Chromate ion
(Chromium (VI)
tctroxidc)
Ferric
hydroxide
V.
Chromium (III)
hydroxide
'OH
Hydroxyl
ions
solid solution
Reduction ofCr(Vl) to Crflll) and precipitation of hydtoxide phases.
2
-------
Figure 3a. Plume capture by a funnel & gate system. Sheet
piling funnels direct the plume through the reactive
gate.
Knowledge of these parameters helps confirm emplacement as
well as address and detect possible:
• loss of reactivity
• decrease in permeability
• decrease in reaction zone residence time
• short circuiting of the reactive zone (i.e., preferential
pathways), and
• funnel wall leakage
Some advantages and disadvantages of reactive
barrier technology:
Advantages-
• actual in situ contaminant remediation, rather than simple
migration control as with impermeable barriers
• passive remediation, no ongoing energy input and limited
maintenance following installation
• no required surface structures other than monitoring wells
following installation
• can remediate plumes even when the source term of the
plume cannot be located
• should not alter the overall ground water flow pattern as
much as high-volume pumping
• contaminantsarenotbroughttothesurface; i.e., no potential
cross-media contamination
• no disposal requirements or disposal costs for treated
wastes
• avoids the mixing of contaminated and uncontaminated
waters that occurs with pumping
Disadvantages-
• currently restricted to shallow plumes, approximately 50
feet or less below ground surface
• plume must be very well characterized and delineated
• limited long-term field testing data is available and field
monitoring is in its infancy
• limited field data concerning longevity of wall reactivity or
loss of permeability due to precipitation
• currently no field-tested applications to remediation of
contaminant source terms
Current applications of reactive barriers to
contaminant plumes:
Permeable reactive subsurface barriers are currently being
used in full-scale field applications for the treatment of plumes
of chlorinated hydrocarbons and chromate. As of this writing, six
full-scale reactive barriers have been installed in the field.
Information on five of these installations is provided in Table 1.
Figure 3b. Plume capture by a continuous trench system. The
plume moves unimpeded through the reactive gate.
Regulatory acceptance of permeable reactive
barriers for subsurface contaminant remediation:
The U.S. Environmental Protection Agency has supported
the development of this innovative in-situ technology through
active collaboration on research involving the National Risk
Management Research Laboratory and the National Exposure
Research Laboratory of U.S. EPA's Office of Research and
Development, through the Remediation Technologies
Development Forum (RTDF) Permeable Barriers Action Team,
and from support provided byU.S. EPA'sTechnology Innovation
Office (TIO). In addition, support has been provided from
several regional offices where sites are testing the technology
at pilot scale. The U.S. EPA recognizes this technology as
having potential to more effectively remediate subsurface
contamination at many types of sites at significant cost savings
compared to other more traditional approaches. The U.S. EPA
is actively involved in the evaluation and monitoring of this new
technology to answer questions regarding long-term system
performance, and in providing guidance to various stakeholder
groups.
As with any remedial technology, adequate site
characterization is necessaryto demonstrate that thetechnology
is suitable for application at a particular site. There are site
characteristics, such as excessive depth to contaminant plume,
fractured rock, etc., which would argue against permeable
reactive barriers as a remedy selection. These situations are
currently topics of research and/or pilot testing. More definitive
information regarding the application of the technology will be
included in a forthcoming EPA Issue Paper.
The Interstate Technology and Regulatory Cooperation (ITRC)
Workgroup (Permeable Barrier Wall Subgroup) is also actively
involved in defining the regulatory implications associated with
the installation of permeable reactive barriers in the subsurface
and in providing guidance on regulatory issues where possible.
Additional sources of information on reactive
barriers:
Remedial Technologies Development Forum, Permeable
Barriers Work Group
http://www. rtdf. org
Ground-Water Remediation Technologies Analyis Center
(GWRTAC)
http://www.gwrtac. org:80/
EnviroMetal Technologies Inc.
http://www. beak. com:80/Technologies/ETI/eti. html
-------
Table 1. Specifications for selected permeable reactive barrier installations.
Site
Installation
Date
Contaminant &
high cone.
Design
Reactive Wail
Type
Funnel
Material
Funnel Length
No. of Gates
Reactive
Material
Reactive Zone
Height
Reactive Zone
Length
Reactive Zone
Thickness
Total Mass of
Reactant
Treatment Wall
Depth
Total System
Length
Special
Features
& Misc.
Cost
Industrial
facility,
Mountain View,
California
Sept. 1995
2 mg L-' cDCE
Excavate & fill
Not Applicable
Not Applicable
Not Applicable
Fe°
5ft
44ft
4.5ft
90 tons
15to20ftbgs
44ft
HOPE atop Fe
to surface
upgradient
directs H2O
through Fe
No Information
Industrial
facility, Belfast,
Northern Ireland
Dec. 1995
300 mg L1
TCE
Reaction Vessel
Slurry Walls
100ft + 100ft
1 Reaction
vessel
Fe°
18 ft in vessel
NA
16 ft in vessel
15 tons
18to40ftbgs
Appmx. 200 ft
Walls direct
H2O to vessel
inlet, gravity
flow to outlet
downgradient
$375 K
Industrial
facility,
Coffeyville,
Kansas
Jan. 1996
400 isg L' TCE
Funnel & Gate
Soil-Bentonite
Slurry
490 ft + 490 ft
1
Fe°
11 ft
20ft
3ft
70 tons
17to28ftbgs
1000ft
$400 K
USCG facility,
Elizabeth City,
North Carolina
June 1995
Wmg L-< TCE
WmgL1 Cr(VI)
Continuous
Trench
Not Applicable
Not Applicable
Not Applicable
Fe°
Appmx. 23 ft
150ft
2ft
450 tons
3to26ftbgs
150ft
Two contam.
treated. Chain
trencher with
immediate Fe
placement
$500 K
Government facility,
Lakewood,
Colorado
Oct. 1996
700 fjg L'
each TCE & DCE
15 ug L1 VC
Funnel &
Multiple Gate
Scalable Joint
Sheet Pilings
1040 ft total
4
Fe°
10-1 5 ft
40 ft each
(4x40 = 160)
Gates differed,
low = 2 ft
high = 6 ft
No Information
10-1 5 to
20-25 ft bgs
1200ft
Largest of its
kind. Gates
installed using
sheet pile box.
No Information
For more information, contact:
Dr. Robert W. Puls (RTDF co-chair)
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
P.O. Box 1198
Ada, OK 74820
Tel: (405) 436-8543
Email: puls@epamail.epa.gov
Authors:
Robert W. Puls, Subsurface Protection and
Remediation Division, NRMRL, ORD, U.S. EPA
Robert M. Powell, Powell & Associates Science
Services, Las Vegas, NV (under subcontract to
Man "Tec/7 Environmental Research Services Corp.)
References for the chemical reactions and
mechanisms:
(1) Roberts, L A.; Totten, L A.; Arnold, W. A.; Burris, D. R.;
Campbell, T. J. Environmental Science & Technology 1996,
30, 2654-2659.
(2) Powell, R. M.; Puls, R. W.; Hightower, S. K.; Sabatini, D. A.
Environmental Science & Technology 1995, 29, 1913-
1922.
Notice: The U.S. Environmental Protection Agency through
its Office of Research and Development funded the research
described here. It has been subjected to the Agency's peer and
administrative review and has been approved as an EPA docu-
ment. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
------- |