© 2001,

AAPG/DEG

, 1075-9565/00/$15.00/0Environmental Geosciences, Volume 8, Number 4, 2001 258–265

258

E N V I R O N M E N T A L G E O S C I E N C E S

Treatment of Contaminated Groundwater Using Permeable Reactive Barriers

JAMI STRIEGEL,* DEE ANN SANDERS,

‡

and JOHN N. VEENSTRA

‡

*

Roberts Schornick Associates, Tulsa, OK 74128

‡

Department of Civil and Environmental Engineering, Oklahoma State University, Stillwater, OK 74078

ABSTRACT

The permeable reactive barrier (PRB) is a relatively new technol-

ogy that can be a cost-effective and low-maintenance remedy

for a contaminated site. However, to use PRBs appropriately,

the remedial manager must understand the technology, geologi-

cal conditions of the site to be remediated, and the nature of the

contaminants. The U.S. Environmental Protection Agency

(USEPA) has sponsored research into PRBs, and the USEPA

and several state regulatory agencies have approved PRB reme-

dies at contaminated sites. This article succinctly presents the

background of PRB technology, guides the new remedial man-

ager through the process of determining if a PRB remedy is ap-

propriate to for given site, discusses the pros and cons of PRBs,

and outlines data requirements and guidelines of design for a

PRB remedy. Summaries of existing sites with PRBs are given,

along with a bibliography of government and environmental

journal references.

Key Words:

permeable reactive barrier (PRB), treatment wall,

groundwater remediation, passive remediation.

INTRODUCTION

Subsurface barriers are commonly used to restrict or con-trol the movement of contaminant plumes in groundwater.Such barriers are typically constructed of highly imperme-able materials, to eliminate the possibility that a contaminantplume can move toward and endanger drinking water wellsor discharge into surface waters. Recently, the concept of sub-surface barriers has been revised to develop a new ground-water remediation technology called permeable reactive bar-riers (PRBs). Rather than restricting the flow of ground-water,PRBs are designed to allow contaminated ground-water toflow through them. When the contaminated water passesthrough the PRB, contaminants are either immobilized orchemically transformed to a less toxic state by the reactivematerial contained within the barrier (USEPA, 1997a).

PRBs are emerging as a viable and cost-effective newtechnology. Numerous laboratory and field studies are cur-rently in progress to quantify specific procedures and long-

•

•

term maintenance requirements associated with the use ofPRBs. The technology has been shown to be effective for arange of contaminants, such as chlorinated solvents, chlori-nated hydrocarbons, other halogenated organic compounds,radionuclides, and metals (RTDF, 2000). Recent applica-tions have also included recovery of light, nonaqueousphase liquids (LNAPLs) (Testa and Winegardner, 2000).

This article examines the technology of PRBs, selectionand design of a PRB system, and regulatory and monitoringconcerns. Summaries of existing PRB systems, along withbrief design and cost data, are included. This study is ex-pected to provide the environmental professional or reme-dial manager with a basic understanding of the technologyand a list of sources for additional information.

PERMEABLE REACTIVEBARRIER TECHNOLOGY

A PRB is a zone of reactive material, placed in an aquifer,which passively degrades or immobilizes contaminants asgroundwater flows through it. PRBs are typically constructedperpendicular to the flow of groundwater (Figure 1).

Natural hydraulic gradients transport contaminants throughthe reactive media within the PRB. The reactive media fa-cilitates reactions that break down contaminants in theplume into harmless byproducts or form insoluble productsthat remain in the barrier as groundwater continues to flowthrough (USEPA, 1995, 1997b). Reactive media can be de-signed to degrade, sorb, precipitate, or otherwise removechlorinated solvents, metals, radionuclides, and other pollut-ants from groundwater. Contaminants will either be degradedor retained in a concentrated form by the barrier material.This type of treatment can permanently contain relativelybenign contaminant residues or provide a decreased volumeof the more toxic contaminants for subsequent treatment(Palmer, 1996; USEPA, 1997b). Good general references onPRB systems include Testa and Winegardner (2000) andGillham and Burris (1997).

Two basic PRB designs are currently being used in full-scale implementation. These designs are called the funnel-and-gate and the continuous trench, and are described asfollows (USEPA, 1998):

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•

A funnel-and-gate system (Figure 2a) consists of imper-meable sheet pilings or slurry walls placed slightly outof perpendicular to the direction of groundwater flow,such that the contaminated water is funneled throughthe gate containing the permeable reactive material.This design results in increased hydraulic gradientsthrough the reduced cross-sectional area of the gate,and the impermeable walls must be designed to preventcontaminated groundwater from flowing around thebarrier.

•

The continuous trench design (Figure 2b) consists of atrench that has been excavated and backfilled with re-active material sufficiently permeable to allow water topass through the barrier under its natural hydraulic gra-dient.

The required thickness of the reactive zone necessary toprovide adequate residence time must be calculated for ei-ther design. This calculation requires that information oncontaminant concentration, contaminant degradation rate inthe presence of the reactive substrate, and groundwater flowrate through the barrier be determined (USEPA, 1997a).

Funnel-and-gate barriers are more cost-effective for largeor deep plumes, due to the amount of reactive material re-quired for a continuous trench. Slurry walls, sheet piles, andother materials used to form the funnel are often easier andmore economical to install than the reactive wall. The num-ber of gates required for a funnel-and-gate system dependson site characteristics. The ratio of funnel areas to gate areasmust be balanced to achieve remedial objectives at leastcost. Plumes with a mixture of contaminants can be fun-neled through a gate with multiple reactive walls in series(Palmer, 1996).

PRB REACTIVE MATERIALS

The effectiveness of a variety of natural and synthetic ma-terials has been evaluated in PRB full-scale applications.

The materials that have shown the best success with a widerange of contaminants are zero-valent iron, calcium carbon-ate (limestone), and organic material (including activatedcarbon). Although all of these reactive materials are effec-tive individually, combinations of materials may prove to beoptimal for certain contamination situations. Design consid-erations in the selection of the PRB reactive material arediscussed in what follows.

Zero-Valent Iron

The majority of installed PRBs use elemental iron, usuallytermed “zero-valent iron,” as the reactive medium (USEPA,1998). Zero-valent iron (Fe

0

) donates the electrons neces-sary to reduce the contaminants, and becomes oxidized toFe

2

�

or Fe

3

�

. This ability can be exploited to remediatemetals that are more toxic and mobile in higher oxidationstates, such as Cr

6

�

(Evanko and Dzombak, 1997; USEPA,1997a). Zero-valent iron can dehalogenate hydrocarbonsand precipitate anions and oxyanions (USEPA, 1998).

Calcite/Limestone

Calcite [CaCO

3

(s)] is a common mineral phase in manylimestone aquifers. Groundwater in contact with calcite,typically in near equilibrium with this solid, is elevated inpH and contains significant concentrations of bicarbonateand carbonate ions. Metal concentrations in calcareousgroundwater are generally controlled by solubility or sorp-tion processes. For example, concentrations of metal ions incalcareous groundwater may be controlled by solubility

FIGURE 1: Permeable reactive barrier (USEPA, 1998).

FIGURE 2: Plume capture strategies (USEPA, 1998).

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E N V I R O N M E N T A L G E O S C I E N C E S

equilibria with metal carbonate, hydroxycarbonate, and hy-droxide solid phases, because many of these solids exhibitlow solubility and rapid precipitation–dissolution kinetics(Benjamin, 2002).

The use of limestone powder and/or gravel treatmentwalls has been proposed for sites with metals contamina-tion, in particular former lead acid battery recycling sitesthat have lead and acid contamination in groundwater andsoil. Neutralization of acidic groundwater and the subse-quent rise in pH promotes immobilization of any dissolvedlead through precipitation and/or adsorption onto minerals.Limestone has been shown to be effective at raising the pHof highly acidic groundwater (at pH 2.3 to 3.5) to 6.0 to 6.7(Evanko and Dzombak, 1997). Experience with this tech-nology, called “anoxic limestone drains,” is limited and alldata are empirical. However, results to date have beenlargely positive, especially when large (#3 to #4) limestonerocks are used. An additional caution is that raising pH doesnot always result in reduction of a metal contaminant. Somemetal salts, notably zinc hydroxides, are soluble at high pHvalues (Sawyer et al., 1994).

Organic Materials

Organic materials can be very effective, either individu-ally or as mixtures, at removing nitrate, sulfate, dissolvedorganics, and dissolved metals from groundwater. Two pro-cesses that appear to be potentially applicable to PRB tech-nology are sulfate reduction combined with metals precipi-tation, and direct sorption.

Naturally occurring organic matter may be a very impor-tant sorbent for metals in soils, sediments, and aquifers(Hem, 1992). In the presence of sulfate, dissolved metalscan be removed from groundwater as a result of chemicalreduction, catalyzed by sulfate-reducing bacteria (SRB),and the subsequent precipitation of sparingly soluble com-pounds. The chemical reduction process involves reductionof SO

42

�

to HS

�

using solid-phase organic carbon as a car-bon source. The organic carbon sources that have been usedinclude straw, hay, peat, wood, leaf mulch, mushroom com-post, and solid municipal waste. A successful PRB installa-tion in Ontario uses municipal compost, leaf compost, andwood chips to remove nickel from an acid mine drainage-contaminated aquifer (USEPA, 1998).

State of the Practice

The USEPA has conducted permeable barrier technologyresearch at a national level, and several regional officeshave supported pilot-scale testing. The Permeable ReactiveBarriers Action Team was established in March 1995 underthe Remediation Technologies Development Forum to ac-celerate the development of cost-effective PRB technolo-gies and promote public and regulatory acceptance of thistreatment option (USEPA, 1997b). Several reactive wall de-

signs and a variety of reactive materials have been evalu-ated in laboratory, pilot, and full-scale applications in theUnited States and Canada (Palmer, 1996; RTDF, 2000). Ta-ble 1 summarizes data from the 19 full-scale projects in-stalled as of December 2000 (RTDF, 2000).

Several advantages and disadvantages in utilizing PRBtechnology have been identified from the laboratory andfieldwork presented in USEPA (1997b) and RTDF (2000)reports. These advantages and disadvantages are summa-rized in Table 2.

SELECTION AND DESIGN OF APRB SYSTEM

Successful remediation of contaminated groundwater us-ing PRB technology requires data collection, evaluation ofthe contaminants, review of the processes capable of reme-diating the contaminants, input and concurrence of regula-tors, and design of a system specifically tailored to the siteand contaminant characteristics. In addition to the quality ofthe design, the quality of the construction techniques andproper maintenance of the system are crucial to the successof the remediation project.

Site Characterization

As with any groundwater remediation technology, a com-plete site characterization is necessary for successful designand installation of a PRB. The barrier design, location, con-struction methodology, and estimated life expectancy arebased on the site characterization information. As a result,the effectiveness of the entire remediation project dependson the quality of the data gathered during the site character-ization. At a minimum, the site data listed in Table 3 shouldbe gathered (USEPA, 1997a).

The PRB must be constructed such that the entire plumepasses through the reactive material and is adequately reme-diated. The design must account for fluctuations in ground-water flow directions, and the system must be designed totreat the entire plume with no underflow or bypass of thewall. The reactive material must be designed to reduce thecontaminant to concentration goals without rapidly plug-ging with precipitates or losing its reactivity. The residencetime of contaminated groundwater in the reactive wall andthe rate of contaminant degradation within the wall shouldbe balanced. Flow through the reactive material is maxi-mized to ensure all contaminated groundwater is captured,while residence time in the wall is maximized for adequatetreatment. Balancing these design parameters is difficult,because they are inversely related. Aquifer heterogeneitiesalso complicate the task of design. Flow and transport mod-eling may facilitate this task (Palmer, 1996; USEPA, 1997a).Achievement of these design criteria may require collectionof additional data following the initial site characterization.

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261

Installation

Several installation techniques are currently available forPRBs. Continuous trench barriers, which consist only of awall of reactive material, may be installed as follows (Palmer,1996; USEPA, 1998):

•

Excavation and backfill

. Excavation and backfill typi-cally involves digging an open trench, stabilizing thesidewalls, placing permeable reactive material as back-fill, and removing the stabilization materials. Sidewall

stabilization can be achieved using materials such as re-movable steel sheet piling and biodegradable polymerslurry.

•

Overlapping caissons

. Overlapping steel caissons aredriven or vibrated down to the desired depth, and thesoil within them is removed with an auger. The excava-tion is backfilled with reactive material, and the cais-sons are removed.

•

Soil mixing

. Large-diameter mixing augers are drilledinto the subsurface, and reactive material is injected

TABLE 1.

Summary of Full-Scale PRB Installations (Adapted from RTDF, 2000).

SiteDate

Installed Contaminant Media Summary

a

Cost

b

Aircraft maintenancefacility, OR

1998 TCE Zero-valentiron

Funnel-and-gate system with two gates, each 50 ftwide, and a 650 ft long funnel. Wall thickness is9 in; installation performed with continuous trencherand sheet piles

$ 600,000

Caldwell Trucking, NJ 1998 TCE Zero-valentiron

Continuous trench 50 ft deep; two 3-in walls, 150and 90 ft long; uses 250 T zero-valent iron; con-structed by hydrofracturing and permeation infilling

1,120,000

Federal HighwayAdministration facility,Lakewood, CO

1996 TCA; 1,1-DCE;TCE;

cis

-DCEZero-valentiron

Funnel-and-gate system with a 1040-ft funnel andfour gates, each 40 ft wide 1,000,000

Former dry cleaner,Rheine, Westphalia,Germany

1998 PCE; 1,2-DCE Zero-valentiron, ironsponge

Continuous wall constructed by mandrel method;wall varies in width from 2 to 3 ft and is 74 ft long.Media is 69 T of zero-valent iron in 33 ft of walland 85 T of iron sponge in 41 ft of wall

123,000

Former manufacturingsite, Fairfield, NJ

1998 1,1,1-TCA; PCE;TCE; DNAPL

Zero-valentiron

Continuous trench 127 ft wide, 25 ft deep, 5 ftthick; constructed with sheet piles 875,000

Industrial site, Coffeyville,KS

1996 TCE; 1,1,1-TCA Zero-valentiron

Funnel-and-gate system; permeable gate is 20 ftlong and 3 ft thick, filled with 70 T of zero-valentiron; funnel is two, 490-ft soil-bentonite walls;depth of system is 30 ft

400,000

Industrial site, NY 1997 TCE.

cis

-DCE,VC

Zero-valentiron

Continuous trench system using 742 T of zero-valentiron; wall is 370 ft long, 18 ft deep, and 1 ft thick 797,000

Industrial site, SC 1997 TCE.

cis

-DCE,VC

Zero-valentiron

Continuous trench system; wall is 325 ft long, 29 ftdeep, and 1 ft thick 400,000

Intersil semiconductorsite, Sunnyvale, CA

1995 TCE.

cis

-DCE,VC; Freon 113

Zero-valentiron

Funnel and gate construction with permeable zone4 ft wide, 36 ft long, and 20 ft deep; barrier is chargedwith 220 T of zero-valent iron

1,000,000

Kansas City plant, KansasCity, MO

1998 1,2-DCE; VC Zero-valentiron

Continuous trench 130 ft long with sheet piles drivento support the side walls. Wall is 4 ft thick, with mediamix designed for variable permeability of aquifer;8320 cu ft of iron were used

1,500,000

U.S. Coast Guard supportcenter, Elizabeth City, NJ

1996 Cr-VI; TCE Zero-valentiron

Continuous trench 150 ft long, 2 ft thick and 24 ftdeep; wall contains 450 T of iron 500,000

100 D Area, Hanford Site,WA

1997 Cr-VI Sodiumdithionate

Wall constructed by injecting sodium dithionateinto a series of five wells to a depth of 100 ft;treated zones overlap, creating barrier 150 ft longand 50 ft wide

480,000

Nickel Rim Mine site,Sudbury, Ontario, Canada

1995 Ni, Fe, sulfate Organiccarbon

Cut-and-fill method used to construct barrier 50 ftlong, 14 ft deep and 12 ft wide. Reactive material ismunicipal compost, leaf compost, and wood chips

30,000

Tonolli Superfund site,Nesquehonig, PA

1998 Pb, Cd, As, Zn,Cu

Limestone Continuous trench method used to construct a trench3 ft wide, 20 ft deep, and 1100 ft long; no otherdata available

Notavailable

Y-12 Site, Oak RidgeNational Laboratory, TN

1997 U, Tc, HNO3 Zero-valentiron

Funnel-and-gate system. Total system is approxi-mately 220 ft long, 25 ft deep and 2 ft thick; gatein middle of wall is 26 ft long; and filled with 80 Tof zero-valent iron

1,000,000

a

Summary information includes all data on dimensions given in the reference. Project citations are not all complete.

b

Cost data varies between projects and must be used with caution. See the original reference for complete details.

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E N V I R O N M E N T A L G E O S C I E N C E S

through the hollow stem. The native soil and the reac-tive material are completely mixed.

•

High-pressure jetting

. Jetting is an established practiceto inject grouts for construction purposes. The tech-nique is also being used to inject grouts to make imper-meable walls. The jetting nozzle can be rotated to formcolumns 3 to 7 ft in diameter, which can be filled withreactive material.

•

Vertical hydraulic fracturing

. Holes are bored to ini-tiate fracture in permeable sands. A gel containing thereactive media is pumped into the fractures.

Funnel-and-gate systems require construction of multiplecomponents. Gates, which contain the permeable reactivematerial, are installed as just described for continuoustrenches. The impermeable funnel sections can be con-structed using sheet piling, slurry walls, grout curtains, orother typical subsurface barrier techniques.

Regulatory Concerns, Compliance, and Performance Monitoring

The USEPA has funded research in PRB technologies forseveral years and recognizes that the technology has the po-tential to effectively remediate contaminated groundwater(USEPA, 1998). Thirteen state environmental agencies havealso approved installation of PRB systems (USEPA, 1998).The advocacy of regulatory agencies makes PRB technol-ogy an especially attractive remedy that a groundwater pro-fessional should consider when designing a groundwatertreatment system.

PRB systems installed under compliance orders from theUSEPA or delegated state agencies are required to have aQuality Assurance Project Plan (QAPP) developed and ac-cepted prior to any groundwater sampling. The QAPP willspecify monitoring necessary for regulatory compliance andevaluation of treatment performance. When locating thewells, selecting the screen lengths, and designing other aspectsof the monitoring well system, the sampling program objec-tives and site conditions should be carefully considered. Theplans for eventual site closure should also be developedprior to design of the PRB (Testa and Winegardner, 2000).

In general, several monitoring wells should be installed todetermine the following (USEPA, 1997a):

•

If regulatory goals are being achieved.

•

If contaminant breakthrough occurs, and if so, how it ischaracterized.

•

If the contaminant is flowing around the wall.

Monitoring wells should typically be located as follows(Powell, 2000; USEPA, 1997a):

•

Immediately downgradient of the reactive zone dis-charge.

•

At each end of the wall.

•

Below the wall.

•

Upgradient of the wall.

Compliance monitoring determines whether regulatorycontaminant concentration requirements are being met. Typ-ically, the compliance monitoring criteria will be set by thestate where the site is located. Normal compliance monitor-ing parameters include the following (Powell, 2000; USEPA,1997a):

•

Contaminants of interest.

•

Potential contaminant daughter (degradation) products.

•

Water quality parameters such as pH, alkalinity, spe-cific conductance, and temperature.

In addition to the contaminants, their products, and theroutine water quality parameters just listed, performance

TABLE 3.

Required site data (adapted from USEPA, 1997b).

Contaminant Properties Aquifer Properties

Source locationChemical and physical

Subsurface geology, includingconfining layers and fractures

properties Direction of groundwater flowContaminant distribution

(depth and areal extent)Hyraulic gradientSoil permeability—both horizontal

Plume location-depth togroundwater, overallnative groundwaterquality, risk, etc.

and vertical heterogeneity, significant permabilitycontrasts

Hydrologic changes with timeConcentration Stratigraphic variations in per-

meabilityAqueous geochemistry

TABLE 2.

Advantages and disadvantages of PRBs (adapted from RTDF, 2000 USEPA, 1997b).

Advantages Disadvantages

Contaminants are remediatedin situ

No surface structures are

Application currently limitedto shallow plumes (

�

50 ftdeep)

required except monitoringwells, but contaminant tobe remediated should befactored into design

Remediation is passive; no

Metals can be concentrated inwall and if reactive materialnot removed metals can bereleased if groundwaterchemistry changes in future

input of energy is requiredExact source definition not re-

Plume must be very welldelineated and characterized.

quired for plume remediationGroundwater flow pattern is

Long-term monitoring data arelimited

only minimally alteredPotential for transfering con-

Field data on longevity ofmedia are limited

taminants to other aquifersor other media is minimal

Discharge of treated ground-

Field data on loss ofpermeability are limited

Control of pH may be requiredwater is avoided,minimizing technical andregulatory concerns

Site specific constraints mayexist, such as subsurfaceutilities, inaccessibility, etc

Biological activity may limitwall effectivess andchemical precipitate plug

S T R I E G E L E T A L . : G R O U N D W A T E R T R E A T M E N T

263

monitoring of permeable reactive barriers should be per-formed. Performance monitoring is to assure that the reac-tive barrier is functioning as designed and to allow evalua-tion of whether it continues to function properly over time.Changes in the monitored parameters should alert the inves-tigator to potential problems with the barrier. Performancemonitoring parameters are discussed extensively in USEPAdocuments (USEPA, 1997a, 1998) and in Powell (2000),and include the following:

•

Hydrologic parameters, including baseline andchanges over time.

•

Hydraulic head measurements.

•

Tracer studies.

•

In situ flowmetering.

•

Measurement of accumulation of precipitates onthe iron surfaces, and the rate of buildup.

•

Scanning electron microscopy.

•

X-ray photoelectron spectroscopy.

•

Oxidation/reduction potential, Eh.

•

Dissolved oxygen.

•

Ferrous iron.

Knowledge of these parameters helps confirm properconstruction, as well as to address and detect the following(USEPA, 1997a):

•

Loss of reactivity.

•

Decrease in permeability.

•

Decrease in reaction zone residence time.

•

Short circuiting of the reactive zone.

•

Funnel wall leakage.

Increases in the hydraulic head across the barrier could indi-cate that plugging is occurring. Coring the barrier and evaluat-ing the buildup of precipitates on the iron surfaces could con-firm this suspicion, although multiple cores can be required tointercept the impacted area. When functioning properly, Eh inthe iron barrier should be low, oxygen should be undetectable,and ferrous iron should be present (Powell, 2000).

Costs

In situ remediation technologies have the potential to pro-vide significant cost savings over ex situ techniques becausethey eliminate the need to excavate and dispose of contami-nated solids or to pump and treat contaminated groundwa-ter. Traditional technologies, such as pump and treat, re-quire an external energy source and associated costs arehigh. In spite of these costs, subsurface residuals frequentlyremain at undesirable levels (USEPA, 1995). Based on theshortcomings of currently available technologies, the use ofPRBs for effective, low-cost, passive remediation of metalcontamination in groundwater is expected to increase(Evanko and Dzombak, 1997).

Although little cost information is currently availablefor PRBs, this new technology appears to be more cost-ef-fective than traditional technologies. The PRB ActionTeam compared PRB systems with conventional pumpand treat systems, and found that PRB systems cost$500,000, not including monitoring costs, versus $7 mil-lion for pump and treat (RTDF, 1997). No details concern-ing the volume or reactive materials utilized for this PRBwere provided. It is also not clear if these costs are totallife-cycle costs.

The case studies summarized in Table 1 show the costdata available as of the end of 2000. It should be noted thatcost data have different bases and cannot be used or com-pared directly. The data listed should be used only for pre-liminary evaluation of costs. More specific information canbe found by reviewing the full RTDF project summaries(RTDF, 2000).

Cost data can also be developed for specific PRB projectsusing unit cost information and traditional cost estimatingtechniques, as follows:

•

Slurry walls and permeable barriers are listed in an esti-mating manual published by the R.S. Means Co.(ECHOS, 1998). The tables contained in this resourcelist the following: (1) a description for each construc-tion component; (2) the base units of measure for thetask; and (3) the unit costs for labor, equipment, andmaterial associated with the task. For slurry walls andpermeable barriers, the tables list line items such as“Normal Soil, to 25

�

, Slurry Wall Excavation” and“Iron Filings,” which represent excavation of soil for aslurry wall and placement of iron filings in an excava-tion. The total unit costs, or cost per cubic yard, listedfor these two items are $1.91 and $1788.00, respec-tively.

•

A detailed cost estimate can be prepared for PRB instal-lation by listing all construction materials and tasks re-quired, listing unit costs for each of these line items,totaling the unit costs, and then multiplying the totalunit cost by the total volume of the PRB. For example,filling a trench with iron filings would cost $1788.64per cubic yard (cy), including labor, equipment, andmaterials costs. Assuming the trench has an approxi-mate volume of 10 cy, the total cost to acquire and placeiron filings in the trench would be ($1788.00/cy)

�

(10 cy)

�

$17,880.00.

•

Hourly output, or the number of cubic yards of materialmoved per hour, is also listed in the tables. Total unitcost can be multiplied by the estimated hourly output tocalculate the cost per man-hour to construct the PRB.

•

Additional detail can be added to the cost estimate, us-ing ER data to define crew, equipment, location, andsafety level requirements.

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E N V I R O N M E N T A L G E O S C I E N C E S

For large PRB projects, it may be useful to prepare a de-tailed cost estimate using unit cost tables, and compare itto costs of actual projects, as listed earlier. This compari-son should provide an order-of-magnitude check for theestimate.

CONCLUSIONS

Despite the relative newness of the technology, PRB sys-tems are generating a great deal of interest on the part ofgroundwater professionals and regulatory officials. A re-view of installed PRB systems shows the following conclu-sions about the technology:

•

PRBs have a favorable cost/benefit ratio compared totraditional (mostly pump-and-treat) systems used forgroundwater remediation. This is true for both capitaland operation and maintenance costs.

•

PRBs effectively mitigate the spread of contamination,even when source definition is not precisely available.Precise definition of the plume is required for effectiveremediation.

•

Of the two types of PRB systems, the funnel-and-gatesystem shows the greatest promise for cost-effective re-mediation of plumes.

•

PRBs cause limited disruption of groundwater flow re-gimes; the funnel-and-gate system does cause moredisruption to groundwater flow than does the continu-ous wall.

•

At this time, PRBs can remediate only shallow (

�

50 ftBGS) plumes.

•

Operational data on PRB systems are limited, becausefew systems have been in place for more than a fewyears. Potential operational problems include plugging,fouling, or exhaustion of the reactive material.

•

PRB research efforts are continuing at various levels,including local and national government, academic, andprivate industry.

•

Cost estimating methods for this technology are cur-rently somewhat crude, but should improve as addi-tional literature is published documenting actual designand installation costs.

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•

•

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S T R I E G E L E T A L . : G R O U N D W A T E R T R E A T M E N T 265

ABOUT THE AUTHORS •Dee Ann Sanders

Dee Ann Sanders is an associate profes-

sor of Civil and Environmental Engineer-

ing at Oklahoma State University. Over a

twenty-five-year career, she has worked for

civil and environmental consulting firms,

the U.S. Environmental Protection Agency,

and the U.S. Air Force. Dr. Sanders’ inter-

ests lie in environmental law, infrastruc-

ture, and the application of environmental engineering principles to

the problems of the domestic petroleum industry. She is a registered

professional engineer in the state of Texas.

Jami A. StriegelJami A. Striegel is an environmental en-

gineer with Atkins Benham, Inc., Environ-

mental Division, Tulsa, Oklahoma. She has

worked in the area of site remediation since

graduating from Oklahoma State Univer-

sity in 1998. She is a registered profes-

sional engineer in Oklahoma.

John N. VeenstraJohn N. Veenstra is professor of Civil

and Environmental Engineering and coor-

dinator of the graduate program in environ-

mental engineering at Oklahoma State Uni-

versity. Professor Veenstra is a registered

professional engineer in both Iowa and

Oklahoma. His areas of research interest

focus on water treatment operations, reme-

diation of soils contaminated with heavy metals, environmental

cleanup of hydrocarbons, and sludge dewatering processes.