EPA/600/A-97/029
NGWA Workshop on Permeable Reactive Barriers in Ground Water
Workshop Organizer: Robert W. Puis, Ph.D., National Risk Management
Research Laboratory, USEPA
Introduction:
Information is required for ground water professionals to better understand the application of in
situ permeable reactive barriers for the remediation of contaminated ground water. This
workshop will describe this innovative technology, provide examples of where it has been
successfully applied, discuss site characterization needs for successful implementation, reactive
materials selection, the conduct of pre-installation feasibility tests, methods of emplacement
(already demonstrated and others currently being researched), and acceptable approaches for
compliance and performance monitoring of emplaced systems.
Workshop Sessions:
I. Permeable Reactive Barrier Design Considerations
David W. Blowes, Ph.D., University of Waterloo, Waterloo, Ontario, Canada
n. Site Characterization
Robert M. Powell, Powell & Associates Science Services
For ManTech Environmental Research Services Corp.
EI. Emplacement Methods For Optimized Design & Cost Effectiveness
Dale S. Schultz, Ph.D., Dupont Central Research & Development
IV.	Compliance and Performance Monitoring
Robert W. Puis, Ph.D., USEPA National Risk Management Research Laboratory
Ada, OK.
V.	Installed Treatment Wall Demonstrations and Applications
John Vogan, Envirometal Technologies Inc., Guelph, Ontario, Canada

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I. Permeable Reactive Barrier Design Considerations
David W. Blowes, Ph.D., University of Waterloo, Waterloo, Ontario, Canada
Introduction:
Permeable reaction walls are an emerging technology for the treatment of contaminated
groundwater. Successful treatment of contaminated groundwater using this technique requires
that the contaminant be rendered innocuous or immobile during transport through the in-situ
treatment zone. The extent of treatment, and the success of the permeable barrier system
depends on the nature of the contaminant, the selection of the reactive material, the physical
design of the treatment system, and site conditions.
Selection Of The Reactive Material
The selection of the reactive material for use in the permeable reactive barrier is
dependent on the type, concentration and mobility of the contaminant to be treated. Much of the
reactive barrier research conducted to date has focused on the use of zero metals, principally iron
to treat halogenated hydrocarbons and dissolved electroactive metals. Elemental iron (Fe°) has
been demonstrated to accelerate the dechlorination of numerous halogenated hydrocarbons,
including the common groundwater contaminants trichloroethylene (TCE), dichloroethylene
(DCE) and vinyl chloride (VC). Fe° has also been demonstrated to reduce electroactive metals,
including hexavalent chromium (Cr(VI), selenium (Se(VI), and uranium (U(VI), resulting in the
formation of sparingly soluble precipitates that effectively remove these metals from the flowing
groundwater.
Although Fe° has been studied extensively, other reactive materialsf, including bimetal
combinations, organic carbon, ferric oxyhydroxides, and surfactant enhanced zeolites and clays
have been used or proposed for use in reactive barriers. Bimetal combinations, such as iron-
palladium mixtures, iron-copper mixtures, and iron-nickel mixtures have been proposed for use
in reactive walls, and have been tested in the laboratory, and in small-scale field trials. The
advantages of these bimetal combinations include more rapid reaction kinetics, and the potential
for treatment using smaller volumes of reactive materials. These advantages become most
significant when dealing with plumes containing high concentrations of contaminants, very deep
plumes, or installation systems which provide limited treatment media volumes. Organic carbon
has been used as a reactive material for the treatment of dissolved metals, acid mine drainage,
and nitrate. In all of these cases, the organic carbon serves as a substrate for facultative bacteria,
which catalyze reduction reactions, resulting in the removal of the contaminant from solution.
Organic carbon is inexpensive, and readily available at most locations. In addition to reactive
barriers that result in a change in the chemical nature of the contaminant, barriers that result in
the adsorption or retardation of contaminants have also been studied. Potential adsorbents
include iron oxyhydroxide phases, added as solid precipitates or as a slurry of colloidal material,
and surfactant enhanced zeolites and clays, which can be engineered to remove metals, anions, or
organic compounds.

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Installation Technique:
Two basic designs have been proposed for the installation of reactive barriers, continuous
walls, which intercept the entire plume width with reactive material, and Funnel and Gate™
systems, which use impermeable barriers to direct contaminated water into a smaller treatment
zone containing reactive materials. The selection of the installation technique is highly
dependent on the site characteristic, and, to a lesser degree on the cost and availability of the
reactive material. Continuous wall systems tend to spread the reactive material over a greater
cross sectional area, and tend to result in little disruption of natural groundwater flow patterns.
Continuous barriers, therefore, may be preferable at sites where walls are installed within an
aquifer, with no underlying impermeable layers. At these sites minimizing the disturbance to the
groundwater flow system may provide greater protection against directing groundwater flow
beneath the reactive barrier, and the reactive materials. Funnel and Gate ™ configuration. Sites
where plumes are contained in aquifers underlain by low permeability fine grained materials, or
bed rock are particularly well suited to the use of Funnel and Gate™ systems, because the
potential for underflow beneath the wall is minimized. Funnel and Gate™ systems are well
suited to remedial techniques that require the use of expensive reactive materials. In these cases,
the costs associated with the installation of the impermeable portion of the barrier is offset by
decreased costs for reactive materials. At sites where inexpensive reactive materials will be used
(e.g. organic carbon), the costs associated with the installation of the impermeable Funnel may
not be warranted.
Site Specific Conditions:
The successful installation of a reactive barrier will rely on an understanding and
incorporation of site specific conditions into the design of the reactive barrier. Site conditions
that may affect barrier performance include the rate, direction and variability of groundwater
flow, the concentration, and the variability of concentration of the contaminant, and the
geochemical nature of the groundwater. The rate and variability of groundwater flow is a critical
design parameter that must be determined by field investigations. The effect of the observed
variability can be assessed through computer modeling techniques. Variations in the
contaminant concentrations also should be assessed through site investigations, and the
robustness of the treatment media assessed through treatability studies and computer simulations
which incorporate this information. The composition of the site groundwater may provide the
potential for enhanced groundwater treatment through the provision of reactive solutes, or in the
case of biologically active walls, essential nutrients. High concentrations of dissolved
constituents within site ground waters may enhance the potential for clogging of the reactive
material or blinding to reactive surfaces.
II. Site Characterization
Robert M. Powell, Powell & Associates Science Services
ManTeeh Environmental Research Services Corp.
A complete site characterization is of critical importance for the installation of a reactive

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barrier. The plume location and extent ground water flow direction, and contaminant
concentrations must be accurately known to achieve the required performance. In addition,
information on stratigraphic variations in permeability, fracturing, and aqueous geochemistry is
needed. The plume must not pass over, under, or around the barrier and the reactive zone must
reduce the contaminant to concentration goals without rapidly plugging with precipitates or
becoming passivated. The barrier design, location, emplacement methodology, and estimated
life expectancy are based on the site characterization information; therefore, faulty information
could jeopardize the entire remedial scenario.
As with any ground water remediation technique, adequate site characterization must be
done to understand the ground water flow patterns and the distribution of the contaminant plume.
This is especially important for the installation of a permeable reactive subsurface barrier since
the entire plume must be directed through the reactive zone of the barrier. To attain a "passive"
remediation system, the barrier must be placed in a location that allows the plume to move
through the reactive zone using the natural ground water gradient. It must also be designed to
eliminate the possibility that portions of the plume could flow around the barrier in any direction.
This requires a complete understanding of the four-dimensional extent of the plume, the width,
depth, length and how these dimensions can be expected to change over the lifetime of the
reactive wall. Information that must be obtained includes the piezometric surfaces and gradient,
the hydraulic conductivity, permeability, porosity, and the other hydrologic parameters typical of
a careful and complete subsurface characterization.
It is also important to understand seasonal changes in flow direction and flux, since the
reactive wall must be designed to accommodate these changes. Beyond general hydrologic
factors, the stratigraphy and lithology of the site will often dictate the type of barrier design
chosen. It is desirable to "key" the barrier into a low-permeability clay layer to prevent
contaminant underflow, for example. If such a layer is not available at a reasonable depth, then a
"hanging" design might be necessary. This type of system must be engineered tp prevent
contaminant underflow. Understanding the vertical variation in stratigraphy is also important for
choosing the zone(s) that the barrier will intersect. Clearly if the contaminant is moving through
a highly permeable layer, the barrier should be placed vertically to encompass this layer.
Therefore, a careful evaluation of the stratigraphic variability at the location of the reactive wall,
and the continuity of this stratigraphy with respect to the upgradient plume, is necessary for
having confidence in the installation.
Historically, plume dimensions have been determined using the installation of monitoring
wells. However, these are expensive, time-consuming to install and sample, and require special
multi-level installations to provide adequate delineation of the plume in the vertical dimension.
The use of push technologies, such as Geoprobe«, Hydropunch«, and cone penetrometers are
rapidly becoming the tools of choice for evaluating shallow plume locations. These can be
driven rapidly and relatively inexpensively, allowing more points to be sampled than could be
accomplished with monitoring wells for the same amount of time and money. Additionally, they
can be used to collect samples over very narrow vertical intervals, allowing better delineation of
the contaminant concentrations in the plume than could normally be acquired with exploratory
monitoring wells. Following this characterization, monitoring wells can be installed where

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appropriate, with much less guesswork.
Information on contaminant concentrations is necessary for any remedial operation,
whether pump and treat or in situ technologies are used, since the remediation technique must be
effective up to the maximum concentrations that will be encountered. This is especially
important for reactive barriers because once emplaced, it is difficult to change the thickness of
the reactive zone. Since permeable reactive barriers are generally placed downgradient of the
plume center-of-mass, it is important that the barrier be designed to accommodate the higher
upgradient concentrations, should they arrive at the barrier unattenuated. This requires sufficient
contaminant characterization to accurately determine this high concentration zone. It is also
clearly desirable to know whether this zone is moving downgradient over time or whether a
steady-state has been achieved that could suggest natural attenuation is occurring. If this is so,
the barrier could be designed for attenuating only the lower downgradient concentration(s) for the
protection of nearby receptors or to eliminate migration beyond site boundaries.
Site/plume geochemical information is needed for reactive barrier implementation and to
further our understanding of the expected lifetime of these systems. It has been shown, for
example, that water passing through a reactive barrier of zero-valent iron undergoes radical
geochemical changes, including a change in pH up to 9 or 10, elimination of oxygen with Eh
reduction to minus several hundred millivolts, and a reduction in carbonate alkalinity. Often,
sulfate is reduced to sulfide and dissolved iron appears. Other changes might be observed,
dependant upon the contaminants being reacted. Some of the geochemical changes can result in
precipitation on the reactant surfaces, potentially reducing the permeability of the reactive zone
over time.
One area of site characterization for reactive barriers that needs further study is microbial
activity. The interactions of native microbial populations, contaminants, and reactive barrier
materials are likely to be quite complex, and have the potential for either beneficial or
detrimental effects on the remediation. Beneficial effects could include enhanced contaminant
degradation; for example the further reduction in Eh due to the presence of sulfate-reducing
bacteria might increase the rates of contaminant reduction. Adverse effects could include the
potential loss of permeability of the reactive zone due to biofouling. Additional laboratory and
field studies are needed to understand these interactions and learn how to enhance the positive
effects and reduce the negative effects of native microorganisms.
III. Emplacement Methods for Optimized Design and Cost Effectiveness
Dale S. Schultz, DuPont Central Research & Development
Permeable reactive barriers can be an attractive alternative to conventional means of
groundwater treatment. The challenge is to optimize the design of these systems and the means
of emplacing them in the ground so as to implement them as cost-effectively as possible.
In this paper, it will be shown how laboratory reaction kinetics data and basic knowledge
of the plume characteristics and the remediation goals may be used to estimate the required

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amount of reactive material per unit cross-section of the plume. The value of this parameter has
important implications regarding the choice of permeable barrier design and emplacement
method. The specific application of granular iron to treat groundwater contaminated with
chlorinated solvents is considered here, but the methodology may be applicable to other types of
media and contaminants.
Thus far, most field applications of permeable reactive barriers have employed the
funnel-and-gate design. That is, the reactive material is placed into excavated regions (gates) and
groundwater is directed to them by means of impermeable walls (funnels). Experience has
shown that the most expensive element is construction of the gates. Therefore, there is
increasing interest in implementing continuous permeable zones across the entire width of
plumes. If such zones can be emplaced at costs similar to those of impermeable walls, then
considerable cost savings would be expected. Emplacement methods being considered include
the use of trenching machines, driven mandrels, high pressure jetting, and deep soil mixing.
Each of these methods offer different capabilities and costs. Recent developmental efforts aimed
at applying these methods to implement permeable reactive barriers are discussed.
IV. Compliance and Performance Monitoring
Robert W. Puis, Ph.D., USEPA National Risk Management Research Laboratory
Ada, OK.
Compliance monitoring typically involves the monitoring of the contaminants of interest
at a particular hazardous waste site discovered to exceed regulatory limits during a remedial
investigation. General water quality monitoring is also often included such as determinations for
major cations and anions and other water quality indicator parameters such as pH, alkalinity,
specific conductance etc. For permeable reactive barriers, similar monitoring requirements are
necessary; however, the placement and design of monitoring wells or monitoring points and the
methods used to sample ground water may be different.
Well placement and design are important to ensure assessment of system performance. In
addition to upgradient and downgradient wells, wells should be located to ensure that
contaminated water is not flowing around or under the barrier wall. Selection of screen length
should be compatible with sampling program objectives and site conditions.
Performance monitoring of permeable reactive barriers includes the monitoring of
physical, chemical and mineralogic parameters over time. It begins with adequate site
characterization to provide a baseline for later comparison and its objective is to evaluate
permeable barrier wall performance as designed over time. A performance monitoring program
should address and be able to detect loss of reactivity, decrease in permeability, decrease in
contaminant residence time in the reaction zone, short circuiting or leakage in the funnel walls
and verification of emplacement. In addition to monitoring the contaminants of concern and
general water quality, the following are also recommended: degradation products, precipitates,
hydrologic parameters and geochemical indicator parameters. Understanding of the mechanisms
controlling contaminant transformation, destruction or immobilization within the reaction zone is

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critical to interpretation of monitoring data.
Geochemical indicator parameters which provide some measure of system performance
include: pH, Eh, alkalinity, dissolved oxygen, and ferrous iron. These parameters provide
indications that iron corrosion is proceeding and provide some indication of the extent of
precipitate formation within the barrier which may eventually decrease wall performance over
time. When included with general water quality monitoring and used in conjunction with
geochemical modeling, these geochemical parameters can support modeling projections
concerning potential precipitate formation. Coring of the reactive mixture and surface
mineralogic analyses of the iron surfaces should be done periodically to assess precipitation
build-up which might eventually lead to system plugging and failure over time. Hydrologic
changes over time should also be closely monitored. Head measurements, tracer tests and in-situ
flow meters can be used to monitor for changes in system permeability and alteration of flow
paths over time.
This workshop session will expand the above discussion and also provide
recommendations on sampling methods, onsite analysis, and sampling materials useful for both
compliance and performance monitoring of this emerging innovative remedial technology.
V. Installed Treatment Wall Demonstrations and Applications
John Vogan, Envirometal Technologies Inc., Guelph, Ontario, Canada
In-situ permeable reactive barriers can provide a cost-effective alternative to pump and
treat techniques for control of dissolved phase contaminant plumes. Six full-scale and five pilot-
scale in-situ systems using permeable iron treatment sections have been installed at several sites
in the past two years for remediation of chlorinated solvents (VOCs). At most sites, a funnel and
gate configuration has been employed where impermeable barrier sections (funnels) consisting of
sheet piling or slurry wall are used to direct the groundwater plume towards permeable iron
treatment sections (gates). Continuous reactive walls and an in-situ reaction vessel have also
been constructed at other facilities. The systems have been installed to depths of about 40 feet to
date, and are treating VOCs in the range of 10's of ppb to 100's of mg/L.
This section of the workshop will present several case histories of these installations to
illustrate how site specific hydrologeologic and geotechnical characteristics were addressed in
system design and installation. Construction of these systems has also involved addressing
concerns common to many types of subsurface construction, including dewatering needs,
unexpected lithological changes, soil disposal options and health and safety issues. Problems
encountered and overcome in this regard will also be described.
Little data has been published regarding the costs of in-situ permeable treatment walls.
As part of this section, capital costs for the full-scale systems installed to date till be presented,
and compared to the estimates developed for alternative treatment remedies. Costs of
constructing the same size of system using different construction techniques will also be
compared.

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A brief summary of operating results to date will conclude this section. The longest
operating in-situ permeable treatment wall, installed at the University of Waterloo test site at
CFB Borden, Ontario has been operating for five years. Commercial installations have been in
place for up to two years. Geochemical data from these installations will be presented in the
context of the performance predicted for these systems in the design phase.
Notice
Although contributions to this abstract have been funded wholly or in part by the United
States Environmental Protection Agency, it has not been subjected to the Agency's peer and
administrative review and therefore may not necessarily reflect the views of the Agency and no
official endorsement may be inferred.

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TECHNICAL REPORT DATA
1. REPORT NO.
EPA/600/A-97/029
2.
3. 3
4. TITLE AMD SUBTITLE
NGWA WORKSHOP ON PERMEABLE REACTIVE BARRIERS IN GROUND WATER
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
ROBERT W. PULS1, DAVID W. BLOWES', ROBERT M. POWELL3, DALE S. SCHULTZ\
AND JOHN VOGANs
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
1USEPA "UNIVERSITY OF WATERLOO
NATIONAL RISK MANAGEMENT RESEARCH LAB. WATERLOO, ONTARIO, CANADA
ADA, OK
'MANTECH ENV. RESEARCH SVCS. CORP. "DUPONT CENTRAL RESEARCH & DEV.
LAS VEGAS, NV
'ENVIRONMENTAL TECHNOLOGIES INC.
GUELPH, ONTARIO, CANADA
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 & REMEDIATION DIV.
P.O. BOX 1198
ADA, OK 74820
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/15
15. SUPPLEMENTARY NOTES
TO BE PUBLISHED: NGWA WORKSHOP PROCEEDINGS, BOOK & CHAPTER
16. ABSTRACT
Permeable reaction walls are an emerging technology for the treatment of
contaminated groundwater. Successful treatment of contaminated groundwater using
this technique requires that the contaminate be rendered innocuous or immobile
during transport through the in-situ treatment zone. The extent of treatment,
and the success of the permeable barrier system depends on the nature of the
contaminant, the selection of the reactive material, the physical design of the
treatment system, and site conditions.
17. KEY WORDS AND DOCUMENT ANALYSIS
A. DESCRIPTORS
B. IDENTIFIERS/OPEN ENDED TERMS
C. COSATI FIELD, GROUP
Permeable Reactive Barrier
Zero-Valent Iron
In-Situ Remediation


18. DISTRIBUTION STATEMENT
Release 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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