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In Situ Permeable Reactive Barriers for GroundwaterContaminationJohn P. Richardson a & John W. Nicklow ba Remedial Project Mgmt. Sect., Illinois Environ. Protection Agency, Springfield, IL, 62794b Dept. of Civil Engrg., Southern Illinois University Carbondale, Carbondale, IL, 62901; E-mail: nicklow@engr.siu.edu; Fax: 618-453-3044; Phone: 618-453-3325.Published online: 24 Jun 2010.

To cite this article: John P. Richardson & John W. Nicklow (2002) In Situ Permeable Reactive Barriers for GroundwaterContamination, Soil and Sediment Contamination: An International Journal, 11:2, 241-268

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Soil and Sediment Contamination, 11(2):241-268 (2002)

1532-0383/02/$.50© 2002 by AEHS

In Situ Permeable ReactiveBarriers for Groundwater Contamination

John P. Richardson1 and John W.Nicklow2

1Remedial Project Mgmt. Sect., IllinoisEnviron. Protection Agency, Springfield, IL,62794; 2Dept. of Civil Engrg., Southern IllinoisUniversity Carbondale, Carbondale, IL, 62901;E-mail: nicklow@engr.siu.edu; Fax: 618-453-3044; Phone: 618-453-3325.

ponents through chemical and/or biologi-cal reactions, adsorbed, or chemically al-tered so that they form insoluble precipi-tates. This article represents a summaryreview of representative literature on per-meable reactive barrier technology. It con-sists of a description of the technology, alist of treatable contaminants, the processesnecessary for its implementation, consid-erations for conducting performance moni-toring, a discussion of the positive andnegative attributes and costs of the tech-nology, and lessons learned during recentapplications. Where conditions are favor-able and time factors are appropriate, thistechnology appears promising. The maincharacteristic in its favor is the lack of theneed to operate pumps or treatment ves-sels, thereby saving operation and mainte-nance costs and allowing the economicvalue of property to be restored duringremediation. Its reliance on natural advec-tive processes to move contaminantsthrough the treatment zone, resulting inlong treatment time frames, can be a dis-advantage under some circumstances.There are also uncertainties about the long-term effectiveness of the reactive media.Regulators need to continue the trend to-ward being more receptive of this technol-ogy, as well as other innovative technolo-gies, so that it can be improved. Thisreceptiveness will benefit all stakeholdersinvolved.

In situ permeable reactive barriers (PRBs)consist of zones of reactive material, suchas granular iron or other typically reducedmetal, lime, electron donor-releasing com-pounds, or electron acceptor-releasingcompounds, installed in the path of a plumeof contaminated groundwater. As thegroundwater flows through this zone, con-taminants are degraded to innocuous com-

KEY WORDS: Permeable Reactive Barriers, Groundwater Remediation.

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INTRODUCTION

Description of Reactive Barrier Technology

PERMEABLE reactive barrier consists of a zone of reactive material, suchas granular iron or other reduced metal, lime, electron donor-releasing

compounds, or electron acceptor-releasing compounds, installed in the path of aplume of contaminated groundwater. As the groundwater flows through this zone,contaminants are degraded and transformed to innocuous components, adsorbed,or chemically altered so that they form insoluble precipitates and leave solution.The barriers typically have been employed in continuous trenches or in a funneland gate configuration, as illustrated in Figures 1a and 1b, respectively. It is

A

FIGURE 1b

Plume capture by funnel-and-gate system (USEPA, 1998a).

FIGURE 1a

Plume capture by a continuous PRB trenched system (USEPA, 1998a).

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preferable that the wall is keyed into an underlying aquitard although a hangingwall—one that does not extend the full depth of the aquifer—may also be em-ployed where conditions are favorable (USEPA, 2000). A continuous barrierconsists of a wall of reactive media that is constructed long enough to encompassthe entire plume or at least the core of the plume. In a funnel and gate system, animpermeable barrier is constructed to intercept the plume and direct it through oneor more gates containing the reactive media. The system must be designed toencompass potential shifts in the direction and magnitude of the three-dimensionalgradient. The thickness of the reactive cells range from a few centimeters up to ameter or more, depending on contaminant concentrations and site conditions. Themost commonly applied permeable reactive barrier consists of granular zero valentiron used to remediate dissolved chlorinated solvents (USEPA, 1999). Therefore,the emphasis of this article is on chlorinated solvent remediation using zero valentiron, although the application of other reactive media to remediate various con-taminants is also discussed.

The first reported use of metals to degrade chlorinated organic compounds wasin 1972 when Sweeny and Fischer obtained a patent for the degradation ofchlorinated pesticides by metallic zinc under acidic conditions. They also foundthat catalytically active powders of iron, zinc, or aluminum could be used todestroy several organic contaminants, including PCE, TCE, TCA, trihalomethanes,chlorobenzene, PCBs, and chlordane. The process involved trickling the contami-nated water through a bed of sand and metal or by fluidizing a bed of powderedmetal in the influent (Gavaskar et al., 1998). The exact mechanism by whichchlorinated compounds are degraded is not fully understood. It is thought that avariety of pathways are involved. The primary reaction apparently involves theremoval of the halogen followed by replacement with hydrogen.

Fe0 + H2O + RCl >> RH + Fe+2 + OH– + Cl– (1)

Another reaction involves replacement of the halogen by a hydroxyl group.

Fe0 + 2H2O + RCl >> 2ROH + Fe+2 2Cl– + H2 (2)

The hydroxyl groups can be formed by the interaction of water and oxygen withthe zero valent iron (Matheson and Tratnyek, 1994). Roberts et al., (1996) devel-oped a potential model for PCE and TCE degradation to dichloroacetylene andchloroacetylene, respectively, through a beta-elimination pathway. Those interme-diates are then quickly transformed to lesser chlorinated acetylene and acetates.

2Fe0 + O2 + 2H2O >> 2 Fe+2 + 4 OH– (3)

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In highly oxygenated groundwater, that process may promote negative effects onthe barrier due to the formation of ferric oxyhydroxide or ferric hydroxide (rust)precipitates. Most contaminated aquifers are oxygen deficient, so such oxidationof the reactive iron is often not a serious problem. Zero valent iron, which is astrong reducing agent, reacts with chlorinated compounds through electron trans-fers, resulting in ethene and chloride. Orth and Gillham (1996) found that etheneand ethane represented 80% of the original mass of TCE, and partially dechlori-nated byproducts such as cis and trans 1,2-dichloroethene, 1,1-dichloroethene, andvinyl chloride represented only 3% of the original mass following treatment withzero valent iron. During the proposed reaction of TCE and zero valent iron, sixelectrons are transferred, something that almost certainly does not occur instanta-neously. However, in order to explain the formation of such a small amount ofbyproducts, it is theorized that the TCE molecule must remain attached to the metalsurface long enough for the six-electron transfer to take place. The TCE is thoughtto attach either through its inherent hydrophobicity or through the formation of astrong chloroethene-iron pi bond (Sivavec and Horney, 1995). Some moleculesbreak away early, accounting for the presence of small amounts of DCE and vinylchloride. That dependence on interaction with the iron surface leads to a directrelationship between iron surface area and reaction efficiency. Smaller iron par-ticles lead to faster reactions, but at the cost of hydraulic conductivity (Johnsonet al., 1996).

As seen in Equation 3, the reaction of the iron with water and oxygen produceshydroxide. Under anoxic conditions, the iron also reacts with water in a slower sidereaction to produce hydroxide.

Fe0 + 2H2O >> Fe+2 + H2 + 2OH– (4)

This reaction often causes the pH of the water in the reactive cell to increase. It canreach a value of 9.0 or higher, contributing to the formation of precipitates that cancoat the metal surfaces, potentially reducing reactivity and hydraulic conductivity.As the hydroxide ions are consumed, soluble carbonate ions are formed. They canbuild up to the point that minerals, such as calcite, siderite, or magnesiumhydrocarbonates, are formed and precipitate out (USEPA, 1999). The naturallyoccurring carbonic acid and bicarbonate in groundwater will often have a bufferingeffect on pH, reducing the potential for such negative effects. There are many suchreactions going on in the reactive zone. Many of them do not reach equilibrium inthe short time the groundwater is within the zone. Also, many of the precipitatesare very fine particles that get carried out of the reactive zone, further limiting theirimpacts on the reactive cell’s performance (Gavaskar et al., 1998).

The first full-scale commercial application of a permeable reactive barrier wasthe use of zero valent iron to remediate chlorinated solvents at a Sunnyvale,

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California, semiconductor manufacturing facility in 1994 (IBC, 1999). The Uni-versity of Waterloo in Canada developed the technology and holds a patent on theprocess through its Envirometal Technology, Inc. (Nyer, Palmer, Carman, Boettcher,Bedessem, Lenzo, Crossman, Rorech, and Kidd, 2001).

Problem Description

Halogenated solvents, metals, petroleum, and radionuclides have contaminated thegroundwater at many industrial sites. Ten of the 25 most common contaminantsfound in groundwater at hazardous waste site are chlorinated solvents. Their lowsolubility, recalcitrance, and tendency to form dense nonaqueous phase liquids(DNAPLs) have made many engineered remediation attempts unsuccessful. From1982 to 1997, pump-and-treat techniques were used at 89% of Superfund sites forgroundwater remediation (Ott, 2000). Pump-and-treat methods, however, rely onthe ability of water to serve as a solvent to mobilize contaminants and on advectioninduced by pumping to move them to the extraction wells for removal. Effortsquickly reach the point of diminishing returns. Pump-and-treat systems are energyand maintenance intensive; therefore, they are expensive to operate (Nyer et al.,2001; USEPA, 1995). Additionally, although chlorinated solvents can exist in thevapor phase in the vadose zone, soil vapor extraction fails to address groundwaterbecause much of the contamination sinks through the aquifer and away from theeffects of the extraction process (USEPA, 2000). In situ bioremediation is oftenlimited by the fact that nutrients and electron acceptors or donors must be activelycirculated to the contaminants in sufficient supply to support organisms to degradecontaminants. That process is also energy and maintenance intensive, becauseinjection wells, pumps, and mixing vessels must be operated and chemicals mustbe constantly replenished. Natural attenuation, a process by which naturally occur-ring physical, chemical, and biological processes reduce contaminant concentra-tions to water-quality objectives, is becoming more accepted as a viable remediationtechnique. However, all contaminants and conditions are not amenable to naturalattenuation, and it is limited to areas with a large enough buffer zone to allow theprocess to take place, something that often does not exist in developed areas(USEPA, 2000).

Variations of the Technology

Most applications of the reactive barrier technology have involved the use ofgranular zero valent iron in continuous trenches constructed across the path of theplume or in gates within funnel walls to direct the plume through the reactive zone.Some variations of that basic employment include: interception trenches that

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collect groundwater and route it to in situ or ex situ treatment vessels, nested wellscontaining reactive media, pressurized jetting of reactive media into aquifer sedi-ments, vertical hydrofracturing, biological barriers, and in situ reduction of natu-rally occurring iron with reagents such as thiosulfate to create zero valent iron(ITRC, 1999).

Another variation involves the establishment of a bioactive zone using organiccarbon sources, Oxygen Release Compound™ (ORC™) or Hydrogen ReleaseCompound (HRC™). A leaf compost/gravel mixture was used to construct asubsurface trench to successfully intercept and treat acid mine drainage from anickel mine to reduce sulfate and dissolved iron and to restore pH and alkalinityto normal ranges. Bacteria growing in the mixture mediate sulfate reduction andmetal sulfide precipitation (Benner et al., 1997; Waybrant et al., 1998). ORC™and HRC™ are proprietary products of the Regenesis Corporation. ORC™, whichconsists of magnesium peroxide formulated to slowly release oxygen at a prede-termined rate, has been used to establish oxygenated zones to treat fuel constitu-ents, such as benzene, toluene, ethylbenzene, and xylene (BTEX) at petroleumsites (Johnson and Odencrantz, 1999; Kao and Borden, 1999). ORC™-containing“socks” were placed in closely spaced wells installed perpendicular to the ground-water direction. A zone of enhanced aerobic biological activity was created nearthe wells. Results using ORC™ have been mixed. Some of the oxygen leavessolution, rendering it inaccessible to groundwater microbes, and close well spacingis required in low permeability aquifers (Chapman et al., 1997). HRC™ consistsof a proprietary food-grade polylactate ester that, when placed in groundwater,degrades through pyruvic acid to acetic acid, releasing hydrogen that serves as anelectron donor in the anaerobic biodegradation of contaminants, and, because it isan organic compound, it consumes oxygen as is breaks down, contributing to theanaerobic conditions favorable for reductive dechlorination. It is employed throughunderground injection in a paste consistency or by placement in perforated canis-ters that are lowered into wells. Several research projects at the pilot and field scalehave demonstrated its potential as a barrier material (Sheldon and Armstrong,2000). The most challenging aspect of establishing permeable reactive barriers bydisseminating reactants in the path of plumes lies in obtaining sufficient and evendistribution of those agents. Their success will always be limited by the hydrogeologyof the site and by the effectiveness of advection to distribute them.

Treatable Contaminants

Table 1 lists contaminants that have been shown to be treated successfully withzero valent iron and other media. Most projects to date have involved treatment ofchlorinated ethenes, namely, PCE and TCE (Gavaskar et al., 1998). However, asTable 1 shows, more progress is being made in applying the technology to addressother organics, as well as inorganics (ITRC, 1999b; Lackovic et al., 2000; Ott,

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2000). Permeable barriers containing lime have also been used to raise the pH ofgroundwater near mining operations. Dichloromethane, 1,2-dichloroethane,chloroethane, chloromethane, chloride, PCBs, and perchlorate are typically nottreatable using zero valent iron, although research into developing appropriatetreatment materials continues (USEPA, 1998a).

TABLE 1Contaminants Treatable

by Reactive Materials in PRBs (USEPA, 1998a)

Organic InorganicCompounds Constituents

Methanes Tetrachloromethane Trace metals ChromiumTrichloromethane NickelDichloromethane Lead

UraniumTechnetiumIronManganeseSeleniumCopperCobaltCadmiumZinc

Ethanes Hexachloroethane Anionic Sulphate1,1,1-trichloroethane Contaminants Nitrate1,1,2-trichloroethane Phosphate1,1-dichloroethane Arsenic

Ethenes TetrachloroetheneTrichloroetheneCis-1,2-dichloroetheneTrans-1,2-dichloroetheneVinyl chloride

Propanes 1,2,3-trichloropropane1,2-dichloropropane

Aromatics BenzeneTolueneEthylbenzene

Other Hexachlorobutadiene1,2-dibromomethaneFreon 113N-nitrosodimethylamine

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STEPS IN IMPLEMENTING THE REACTIVE BARRIER TECHNOLOGY

Site Characterization

The first step taken when contemplating installing a permeable reactive barrier isthe development of a conceptual site model, a three-dimensional representation ofthe site and the circumstances that exist with regard to the contamination. The mostimportant information needed to determine whether such a system is feasible andto design the system is data on site hydrogeology. Borings, monitoring wells, andpiezometers must be strategically placed to gather such information as stratigra-phy, aquifer heterogeneity, groundwater levels and flow direction, temperature,flow velocity, porosity, hydraulic conductivity, depth to aquitard, and aquitardthickness, continuity, and competence. The thickness of the water bearing unit(s)must be accurately determined in order to estimate the height of the barrier. Thepresence of low-permeability layers may lead to concentration differences verti-cally. Also, the permeable barrier will have little utility in a low permeability layer.In such a heterogeneous aquifer, a pea gravel zone is installed on the upgradientside to equilibrate flow so it is more evenly distributed in the reactive cell. Thedepth to groundwater is very important because the barrier should extend at least0.6 m (2 ft) above the seasonally high groundwater elevation so contaminants donot pass over the top of it. Flow direction and hydraulic gradient should bemeasured over several seasons of the year, because they can fluctuate seasonally.In some cases where sites are located near large lakes or streams, flow directioncan vary up to 180 degrees over the year as the gradient is deflected or reversedduring times of high surface water. The preconstruction gradient is used as astandard to measure the hydrologic performance of the barrier. Groundwatertemperature, although relatively constant at a given site, will vary geographically.Temperature information is critical to the calculation of residence time and barrierthickness because it affects reaction rates. Porosity and hydraulic conductivity areimportant parameters to measure because the hydraulic conductivity of the perme-able barrier needs to be at least five times higher than that of the aquifer in orderto create an adequate capture zone and allow for some conductivity losses due tocorrosion (Gavaskar et al., 2000). That difference in conductivity is especiallyimportant when a funnel and gate system is used because the presence of the funnelincreases the flow rate through the reactive cell. The presence of an extensive,competent aquitard is preferred in most cases to ensure that contaminants do notpass beneath the treatment zone (ITRC, 1999; USEPA, 1998a).

Contaminant Concentrations in Groundwater

Information on contaminants and their concentrations is critical for the feasibilitystudy, batch and column tests, barrier design and placement, and performancemonitoring. Free-phase liquids are not treatable (ITRC, 2001). Because the design

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of the barrier is strongly dependent on contaminant distribution, the determinationof the extent of the plume is important. Monitoring wells and, in the case of volatilecompounds, soil gas surveys are used to estimate the vertical and horizontaldimensions of the plume or plumes. Constituents should be analyzed using USEPASW-846 methods. The concentrations are plotted on isopleth maps in both plan andcross-sectional views. The maps can assist in locating source areas. The width ofthe plume will determine whether a continuous barrier is installed to intercept theentire plume or if funnel walls are used to capture the groundwater and route itthrough the treatment zone. If more than one plume is identified, a separate barriermay be needed for each one (Gavaskar et al., 1998; USEPA, 1998a).

Inorganic Characteristics of Groundwater

Preconstruction monitoring of several parameters and inorganic constituents thatare prone to forming precipitates or other undesirable species must be conducted,preferably over a period of 1 year. Values for redox potential, pH, and dissolvedoxygen can be used to predict whether conditions at the site are favorable for theformation of precipitates. Calcium, iron, magnesium, manganese, aluminum, barium,chloride, fluoride, sulfate, and bicarbonate (alkalinity) have a potential to react inthe conditions created in the treatment zone. At a site in Colorado, precipitation ofcalcite and siderite in the treatment cell was thought to be the cause for the decreasein permeability that led to a backup of groundwater and subsequent movement ofpart of the plume around and over the treatment zone (USEPA, 1999). Also, carbonin humic and fulvic substances may be reduced to methane, while sulfate may bereduced to bisulfide, and nitrate may be reduced to nitrogen gas or ammonia(Gavaskar et al., 1998).

Physical Setting

The site conditions at the surface will also impact the barrier design and construc-tion method selection. The barrier is normally constructed at the leading edge ofthe contaminant plume. However, if the plume is already at or beyond the propertyboundary, it may have to be installed within the plume. That must be consideredduring waste disposal because the spoils and removed groundwater may be hazard-ous waste. Physical constrictions such as buildings or other structures may alsolimit where the barrier can be placed, as well as the construction technique to beused. For example, constructing a soil-bentonite slurry wall funnel system requiresa large mixing area, which may not be available at some sites (Gavaskar et al.,1998).

Establishing Remediation Objectives

In most cases, the remediation objectives will be site-specific health-based cleanupstandards established by state regulatory agencies. The goal may be complete

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remediation of the aquifer, something that may take many years, or it may be tomeet state standards at the property boundary, as long as restrictions are in placethat exclude exposure pathways on-site. The appropriate regulatory authority andother stakeholders should be consulted early in the planning process to clearlydetermine the remediation goals. Treatability studies or other research can then beperformed to determine whether those goals can be attained before devoting furtherresources to planning for the permeable barrier.

Reactive Media Selection

Reactive media should have the following properties:

• Sufficient reactivity to degrade the contaminants using an economicallypractical barrier thickness.

• The ability to remain reactive for several years to decades under the site-specific geochemical conditions with a minimal amount of maintenance.

• A particle size sufficiently greater than that of the aquifer material so thatan effective capture zone can be created and maintained.

• Environmentally acceptable reaction products.

• Easy construction and availability at a reasonable price.

The most common reactive medium used thus far has been granular zero valentiron. It has been shown to be effective at degrading chlorinated solvents andreducing metals, and it is often economical because it is often a byproduct of metalcutting or grinding operations (Gavaskar et al., 1998). If byproduct iron is used, itshould be checked to ensure it is free of cutting oils and grease, which will destroyits reactivity. The iron should also be at least 95% zero valent iron, with nohazardous levels of trace metal impurities and a minimal oxide (rust) coating (Ott,2000). It should consist of uniform-sized particles to minimize packing (USEPA,1998a). One of the most important parameters to consider is specific surface area,or surface area per unit mass. In a study of iron from 25 different sources, thespecific surface area varied four orders of magnitude (USEPA, 1998a). Thatvariability lends credibility to the need for treatability studies. As indicated earlier,smaller particles, with greater specific surface area, were found to be more efficientat degrading contaminants than larger particles. However, there are tradeoffs:smaller particle size means lower hydraulic conductivity and a greater potential forclogging due to precipitate formation. Therefore, sand-sized particles are generallyused. Alternatively, the iron particles can be mixed with coarse sand to increaseconductivity. In order to minimize oxidation and precipitate formation in thereactive zone, a mixture of pea gravel or sand along with 10 to 15% iron particles

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can be placed upgradient of the treatment cell. The pretreatment area is “sacrificed”to remove oxygen and precipitate-forming species from the water before it reachesthe actual treatment zone (Gavaskar et al., 1998; USEPA, 1998a).

Another form of zero valent iron that shows promise is colloidal iron, havingparticles in the 1 to 3 µm size range. It can be injected into aquifers through wellsto create barriers. It will also travel with groundwater and can penetrate fracturesin bedrock. However, as of 1998 it had not been tested at the field scale (Gavaskaret al., 1998). Because distribution of the colloidal iron relies on advection, it wouldbe subject to the restrictions of other active technologies such as pump-and-treatand enhanced in situ bioremediation. It would not be amenable to low-permeabilityaquifers with high silt and clay content.

Other metals, including stainless steel, brass, mild steel, galvanized steel, andzero valent copper, aluminum, magnesium, tin, zinc and manganese, as well as thebimetallic mixtures iron-copper, iron-palladium, and iron-nickel, have also beenevaluated (Fennelly and Roberts, 1998; Ott, 2000). Most of them have not beenused in general practice due to poor reaction rates and/or their expense (Gavaskaret al., 1998). Iron-palladium has demonstrated quite fast reaction kinetics due tocatalysis by the palladium. However, it has not proven economical due to theexpense of palladium, and long-term column tests cast doubts on its ability tomaintain reactivity over the long term without regular flushing with an acidsolution (Muftikan et al., 1996).

Treatability Testing

Treatability testing is done to finalize selection of the reactive medium, estimatethe half-life of the contaminant so the barrier thickness can be determined, andevaluate conditions that could affect the longevity of the barrier. Batch testingconsists of placing a measured amount of the contaminant solution at a knownconcentration in containers with selected media and measuring the degradationafter preselected time intervals. Although that test is adequate as a screeningprocess for identifying recalcitrant compounds, it does not yield all the data neededto design the reactive barrier.

In order to get more applicable data, tests are generally performed in columns.The columns are glass or plexiglass tubes that are packed with reactive media, asshown in Figure 2.

Sampling ports are installed along the length of the columns to remove samplesfor analysis to determine concentrations along the column. Spiked water or con-taminated water from the site is passed through the column(s) at the same velocityas that measured during site characterization. Ideally, groundwater velocities shouldbe determined in various seasons so that the highest anticipated velocity can beused in the column tests. Column tests yield more reliable results than batch tests,primarily because they are dynamic and better simulate actual aquifer conditions.

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FIGURE 2

Typical column set up (Gavaskar et al., 1998).

If groundwater velocities are high, the column is made long enough or multiplecolumns in series are used to produce the needed residence time. The effects ofnonlinear sorption can be examined by pulling samples from the various portsalong the column (Gillham and O’Hannesin, 1994). A final advantage of columntesting over batch testing is that while reaction products and byproducts accumu-late in the batch reactor they are generally washed through the column, as theywould be in the aquifer (Gavaskar et al., 1998).

The major use of the column test is to determine the half-lives of contaminantsand byproducts. For chlorinated solvents using zero valent iron, first-order kinetics

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are used to estimate half-lives. Contaminant concentrations from each columnprofile are plotted as a function of distance along the column. Then, using thecalculated flow velocity, the distance is converted to residence time. A degradationprofile is developed for each contaminant by plotting concentration against resi-dence time. The first-order degradation rate constant, k, is developed using theintegrated, first-order rate equation

C = Coe–kt (5)

where C = concentration at time t and Co= initial concentration. The degree towhich degradation follows first-order kinetics is evaluated by plotting the normallog of C/Co against residence time. A straight line indicates first-order kinetics.The rate constant for each contaminant is then used to find the half-life, t1/2, for therespective contaminant using

t1/2 = ln(2)/k = 0.693/k (6)

The required residence time, tw , for design purposes can be estimated bydetermining the number of half-lives needed to reduce the measured maximumconcentration to the remediation objective or it can be estimated using

tw = (1/k) ln(Co/C) (7)

It must be noted that the required residence time is based on the longest timerequired to degrade all contaminants to the remediation objectives. For example,in the case of TCE the time required to degrade the parent compound plus all ofits byproducts, such as cis-1,2-DCE and vinyl chloride, must be considered (Gavaskaret al., 1998; USEPA,1998a). Table 2 lists some common organic contaminants andtheir half-lives as determined through column testing and batch testing with zerovalent iron. Johnson, Scherer, and Tratnyek (1996) compiled data from numerousbatch and column studies using zero valent iron to analyze the reaction kinetics.They found that there was a high degree of variability in raw dechlorinationreaction data. Further analysis indicated the variability was due to differences iniron surface area concentration. As expected, reactions in media with greaterreactive surface area displayed shorter half-lives. Therefore, to better compare thedata from the various studies, average reaction rates were normalized to ironsurface area concentrations to develop a representative rate constant, referred to asKSA. Those KSA data were used to develop the half-life values contained in Table 2.

In order to go from laboratory to field, some safety factors must be employed.Treatability testing is generally conducted at room temperature (25°C). Groundwa-ter is typically 10 to 15°C. In one study, degradation rates for TCE decreased bya factor of 1.4 at groundwater temperatures of 8 to 10°C (Gavaskar et al., 1998).

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TABLE 2Treatability Test Results

from Batch and Column Studies

Compound Typical Half-Life (hours)

Tetrachloroethene 0.14–1.16a, 0.5–2.0b

Trichloroethene 0.92–23.1a, 0.5–2.0b

1,1-dichloroethene 5.83–77.0a

Cis-1,2-dichloroethene 11.95–28.88a, 0–6.0b

Vinyl chloride 10.66–19.8a, 2.0–6.0b

Carbon tetrachloride 0.003–0.02a, 0.5–1.0b

Trichloromethane 0.42–3.65a, 1.0–3.0b

1,1,1-trichloroethane 0.6a, 0.5–2.0b

1,1-dichloroethane 10–24b

Tribromomethane 0.04a

Hexachloroethane 0.01–0.35a

1,1,2,2-tetrachloroethane 0.05a

1,1,1,2-tetrachloroethane 0.05a

1,2,3-trichloropropane 64.17–495a

a (Adapted from Johnson, et al., 1996.) Based on average reaction ratesfrom batch and column studies normalized to iron surface area concen-tration (m2 per L solution) (kSA).

b (Adapted from USEPA, 2000.) From column studies; not normalized.

As explained earlier, degradation rates are directly proportional to the specificsurface area (surface area per unit volume of groundwater) of the reactive medium.Due to different settling conditions and the nature of barrier material placement,the bulk density of material is less than that in the packed columns. Therefore, itis suggested that a correction factor of 1.5 be applied to lab-derived half-lives asa safety measure. Other safety factors, based on such things as uncertainty aboutgroundwater flow velocities or contaminant concentrations, may also be applied ona site-specific basis (Gavaskar et al., 1998).

The residence time information and groundwater velocity data are used todetermine the thickness of the reactive barrier. After half-life values are modifiedwith appropriate safety factors and residence time is determined, the thickness ofthe barrier is calculated by:

b = Vx tw (8)

where b = flow through thickness, Vx = groundwater velocity, and tw = residencetime. Because the hydraulic conductivity of the reactive cell is designed to begreater than that of the aquifer in order to maximize capture zone size, groundwatervelocity through the cell is greater than in the aquifer. Hydrogeologic modeling isgenerally used to predict the cell groundwater velocity that is used in Equation 8.

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Another safety factor may be applied to account for seasonal variations in ground-water velocity (Gavaskar et al., 1998).

Modeling to Support Barrier Design

Using the information gathered during the site characterization and treatabilitystudy, hydrogeologic modeling is used to evaluate several design configurations,site parameters, and performance scenarios. Modeling assists in determining thewidth of the barrier and the funnel walls (if necessary) in relation to the plume size,estimating capture zone size, determining the best location for the barrier, the bestlocations for monitoring wells to assess performance, evaluating the effects ofaquifer heterogeneity, buried utilities, buildings and seasonal fluctuations on thesystem, and assessing the potential for underflow, overflow, or flow around thebarrier (ITRC, 1999b). The most important role of the modeling is enabling thedesigner to simulate different barrier configurations and scenarios to aid in opti-mizing the system. For example, the capture zone width is maximized by increas-ing flow through the barrier. However, maximizing flow also increases velocityand decreases residence time. Thus, the reactive barrier must be made thicker and/or wider, increasing cost (Gavaskar et al., 1998). The objective is to balanceperformance, or capturing and treating as much groundwater as possible, witheconomic effectiveness, or making the system no larger than it needs to be.

For most applications, commonly available computer codes such as MODFLOWand MODPATH are sufficient for developing groundwater models as design tools.Geochemical models such as PHREEQ are used to predict the reactions involvinginorganic parameters and their potential to impact barrier performance. Groundwa-ter flow models and particle tracking codes are used to create flownets through thebarrier system to predict travel paths and residence times.

Most barrier applications to date have assumed homogeneous aquifer conditionsto simplify modeling. However, more sophisticated models are needed whenheterogeneities are encountered. When dealing with heterogeneous aquifers, mod-eling will help designers place the permeable barrier in the zone where the mostflow is taking place (Gavaskar et al., 1998).

Reactive Wall Construction

Various geotechnical construction techniques, as well as some specialized meth-ods, are available to install permeable reactive barriers and funnel walls. Table 3lists several methods, depth capabilities, and their approximate unit costs as of2000. Modeling research has shown that, given a constant funnel wall width, thecapture zone size increases with gate width (as expected). Also, modeling resultsagree that the hydraulic capture zone increases with the ratio of the conductivity

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TABLE 3Summary Table of Various Techniques

for Barrier Emplacement (after Gavaskar et al. , 2000)

Maximu VendorEmplacement m QuotedTechnique Depth (ft) Cost Comments

Impermeable BarrierTechniques

Soil-Bentonite Slurry Wall Requires a large working areato allow for mixing of backfill

Standard backhoe excavation 30 $2–$10/ft2 Generates some trench spoilModified backhoe excavation 80 $2–$10/ft2

Clamshell excavation 150 $6–$175/ft2 Relatively inexpensive when abackhoe is used

Cement-Bentonite Slurry Wall Generates large quantities ofStandard backhoe excavation 30 $4–$22/ft2 trench spoilModified backhoe excavation 80 $4–$22/ft2 More expensive than otherClamshell excavation 200 $16–$55/ft2 slurry walls

Composite Slurry Wall 100+ NA Multiple-barrier wallHDPE Geomembrane Barrier 40–50 $38/ft2 Permeability less than 1 × 10–7

Steel Sheet Piles 60 $15–$30/ft2 No spoils producedSealable-Joint Piles 60 $15–$30/ft2 Groutable joints

Permeable or ImpermeableBarrier Techniques

Caisson-Based Emplacement 50 $50–$300/ Relatively inexpensivevertical ft

Mandrel-Based Emplacement 40–50 $10–$25/ft2 Relatively inexpensive and fastproduction rate; a 3–5 in-thickzone can be installed in asingle pass

Continuous Trenching 25 $5–$12/ft2 High production rateHigh mobilization cost

Jetting 200 $40– Ability to install barrier around$200/ft2 existing buried utilities

Deep Soil Mixing 150 $80– May not be cost-effective for$200/ft2 permeable barriers; columns are

3 to 5 ft in diameter

Hydraulic Fracturing 80–120 $2,300 per Can be emplaced at deep sitesfracture Fractures are only up to 3 in thick

Vibrating Beam 100 $8/ft2 Driven beam is only 6 inwide

(a) Does not include mobilization cost.

NA = not available.

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of the permeable reactive cell to the aquifer conductivity. Generally, the ratio isabout 5:1. Increasing it beyond 10:1 produces little increase in capture zone size(Starr and Cherry, 1994).

Continuous vs Funnel and Gate

A continuous permeable reactive barrier is relatively simple to build, and it is lessexpensive than a funnel and gate system, because an impermeable wall is notconstructed. In addition, because the groundwater flows over a wider cross-sectional area, velocity is lower than in a funnel and gate design. That translatesinto a thinner barrier, and, depending on the length of the barrier, saves moneybecause reactive media must be installed. Most new systems utilize continuousreactive walls (USEPA, 2000). As of the end of 1999, 20 continuous wall systemshad been installed and five funnel and gate systems were in place (ITRC, 2001).However, there are circumstances under which a continuous wall is not feasible.One such situation is when the plume is very wide. In that case, an impermeablewall is constructed to “funnel” the plume through the permeable reactive cellplaced in the gate—an opening in the wall. In heterogeneous aquifers, a funnel andgate system utilizing pea gravel in the inlet and outlet sides of the gate is used toequalize flow and create a high conductivity zone to make plume capture moreeffective. In both types of systems, the reactive barrier should be keyed into anaquitard to prohibit contaminants from flowing underneath the treatment zone(Gavaskar et al., 1998).

Trench Method

In the trench method of reactive barrier installation, a backhoe, clamshell, orcontinuous trencher is used to excavate a linear trench that is backfilled withreactive media that is usually mixed with sand. Each method has advantages anddisadvantages, and selection is based on site conditions. The most popular tech-nique employs a backhoe. It can reach a depth of 9.1 m (30 ft), although specializedextensions allow it to excavate down to 24.4 (80 ft) deep. Buckets up to about 1.7m (5.5 ft) wide are available. It has a fast production rate and can remove largestones and small boulders. Sheet piles are sometimes used in conjunction with abackhoe to hold the trench open or to separate the reactive media from a pea gravellayer during construction. The sheet piles are driven in place, then the necessarysoil is removed and replaced with the desired material before the sheet piles areremoved. The clamshell, another trenching tool, is capable of excavating down to61 m (200 ft) deep. It can work in small areas as long as the boom can reach overthe trench, and it can excavate through all but very hard material and rock.However, its production rate is low compared with the backhoe. An innovative

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technique capable of installing a permeable reactive wall as well as a high-densitypolyethylene (HDPE) funnel wall to a depth of 10.7 to 12.2 m (35 to 40 ft) involvesthe use of a continuous trencher. It has a large chainsaw-like excavator with a built-in trench box that cuts a one-foot- to two-foot-wide trench. An overhead hopperfeeds reactive media into the hole and, if desired, unrolls the HDPE liner material(Gavaskar et al., 1998). The major disadvantage of this construction method is thatdisposal of spoils can be expensive if they are contaminated, especially if theycontain listed Resource Conservation and Recovery Act (RCRA) hazardous waste.

Caisson Emplacement

Caissons, which are used in the construction industry, consist of hollow metalcylinders that are driven into the ground. The soil from within the caisson isaugered out and replaced with reactive media. Caissons up to 2.4 m (8 ft) indiameter can be used. They are placed side-by-side, slightly overlapping, to createa continuous wall (Nyer et al., 2001). In a funnel and gate system the reactive cellmust generally be installed more than one caisson thick. They can be used forreactive barriers down to 15.2 (50 ft) deep. Beyond that depth it becomes toodifficult to drive the caissons. The depth is also limited by consolidated sedimentsand the presence of cobbles. However, it is relatively inexpensive and has beenused in several applications (Gavaskar et al., 1998).

Mandrel Emplacement

Mandrel emplacement is similar to the caisson method, but the mandrel has asacrificial drive shoe on the bottom, so soil is displaced as it is driven into theground. The mandrel is then backfilled with reactive media and withdrawn, leavingthe media in the void created. Reactive barriers can be installed to a depth of nearly61 m (200 ft). One disadvantage is that the cells created are small, typically 51 mmby 127 mm (2-in by 5-in) in size. Therefore, several insertions are needed toconstruct a barrier. Each time a mandrel is driven into the ground, some compac-tion of adjacent soil takes place, leading to decreased hydraulic conductivity. Also,subsurface obstructions can cause the mandrel to veer of course or be stopped.However, it is quite inexpensive and no spoils are generated (Gavaskar et al.,1998).

Funnel Wall Construction

The funnel wall consists of a vertical low-permeability structure designed tointercept contaminated groundwater and route it through a “gate”, which is an

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opening in the wall where the permeable reactive cell is located. The wall isgenerally keyed at least 1.5 m (5 ft) into the underlying aquitard (Gavaskar et al.,1998). The most efficient apex angle for the walls is 180 degrees from each otheron either side of the gate (straight wall), as shown in Figure 3a. However, asillustrated in Figure 3b, other configurations may be used depending on site-specific conditions, such as the presence of buildings and utilities. The best systemorientation for maximum discharge is when the gate is perpendicular to the

FIGURE 3b

Other possible funnel-and-gate system configurations (Gavaskar et al., 1998).

FIGURE 3a

Funnel-and-gate system with straight funnel (Gavaskar et al., 1998).

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groundwater flow direction. As mentioned earlier, in a funnel and gate system, thefunnel size, hydraulic capture zone size, and discharge through the gate areinversely proportional to the residence time. Therefore, the system must be de-signed in an effort to optimize both. This can be done by making the gate largeror by using multiple gates. At sites with a very small source area, it may be feasibleto completely surround the source with an impermeable wall, with one outlet gatecontaining a reactive cell (Starr and Cherry, 1994). Although treatment may takeplace slowly, the risk from exposure to the contaminants would be very minimaldue to decreased dissolution and transport.

Sheet Piles

Sheet piles consist of interlocking steel panels that are driven or pushed into theground to provide support for construction projects. In reactive barrier projects,they can be used to create the funnel walls to channel groundwater through thereactive cell. They can be installed to a depth up to about 18.3 m (60 ft), exceptin rocky soils. The main limitation in this technology has been in sealing the joints.The University of Waterloo has developed sealable-joint sheet piles that performwell if they can be installed without damaging the joints. The life of the sheet pilescan range from 7 to 40 years, depending on groundwater conditions. That is a factorthat must be carefully considered, so that it does not render the system ineffectivebefore treatment is complete (Gavaskar et al., 1998).

Slurry Walls

Slurry walls, the most commonly used low permeability barrier, are constructed byexcavating a trench with a backhoe or clamshell. While digging, the trench is keptfilled with a slurry, usually consisting of bentonite and water, that holds the holeopen and forms a “skin” along the walls. As the excavation advances, the trenchis backfilled with a thicker soil-bentonite-water mixture, cement-bentonite-watermixture, or water-bentonite-cement-aggregate mixture. The mixture sets up toform a very low permeability barrier to direct groundwater through the reactive celllocated in a gate. The gate is often constructed using caissons, because they can bedriven through the wall, forming a good seal between reactive cell and funnel wall.Although the soil-bentonite wall is cheaper, it requires a large mixing area, becausesoil from the trench is mixed at the surface and placed back in the trench. Suchspace may be limited at industrial facilities. Most other methods involve com-pletely replacing the soil so no on-site mixing is required. However, that becomesa disadvantage when the funnel wall is installed in contaminated material, due tohigh spoil disposal costs (Gavaskar et al., 1998).

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Innovative Techniques

A few innovative techniques have been used for constructing funnel walls, but dueto their expense their use has been limited. One such method involves jet grouting,a technique in which a specialized drill stem equipped with spray nozzles is usedto inject a bentonite-cement-water slurry directly into the soil. The wall can be aseries of overlapping columns or a vertical curtain, depending on whether the drillstem is rotated as it is withdrawn. This method can also be used to create apermeable treatment zone by injecting a reactive medium such as iron particles ina guar gum slurry. The guar gum biodegrades to leave the reactive medium.Depending on geology, a wall up to 39.6 m (130 ft) deep can be constructed(USEPA, 2000).

Another method that can be used in soft soil is deep soil mixing. It utilizesmixing paddles on a hollow drill stem with a slurry injection port in the center. Itis augered to the desired depth, then rotated to mix soil with bentonite slurry as itis withdrawn to form a column. It is capable of creating a very low permeable wallto a depth of about 36.6 m (120 ft) (Gavaskar et al., 1998; Nyer et al., 2001).

PERFORMANCE MONITORING

Monitoring of permeable reactive barrier performance serves several purposes.The main purpose is, of course, to ensure the barrier is meeting remediation goals,which are generally federal, state, or local cleanup standards, and the protection ofdowngradient groundwater resources. Strategically placed monitoring points willindicate whether the entire plume is being captured, whether underflow or over-flow are occurring, whether the contaminant is being degraded, and whetherundesirable contaminant byproducts are leaving the treatment zone. Diligent moni-toring is necessary so that trend analysis can be used to predict when media needto be regenerated or replaced in order to prevent system failure. It will also showthe effects the barrier is having on the natural groundwater geochemistry. That willgive insight into whether permeability-reducing precipitates may be forming.

Figure 4 shows typical monitoring well arrangements depending on barrierconfigurations. Wells are generally constructed of 25- or 50-mm (1-in or 2-in)-diameter PVC.

Long-screen wells or well clusters with screens at discrete depths should beused, depending on heterogeneity of contaminant concentrations within the aqui-fer. Wells are placed at the ends of reactive cell and funnel walls to detect bypassand within the downgradient edge of the reactive cell to detect breakthrough(USEPA, 1998a). Wells should also be placed further downgradient to monitor therebound of aquifer geochemistry and to monitor for elevated dissolved iron fromthe reactive cell. Water levels are usually measured weekly or monthly for the first

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FIGURE 4

Various monitoring well configurations for evaluating barrier performance (Gavaskaret al., 1998),

quarter, then quarterly thereafter. Contaminant concentrations are measured quar-terly and inorganic parameters are checked annually or biannually (Gavaskar et al.,2000). It is very important that low-flow purging and sampling methods are usedclose to the downgradient side of the reactive cell so localized velocity increasesdo not pull contaminants into the well without adequate treatment, leading to thefalse assumption that the system is not working. The parameters and analyticalprocedures are the same as those during site characterization (i.e., contaminants

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and their byproducts, field parameters, and inorganics such as sulfate, iron, chlo-ride, hardness, etc). Hydraulic head on the upgradient and downgradient sides ofthe reactive cell should also be monitored to detect a possible decrease in perme-ability (Warner et al., 1998).

Regardless of how much effort goes into planning and installing the permeablereactive barrier system, a contingency plan must be developed in case the systemfails to meet the established objectives. Plans may call for adding additional funnelwall length, increasing the thickness of the reactive cell, adding additional reactivecells either in sequence or in another gate cut through the funnel wall, or installinga groundwater extraction system to gain hydraulic control of the plume until thesystem can be upgraded or repaired.

ECONOMICS

The primary variables that will affect costs of a PRB system are the length of timethe medium will remain viable from a reactivity and hydraulics standpoint and thetype and frequency of maintenance. Because this technology has been in full usefor less than 8 years, the lack of sufficient historical information needed to makeaccurate predictions regarding longevity should be considered an uncertainty incost evaluations. Because of that uncertainty, it is recommended that multipleeconomic scenarios, based on varying longevity, be used to compare competingtechnologies. For example, the time until media replacement can be varied between5 and 30 years. The capital costs for a pump-and-treat system may often be lowerthan those for a PRB system, but over the life of the remedial action, the PRBtechnology will often be cheaper due to much lower operation and maintenancecosts. The longer it takes to remediate a plume, the greater the savings of the PRBsystem. An additional economic variable that must be factored into comparisonsis intangible costs, such as the ability to make full economic use of impactedproperty due to the absence of aboveground structures (Gavaskar et al., 2000). Unitcosts and total costs vary from one project to another, depending on site conditions.However, generalizations can be made that are applicable to most sites.

Capital Costs

The first costs to consider are preconstruction costs, including the preliminary siteassessment, site characterization, sample analysis, PRB modeling and design,procurement of materials and construction contractors, and regulatory review.Those costs can approach 50% of the total capital investment (Gavaskar et al.,2000).

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The next expense is that of the reactive medium. Granular zero valent iron hasbeen used for most permeable reactive barrier projects, because it is relativelyinexpensive and it has proven to be versatile. Depending on source, it costsapproximately $350 per ton (ITRC, 2001). The total cost for reactive mediadepends on the type and concentrations of contaminants being treated, theremediation objectives, and groundwater velocity, which affects residence time(Gavaskar et al., 1998).

The third major capital expense is that of installing the permeable reactive zoneand, where applicable, the funnel walls. Table 3 lists approximate 2000 unit costsfor the respective available technologies. The total cost for emplacement willdepend on plume and aquifer depth, plume width, and how difficult installation isdue to site geology (Gavaskar et al., 1998).

If zero-valent iron technology is used, a cost to consider is the license fee thatmust be paid to Envirometal Technology, Inc. (ETI), which holds a patent on thatprocess. The fee is up to 15% of material and construction costs (Gavaskar et al.,1998).

If the barrier is installed within the contaminant plume, disposal of excavatedspoils and water from dewatering operations can also be significant, depending onthe emplacement technique used. For example, the use of a bentonite-cement slurrywall would require disposal of all of the soil excavated from the trench. Soilcontaminated with chlorinated solvents may be determined to be listed RCRAhazardous waste. If so, its disposal cost per cubic yard could approach that of zerovalent iron.

Operation and Maintenance Costs

The major portion of operation and maintenance costs will consist of performancemonitoring expenses. The regulatory authority will most likely require samplingand analysis of groundwater and measurement of hydraulic conditions at leastquarterly. The monitoring will take place for as long as the upgradient concentra-tions exceed applicable standards, which may be many years to decades. The totalcost of monitoring will depend on the number of wells, parameters analyzed, andthe frequency of sampling (Gavaskar et al., 2000).

When monitoring indicates a decrease in system efficiency, such as failure tomeet established objectives or a decrease in permeability, the reactive medium willrequire replacement or regeneration. The amount of maintenance that zero valentiron PRB systems need is not well known, because the technology has only beenin full-scale use since 1994 (IBC, 1999). The systems that have been put into usehave required little maintenance. If precipitates accumulate so that hydraulicconductivity is decreased significantly or if reactivity decreases, it may be neces-sary to flush the reactive cell with reagents such as acid or replace the medium(ITRC, 2001).

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Closure

A final cost consideration involves closure and decommissioning of the remediationsystem. Some barriers, such as those using activated carbon or zeolite, designed toadsorb contaminants, may require removal after their useful life has expired toeliminate the potential for future desorption. In most cases, zero-valent iron bar-riers can be left in place unless long-term disruptions in groundwater flow causedby eventual plugging from oxidation would be problematic (ITRC, 2001).

LIMITATIONS OF REACTIVE BARRIER TECHNOLOGY

Most of the literature describing permeable reactive barriers focuses on the positiveattributes of this technology. While it is agreed that it has advantages over manyremediation methods, and it is a quite effective technology, some drawbacks havebeen observed. One limitation of the technology lies in the potentially long-timerequirements for remediation to be complete. Although the fact that it is passiveand does not require energy input after installation is attractive, as it relies on theslow dissolution and advective transport of contaminants in groundwater,remediation takes several years to decades to complete. It is often cheaper over thelong term but no faster than pump-and-treat. In situations such as real estatetransfers or where lawsuits are pending, a quicker solution may be needed. Thatdisadvantage is more pronounced in low-permeability aquifers such as those foundin glacial till. The reduction in permeability and reactivity due to precipitation ofinorganics such as carbonates and the oxidation of the iron (i.e., the formation ofrust) will eventually reduce the effectiveness of the metal reactive medium, requir-ing flushing with reagents and/or medium replacement. Such maintenance willgreatly add to the cost of the system. A drawback that represents a paradoxicalsituation for this technology is its relative newness. Since the first full-scale systemwas installed in 1994, not enough time has elapsed to evaluate its long-termeffectiveness. Site managers are reluctant to use it without more data on itseffectiveness, but the data cannot be generated until it is used more.

LESSONS LEARNED

Most failures of PRB systems have been due to incomplete capture of the plumeand/or failure to maintain an adequate residence time. For example, at one siteundetected heterogeneities in contaminant concentrations led to construction of aniron wall too thin to provide the residence time needed to treat the higher concen-trations. Those problems can be avoided by a thorough site characterization.Careful monitoring is the most recommended surveillance. Data can be used todevelop a trend analysis to detect a decrease in performance. Collecting core

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samples from the reactive cell is helpful for assessing the degree of corrosion in aniron PRB wall (ITRC, 2001).

CONCLUSIONS

The principal behind permeable reactive barriers involves strategically placing awall of medium or a bioactive zone in the path of a contaminant plume that willreact with the contaminant to destroy it, adsorb it, or cause it to precipitate out ofsolution. The medium must be appropriate for the contaminant, reactive enough sothat a large amount is not required, readily available at a reasonable cost, and itmust not produce undesirable byproducts. It must also be porous enough to allowpreferential flow through the reactive cell, yet have a large specific surface area tofacilitate good contact with contaminants. The most common application of thistechnology to date has involved the use of zero valent iron to remediate chlorinatedsolvents. That is apparently due to the high frequency of occurrence of chlorinatedsolvents in groundwater, the proven effectiveness of the iron to treat those com-pounds, and its availability at a reasonable cost. Research is continuing to exploitthe capabilities of other media. It is imperative that the site hydrogeology be verywell characterized and understood if this technology is to be successful. Ideally, theaquifer should be shallow, with a competent aquitard into which the treatment celland funnel walls can be anchored to prevent underflow, although hanging walls canbe effective where conditions allow. Where conditions are favorable and timefactors are appropriate, this technology appears promising. The main characteristicin its favor when compared with more conventional technologies is the lack of theneed to operate any pumps or treatment vessels, thereby saving operation andmaintenance costs and allowing full use of the surface of the affected property. Itsreliance on natural advective processes to move contaminants through the treat-ment zone, resulting in long treatment time frames, can be a disadvantage undersome circumstances. The potential for depletion of the media’s reactivity, coatingof media surfaces, and plugging by precipitates should be monitored closely toevaluate its long-term effectiveness. Regulators need to continue the trend towardbeing more receptive of this technology, as well as other innovative technologies,so that it can be improved. This receptiveness will benefit all stakeholders in-volved.

ACKNOWLEDGMENTS

The authors would like to thank the anonymous reviewers for their valuable andinsightful comments.

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