Performance Evaluation of aPermeable Reactive Barrier UsingReaction Products as TracersS T A N M O R R I S O N *

Environmental Sciences Laboratory, 2597 B 3/4 Road,Grand Junction, Colorado 81503

A method incorporating laboratory analysis of constituentsthat formed as reaction products was developed andused to determine the flux of groundwater through azerovalent iron-based permeable reactive barrier (PRB)installed to treat U-contaminated groundwater. Concentrationsof three nonvolatile constituents (Ca, U, and V) thatformed as reaction products in the PRB were analyzed in279 samples. Areal distributions of the reaction productsindicate that groundwater flowed through all portions of thePRB and that nearly the entire volume of reactive materialis treating the groundwater. Almost 9 t of calciumcarbonate precipitated in the PRB during the first 2.7 yrof operation, but only 24 kg of combined U- and V-bearingminerals precipitated during the same period. Concentrationgradients of Ca, U, and V dissolved in the groundwaterindicate that a hydraulically upgradient portion of the PRBlost some reactivity during the first 2.7 yr of operation.Calculations that partially couple porosity changes to ZVIreactivity suggest that loss of reactivity may be morelimiting than porosity reduction for long-term performanceof the PRB. Calculations using groundwater concentrationgradients and solid-phase concentrations indicate that themean groundwater flux ranged from 11 to 24 L/min,considerably less than the design flux of 185 L/min. Fluxvalues calculated with all three constituents were in goodagreement. This method provides a more accuratedetermination of groundwater flux than is possible withflow sensor measurements, dissolved tracers, or Darcy’slaw computations.

IntroductionWithdrawals of groundwater in the United States increasedfrom 26 to more than 75 billion gal/d from 1950 to 1995 (1).More than 20,000 sites nationwide have groundwater con-tamination (1). Few sites with contaminated groundwater,particularly those with inorganic contamination, have beenremediated to regulated concentration standards largelybecause of the high costs using existing technologies (2-5).Because of the high cost of groundwater cleanup, the U.S.Environmental Protection Agency advocated the develop-ment of cost-effective innovative technologies (6). To helpreduce costs, a passive remediation technology termedpermeable reactive barrier (PRB) was introduced in 1991 ata field demonstration site at the Canadian Forces Base Bordenin Ontario (7). Since 1991, more than 70 PRBs have beeninstalled at sites with contaminated groundwater (8).

A PRB is an engineered zone of reactive material placedin the subsurface that causes destruction or containment ofcontaminants dissolved in the groundwater flowing throughit. Most PRBs use zerovalent iron (ZVI) as the reactive material.ZVI degrades chlorinated solvents (9) and stabilizes metalsand radionuclides (10, 11). The corrosion of ZVI causeschanges in the hydrogen ion and electron potentials in thetreated groundwater. These changes trigger reactions withdissolved contaminants, causing their destruction or con-tainment by immobile mineral phases.

Analytical results of samples from PRBs that have beenin place for several years generally indicate that concentra-tions of contaminants leaving PRBs remain low. However,the flow regime through PRBs is difficult to quantify, andestimates of the groundwater flux are usually imprecise.Groundwater flux measurements are typically based onDarcy’s law using water table gradient and hydraulicconductivity estimates, results from downhole flow velocitymeters, or dissolved tracer tests. Often, these methods provideconflicting results. In comparisons at the same measurementstation, Wilson et al. (12) observed that three different typesof downhole flow velocity meters rarely provided the samevelocities and flow directions. Measurements with flowmeters indicate that the flow velocity within boreholes isheterogeneous. Sharp gradients and uncertainties in thehydraulic conductivity field and water table gradients makeapplication of Darcy’s law imprecise. Dissolved tracers havebeen used to define local flow velocities. Because they canbe deployed at only a few widely spaced locations to avoidoverlap of the tracer plumes, large uncertainties exist in usingthese data to determine the mean flux of groundwater througha PRB.

A PRB at Monticello, UT, has operated for 2.7 yr to removeU, V, and other contaminants associated with an abandoneduranium ore processing mill (13). An evaluation of columnstudies and field data from the Monticello site indicates thatas ZVI irreversibly corrodes, pE values decrease, pH valuesincrease, carbonate minerals precipitate, iron redistributes,and U and V are removed from solution (14). Because someelements remain in the PRB as the groundwater passesthrough, they can be used as tracers to indicate former flowpaths. If concentration gradients in the groundwater are alsoknown, a mean groundwater flux can be determined. Thepurpose of this study was to develop a method to determinethe mean flux of groundwater through the Monticello PRBand to establish if the groundwater flow was evenly distrib-uted or preferred certain paths. Data used to accomplishthis goal include results of laboratory analyses of 279 PRBcore samples for Ca, U, and V concentrations and 10groundwater sampling events during the initial 2.7 yr ofoperation.

Field Site Description. The Monticello PRB was installedin June 1999 to remediate groundwater contaminated byseepage from uranium mill tailings (13). Sheet piling con-struction was used to install the PRB; two impermeable wingwalls were constructed of soil-bentonite slurry. The slurrywalls are 29.6 and 73.2 m long, and the PRB is 31.4 m inlength (Figure 1). The three parallel zones of the PRB arecomposed of, from upgradient to downgradient, 0.6 m of1.3-cm gravel mixed with 13 vol % 0.85-4.75-mm ZVI (gravel/ZVI zone), 1.2 m of 0.85-2.36-mm ZVI (ZVI zone), and 0.6m of 1.3-cm gravel (Figure 1). Both the PRB and the slurrywalls are keyed into impermeable shale bedrock at depthsof 3.4-4 m. The system nearly spans the width of an alluvialvalley, but small gaps that were left at each end of the slurrywalls because of landowner concerns permit some of the

* Phone: (970)248-6373; fax: (970)248-7628; e-mail:stan.morrison@gjo.doe.gov.

Environ. Sci. Technol. 2003, 37, 2302-2309

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groundwater to bypass the PRB. Morrison et al. (13) provideadditional details of the construction.

Methods SectionField Study. Seventy vertical 4.1-cm-diameter cores wereobtained in February 2002 by pushing a core barrel into thesubsurface with percussion using a model 8MR1 Geoprobe.Core recovery averaged 57% ( 3%; uncertainty nomenclatureused throughout this paper indicates (2 standard errors (SE).The range defined by 2 SE provides approximately a 95%confidence of containing the true mean. Core recovery wasapproximately the same in the gravel/ZVI zone as in the ZVIzone. The recovered portions of the cores were intact andshowed little disruption from the drilling vibrations. Thereason for less than 100% recovery was likely because of thebuildup of friction and the inability of the equipment to pushthe core up the full length (1.2 m) of the core barrel. Eachcore collected from the saturated zone was sectioned into15.2-cm lengths with a hand saw and placed in plasticzippered bags for transport to the laboratory; 614 sampleswere collected.

Groundwater samples were collected 10 times at nearlyequal time intervals since installation of the PRB in June1999. The wells are 2.5 cm diameter with 1.5-m-long wellscreens made of poly(vinyl chloride) positioned at the bottomof the alluvium or reactive media. Samples were collectedafter purging 1 L (about 1 bore volume) with a peristalticpump and pH values had stabilized. Measurements of pHvalues were made in a flow-through cell. Samples for analysisof Ca, U, and V were filtered (0.45-µm filter) into plasticlaboratory bottles and preserved with nitric acid at a pHvalue of less than 2.

Sampling Strategy. Random sampling within segments(15) was used to select transect locations for coring. Thissampling plan combines aspects of simple random samplingbut ensures that sample locations are spread along the entirelength of the PRB. A transect line trending perpendicular tothe front edge of the PRB was located randomly within eachof 10 equally proportioned segments of the PRB. Cores werecollected at six random locations along each transect line,four in the ZVI zone and two in the gravel/ZVI zone. Sampleswere not collected at 5 of these 60 locations because of drillingproblems. Fifteen additional cores were collected at inter-mediate locations throughout the area of the PRB for a totalof 70 core holes (Figure 2). Four samples from each core hole

were selected randomly from the population of 15.2-cm-long samples and were digested and analyzed for Ca, U, andV concentrations.

Core Sample Preparation and Analysis. The contents ofthe plastic bags were placed in aluminum pans and dried ina convection oven at 105 °C. Weights were determined towithin 0.1 g before and after drying to measure moisturecontents. Average moisture contents of the gravel/ZVI andZVI zones were 9.5 and 15.6%, respectively; these values donot represent in situ moisture contents because water waslost during field sampling. Analytical results of seven samplesfrom the upper portion of three cores in the ZVI zone wereomitted from the data because their low moisture contents,ranging from 0.57 to 4.45%, indicated that they were fromthe unsaturated zone. Dry weight density was determinedusing a constant volume of 201 cm3 for the 15.2-cm-longcore samples. Three samples of parent (material prior to usein the PRB) gravel/ZVI and three samples of parent ZVI wereprocessed in the same manner as the core samples.

Half-gram splits of 279 dried core samples were digestedwith 10 mL of concentrated nitric acid in a microwave ovenat a power level of 1000 W with Method 3051 (16). One ofevery 20 samples was digested in duplicate. Method 3051digested more than 90 wt % of the ZVI and removed all theCa, U, and V. Several samples were tested with a totaldigestion method that involved five sequential steps withhot concentrated acids to confirm the total removal of Ca,U, and V concentrations. Nitric acid microwave digestionswere used instead of total digestions because of the highercost of the total digestion method and the equivalent removalof Ca, U, and V.

Semiquantitative values of carbonate concentrations indried samples were determined by measuring carbon dioxidegas emission after treating with hydrochloric acid. One gramof sample was placed in a 60-mL plastic syringe, and 10 mL

FIGURE 1. Schematic of Monticello PRB and slurry walls.Reproduced with permission from Academic Press (13).

FIGURE 2. Core and well locations. Width scale is expanded bya factor of 10.

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of 5% hydrochloric acid was introduced via a second syringe.Carbon dioxide pushes the syringe out against atmosphericpressure. The distance the syringe is displaced is a measureof the carbonate concentration in the sample. The systemwas calibrated with a variety of calcite masses, and thecalibration curve was used to convert values of syringedisplacement to carbonate content in the PRB samples. Themethod has a detection limit of about 0.5% calcite.

Uranium concentrations in the digestates were analyzedby inductively coupled plasma mass spectrometry withMethod 6020 (16); Ca and V were analyzed by inductivelycoupled plasma atomic emission spectrometry with Method6010B (16). Preserved groundwater samples were analyzedwith these same methods.

ResultsParameter Estimation. Mean density was estimated fromfrequency distributions after excluding outliers that are morethan 2 standard deviations (SD) from the mean (Figure 3).Outliers accounted for 5 and 6% of data collected in thegravel/ZVI zone and the ZVI zone, respectively, and resultedfrom imprecise cutting of the core in the field. Mean densitiesare 1.851 ( 0.022 g/cm3 for cores from the gravel/ZVI zoneand 2.238 ( 0.018 g/cm3 for cores from the ZVI zone. Thedensity values measured in the PRB are similar to laboratory-determined values on parent material: 1.74 g/cm3 for gravel/ZVI and 2.21 g/cm3 for ZVI.

Saturated thickness is estimated at 2.01 ( 0.07 m fromfrequency distributions of water levels in 40 monitor wellsaveraged over five evenly spaced sampling events thatrepresent the variation in water table elevations because ofseasonal effects. On the basis of as-built geometry, the totalvolumes (Vt) of the saturated gravel/ZVI and ZVI are 37.9and 75.7 m3, respectively. On the basis of the mean densities,the masses (dry weight basis) of the gravel/ZVI and ZVI zonesare 70.2 and 169.4 t, respectively. Mean concentrations ofCa, U, and V measured in three parent gravel/ZVI samplesare 2371, 0.22, and 34.7 mg/kg, respectively. In the parentZVI, the mean concentrations of Ca, U, and V are 21.0, 0.06,and 60.7 mg/kg, respectively. Laboratory-determined effec-tive porosity values are 42% for gravel/ZVI and 70% for ZVI.Hydraulic conductivity of the parent ZVI is 0.062 cm/s asdetermined from falling head permeameter testing. Hydraulicconductivity of the alluvial aquifer is about 0.013 cm/s basedon pump testing.

Spatial Distributions. Spatial distributions were examinedon contour maps using means of the concentrations insamples from four random depths in each core hole. Meanconcentrations of samples from individual cores from thegravel/ZVI zone averaged from 15.1 to 46.8 g/kg of Ca, from70 to 596.9 mg/kg of U, and from 30.2 to 1168.3 mg/kg of V.These concentrations are relatively evenly distributed along

the full length of the PRB (Figures 4-6). Bivariant plots (notpresented) of Ca, U, or V concentrations in the gravel/ZVIzone indicate no relationship between concentrations anddistance from the hydraulically upgradient front of the gravel/ZVI zone. However, concentrations of Ca, U, or V in samplesfrom a single core vary by as much as a factor of 10. Bivariantplots (not presented) of Ca, U, and V concentrations indicatea lack of correlation with depth.

Mean concentrations of Ca in core samples from the ZVIzone range from 0.8 to 33.5 g/kg (Figure 4). Uraniumconcentrations are near those of the parent material (0.06mg/kg) in samples from throughout the ZVI zone, except intwo cores collected less than 0.25 m from the contact withthe gravel/ZVI zone (Figure 5). The highest concentration ofU in samples from the ZVI zone is 10.5 mg/kg, which issignificantly less than concentrations in most samples fromthe gravel/ZVI zone. Vanadium concentrations are near theconcentration in the parent sample (60.7 mg/kg) in samplesfrom throughout the ZVI zone (Figure 6).

Concentrations of Ca, U, and V in the PRB. Theconcentrations of Ca, U, and V are log-normally distributedin samples collected in the PRB. The frequency distributionsof the logarithmic concentrations are exemplified by U (Figure7). The geometric means (Cs) of concentrations in samplesfrom the gravel/ZVI zone are 22.7 g/kg of Ca, 162 mg/kg ofU, and 107 mg/kg of V (Table 1). The coefficient of variation(η) indicates the relative spread of the data distribution. Asindicated by a smaller η, Ca has a tighter distribution thanU or V in samples from the gravel/ZVI zone.

Mean values for U and V concentrations in samples fromthe ZVI zone were not calculated because they are close tothe concentrations in samples from the parent ZVI. Thegeometric mean of Ca concentration in samples from theZVI zone is 11.0 g/kg (Table 1).

Concentrations of Ca, U, and V in Groundwater. Thegroundwater concentrations of Ca, U, and V (Cw) in samples

FIGURE 3. Frequency distributions of dry weight density.

FIGURE 4. Contour map of means of solid-phase Ca concentrationsin samples from four random depths (g/kg).

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collected hydraulically upgradient of the gravel/ZVI zone forthe 2.7-yr period was estimated by averaging concentrationsfrom the 10 nearly evenly spaced time intervals; Figure 2

presents well locations. The concentrations used for eachtime (Ci

t) are the means of the concentrations in samplesfrom the five upgradient wells (i):

Similarly, mean concentrations in samples collected hy-draulically downgradient of the gravel/ZVI zone were cal-culated from 10 sampling events in 5 downgradient wells.The upgradient concentrations used for the ZVI zone areequivalent to the downgradient concentrations used for thegravel/ZVI zone. The downgradient concentrations for theZVI zone are means from the 10 sampling events using themeans of samples from the 10 downgradient wells shown inFigure 2. Concentration gradients (∆Cw) are the differencesbetween the mean upgradient concentrations and the meandowngradient concentrations in each of the two zones; thestandard errors were calculated from the means for eachtime period (Table 2).

DiscussionData collected during this investigation were used to

(i) evaluate mass-balance evidence for determining reac-tion mechanisms,

(ii) evaluate the tendency for groundwater flow to bypasssome areas of the PRB,

(iii) determine the mass and volume of mineral precipi-tation that has accumulated in the PRB in 2.7 yr,

(iv) determine the loss of porosity caused by mineralprecipitation and potential for loss of permeability, and

FIGURE 5. Contour map of means of solid-phase U concentrationsin samples from four random depths (mg/kg).

FIGURE 6. Contour map of means of solid-phase V concentrationsin samples from four random depths (mg/kg).

FIGURE 7. Frequency distributions of log U concentrations.

TABLE 1. Geometric Means (Cs) ( 2 Standard Errors (SE) andCoefficients of Variation (η) for Ca, U, and V Concentrationsin Core Samplesa

constituent -2 SE Cs +2 SE η

Gravel/ZVI ZoneCa (g/kg) 20.3 22.7 25.2 0.054U (mg/kg) 124 162 212 0.288V (mg/kg) 83 107 138 0.291

ZVI Zoneb

Ca (g/kg) 9.25 11.0 13.0 0.117a Concentrations in the parent materials were subtracted, so the

values represent the concentrations that resulted from precipitationfrom groundwater. b U and V are not presented because of the largenumber of nondetectable values.

Cw ) ∑t)1

10(∑i)1

5

Cit

5)

10(1)

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(v) provide a preliminary assessment of PRB longevity.Calcium, U, and V were selected for this study because theyare nonvolatile and their concentrations decreased signifi-cantly in groundwater as it flowed through the PRB.Concentrations of U and V are useful for this method ingroundwater systems in which they are contaminants.Calcium (and CO2) is useful for evaluations of most ZVI-based PRBs, including those used to treat organic contami-nants, because Ca is present in all groundwater systems.

The methodology used in the study is easy to implement,and the results are more accurate than commonly usedmethods (e.g., tracers and flow sensors) for evaluatinggroundwater flow direction and flux through PRBs. Becausethe constituents are nonvolatile and do not degrade, solid-phase samples can be collected at one time without theaddition of preservatives and analyzed later.

Reaction Mechanisms. Two mechanisms have beensuggested for U uptake by ZVI: reductive precipitation (14,17, 18) and adsorption on oxidative reaction products (19).The reddish-orange color of some of the cores collected fromthe gravel/ZVI zone indicates that some ZVI has oxidized. Acalculation was made to determine if sufficient amorphousferric oxyhydroxide (AFO) adsorption sites may have beenavailable to account for the observed concentration of solid-phase U. The mean concentration of U in the gravel/ZVIzone (162 mg/kg) when normalized to the concentration ofFe in the gravel/ZVI zone (0.00022 mol of U/mol of Fe) isabout half the maximum possible adsorption density (0.0004mol of U/mol of Fe) for U on AFO; adsorption data fromMorrison et al. (20) were used to calculate the maximum Uadsorption density at the dissolved carbonate concentrationsand pH values of the Monticello groundwater samples. Themaximum adsorption density is possible only if all the ZVIin the gravel/ZVI zone has been oxidized to AFO; visualinspections and results of qualitative magnetic separationsof core samples indicate that much of the original quantityof ZVI is still present in samples from the gravel/ZVI zone.A better quantification of the amount of AFO is needed toconfirm that adsorption is a viable mechanism.

Relative Reaction Rates and Preferential Flow. All the Vand 99.2% of the U concentrations in the PRB were depositedin the gravel/ZVI zone, but substantial quantities of Ca weredeposited in both gravel/ZVI and ZVI zones. This distributionsuggests that the uptake reactions for U and V are rapidrelative to those for Ca, a finding that is consistent with resultsof column experiments (14).

Tracer tests conducted at the Monticello PRB indicateheterogeneity that caused preferential flow on a local(decimeter to meter) scale (21). However, the spatial dis-tributions of Ca, U, and V concentrations in the solid phasessuggest that groundwater has moved through the PRBrelatively uniformly along its length. Flow has not bypassedany substantial portion of the PRB. Thus, nearly all the ZVIis being utilized to remove contaminants.

Despite the large mass of calcium carbonate deposited inthe PRB, the cores appear to have substantial permeability.Water flowed freely from the cores during extraction fromthe sample tubes. No hardpan was encountered in the PRB,indicating that calcium carbonate had not completelycemented any portions of the PRB.

Total Masses and Volumes of Minerals Deposited. Totalmasses of Ca, U, or V deposited in the PRB solids (Ms) duringthe first 2.7 yr of operation are calculated as the product ofthe mean solid-phase concentration in the gravel/ZVI or ZVIzone (Cs) and the mean dry weight mass of the ZVI in thezone (Mz) (Table 3):

Calcium occurs in carbonate minerals (CaCO3) as indi-cated by approximately stoichiometric amounts of Ca andCO2 and by mineral composition determined with an electronmicroprobe on four samples. Electron microprobe analysisidentified calcium carbonate in ZVI corrosion rims in all foursamples. Calcium carbonate minerals were identified bystoichiometric concentrations of calcium and, in some grains,by a distinct radiating crystal fabric characteristic of aragonite.The mineralogy of U and V was not determined.

To estimate the mass and volume of mineral matter thatwas deposited, Ca, U, and V were assumed to deposit ascalcite (CaCO3), uraninite (UO2), and vanadium trioxide (V2O3)with mineral densities of 2.71, 8.0, and 4.87 kg/L, respectively(densities from refs 22 and 23). Calcite deposited in the PRBduring the 2.7-yr period has a mass of 8.8 t and occupies 3.2m3 (Table 4). The volume of calcite deposited in the gravel/ZVI zone (1.5 m3) represents 9.3% of the pore space availableat the time of installation (15.9 m3). The volume occupiedby U and V minerals (1.6 and 2.3 L, respectively) is minor ascompared to that occupied by calcite. The volume of calcitedeposited in the ZVI zone (1.7 m3) represents 3.2% of theinitial pore space (53.0 m3).

Groundwater Flux. Unfiltered samples were collectedwith the filtered (0.45 µm) samples from five downgradientwells in one sampling event (July 2002). Concentrations ofCa and alkalinity in the unfiltered samples were similar toconcentrations in the filtered samples, indicating that noparticulates containing these constituents were exiting thePRB (Table 5). Similarly, loss of U and V by particulatetransport was insignificant. Calcium, U, and V do not volatilizeand, therefore, are conserved within the chemical system.Thus, any mass removed from the groundwater must reside

TABLE 2. Geometric Means ( 2 SE and Coefficients ofVariation (η) for Ca, U, and V Groundwater ConcentrationGradients (∆Cw) across Gravel/ZVI and ZVI Zones

constituent -2 SE mean ∆Cw +2 SE η

Gravel/ZVI ZoneCa (mg/L) 16.6 47.0 77.4 1.020U (µg/L) 241 334 427 0.442V (µg/L) 300 330 360 0.141

ZVI Zonea

Ca (mg/L) 85.0 118.4 152.0 0.445a U and V are not presented because of the large number of

nondetectable values.

TABLE 3. Mass of Ca, U, and V Deposited in PRB (Ms) duringthe First 2.7 yr of Operation Based on Equation 2

mass (Ms)

constituent gravel/ZVI zone ZVI zone total

Ca (t) 1.6 1.9 3.5U (kg) 11.4 ∼0 11.4V (kg) 7.5 ∼0 7.5

TABLE 4. Mass and Volume of Minerals Deposited in the PRBduring First 2.7 yr of Operation

mineral gravel/ZVI zone ZVI zone total

calcite (t) 4.0 4.8 8.8calcite (m3) 1.5 1.7 3.2uraninite (kg) 12.9 ∼0 12.9uraninite (L) 1.6 ∼0 1.6V trioxide (kg) 11.0 ∼0 11.0V trioxide (L) 2.3 ∼0 2.3

Ms ) CsMz (2)

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in the solid phases of the PRB. Mean groundwater flux (Qw)for the investigation period (t ) 2.7 yr) was calculated fromthe total mass (Table 3) and the mean groundwater con-centration gradients (Table 2) for Ca, U, and V:

Mean groundwater fluxes calculated with eq 3 range from11.3 to 24.0 L/min (Table 6). The calculated means fromthese four independent measurements are reasonably con-sistent, providing confidence that the true groundwater fluxis close to this range. The 2 SE values for the means of boththe solid-phase concentrations and the groundwater con-centration gradients expand the calculated range of ground-water flux to 7.2 to 75.0 L/min. Data for Ca concentrationsin the ZVI zone and V concentrations in the gravel/ZVI zoneare best suited for calculating groundwater flux because theirdistributions are the most constrained. The groundwater fluxcalculated with these data ranges from 7.2 to 22.8 L/minwith a combined mean of 13.5 L/min.

Calculated values of groundwater flux using the Cadistribution in the gravel/ZVI zone have a relatively widerange, even though the concentration distribution in the solidphase has a low coefficient of variation (Table 1). This widerange is attributable to an increase in the Ca concentrationsin groundwater exiting the gravel/ZVI over time (the gradientdecreased), resulting in a high coefficient of variation for thegroundwater concentration gradient (Table 2). The decreas-ing Ca concentration gradient indicates that the gravel/ZVIzone was losing reactivity during the 2.7-yr period. Loss ofreactivity is also indicated by an increase in the mean Uconcentration exiting the gravel/ZVI zone from 0.2 to 185µg/L during the 2.7-yr period.

Number of Samples Required. Coring and the numberof samples collected and analyzed share the major costs ofusing solid-phase concentrations to assess performance ofa PRB using the methods described in this study. Distributionsof solid-phase concentrations developed during this studywere used to determine the number of samples required toestimate the true means within a 95% confidence limit. Thecoefficient of variation (η), presented in Table 1, is used to

estimate the number of samples (n) required for a desiredvalue of relative error (dr) (15):

where Z1-R/2 for the 95% confidence interval is equal to 1.96(Table 7). For U, 32 samples are required to have 95%confidence that the sample mean is within 10% of the truemean. To have the same confidence of being within 5% ofthe true mean, 128 samples are needed. Because of the tightdistribution of Ca concentrations in the gravel/ZVI zone,only two samples are needed to have 95% confidence of beingwithin 10% of the true mean. Thus, if the distributions areknown or can be reliably assumed, the number of samplesrequired may be considerably fewer than were collected forthis study. Additional samples would be required to ad-equately define preferential flow paths through a PRB.

Longevity. This study identified two mechanisms thatcan limit the longevity of PRBs: reduction of reactivity andreduction of porosity. Deposition of carbonate minerals onZVI surfaces could affect both processes. Reactivity decreasesas a PRB ages, causing a decrease in the rate of porosity loss.A simple model of PRB performance was formulated thatincorporates a partial coupling between reactivity loss andporosity loss and is supported by the data from this study.

The rate of removal of U from groundwater by ZVIdecreases with decreased surface area. If reactivity decreasesbecause of mineral precipitation on ZVI surfaces, then therate of reactivity would likely be rapid initially because of thehigher density of available surface sites and would decreaseexponentially as reaction sites are coated. A first-order rateexpression is used to portray this loss of ZVI reactivity,expressed as the loss of ZVI surface area (Sz) over time (t):

where λ is a rate constant. Equation 5 is used to model theloss of ZVI surface area in the gravel/ZVI zone as indicatedby the decrease in the gradient of dissolved U over time(Figure 8). The rate constant was used as a fitting parameterto provide reasonable consistency between the model andthe field data.

By using an initial surface area of 1 m2/g for ZVI that wasestimated from data provided by Johnson et al. (24), the initialsurface area of ZVI (Sz0) is 291 and 2240 m2/L in the gravel/ZVI and ZVI zones, respectively. Effluent from the gravel/ZVI zone exceeded the Monticello standard of 30 µg/L afterabout 0.5 yr of PRB operation. At that time, the gravel/ZVIzone had about 220 m2/L of reactive surface area based oneq 5; the surface area of the ZVI zone would decrease to 220m2/L in about 6 yr from the time of installation (Figure 9).

Porosity (Φ) decreases as mineralization coats ZVI sur-faces. Assuming that the change in surface area is linearlyrelated to the change in porosity, the porosity is given by

TABLE 5. Comparison of Filtered and Unfiltered Results forCalcium and Alkalinity for Downgradient Samples Collected inJuly 2002

calcium (mg/L) alkalinity (mg/L CaCO3)

well filtered unfiltered filtered unfiltered

1 71 66.5 125 1252 150 201 232 naa

3 96.8 98.3 141 1534 59 58.2 98 925 104 101 173 170

a na, not analyzed.

TABLE 6. Groundwater Flux through the PRB during First 2.7 yrof Operation (L/min)

groundwater flux (Qw)

constituent zone lowa mean higha

Ca gravel/ZVI 12.9 24.0 75.0Ca ZVI 7.2 11.3 18.3U gravel/ZVI 14.5 24.0 43.6V gravel/ZVI 11.4 16.0 22.8

a Low and high values incorporate the combined uncertainty of 2 SEfor solid-phase concentrations and aqueous-phase concentrationgradients.

Qw ) Ms/t∆Cw (3)

TABLE 7. Number of Samples Required for 95% ConfidenceLevel

constituent/zone η dr

samplesrequired (n)

samplescollected

uranium/gravel/ZVI 0.288 0.10 32 1190.05 128

vanadium/gravel/ZVI 0.291 0.10 33 1190.05 131

calcium/gravel/ZVI 0.054 0.10 2 1190.05 5

calcium/ZVI 0.117 0.10 6 1630.05 21

n ) (Z1-R/2η/dr)2 (4)

-dSz/dt ) λSz (5)

Φ ) Φ0 - (Tc)(Sz0 - Sz) (6)

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where Φ0 and Sz0 are initial values and Tc is a proportionalityconstant equivalent to the thickness of an evenly distributedcoating of mineral; Tc is determined by fitting eq 6 to 9.3%porosity loss in the gravel/ZVI zone and 3.2% loss in the ZVIzone in the first 2.7 yr.

Failure of the PRB is expected if hydraulic conductivity(K) of the PRB approaches that of the alluvial aquifer.Hydraulic conductivity is assumed to be related to Φ3/(1 -Φ)2 as indicated by the theoretical analysis of pore flowdeveloped by Carmen (25). According to this relationship, Kis expected to decrease rapidly initially but approach anasymptote at about 0.034 cm/s for the gravel/ZVI zone and0.052 cm/s for the ZVI zone (Figure 10). Thus, the K of thePRB is not expected to decrease to the value of the alluvialaquifer (0.013 cm/s).

Unfortunately, the chemical reactions occurring at mineralsurfaces leading to loss of reactivity are poorly understood,as is the effect of mineral precipitation on K. A betterunderstanding of these processes is needed to have confi-dence in predictions of PRB longevity.

Design Implications. The precipitation of noncontami-nant-bearing minerals is an important factor in the longevityof PRBs. Calcite mineralization is evident throughout theMonticello PRB, but contaminants are confined to the gravel/ZVI zone, indicating that contaminants precipitate with lessresidence time. Therefore, design strategies that decreaseresidence time could be effective in prolonging the life spanof the PRB.

A design that could improve the performance of somePRBs includes a series of buried perforated pipes parallel to

and spanning the length of the PRB. Valves could be usedto initially direct contaminated groundwater to a perforateddistribution pipe near the hydraulically downgradient edgeof the PRB. If the distribution pipe is close to the PRB exit,the residence time in the ZVI zone is less than it would befor groundwater passing through the entire width of the PRB.The shorter residence time would result in a smaller increasein pH values and less precipitation of calcium carbonate. Ashydraulically downgradient zones become less reactive,hydraulically upgradient zones in the PRB could be activatedby directing flow through a different distribution pipe. Thisdesign takes advantage of the finding that some contami-nants, such as U and V, are removed from solution fasterthan calcium carbonate.

AcknowledgmentsInstallation and 2 yr of monitoring of the PRB at Monticellowas funded by the DOE Office of Science and Technologythrough the Accelerated Technology Deployment Program.Funding for this study was provided through the DOE GrandJunction Office Monticello Program Operable Unit III. Thisproject was possible through the coordinated efforts of KristenMcClellen (MFG, Inc.), Joel Berwick (DOE Grand JunctionOffice), Paul Mushovic (U.S. Environmental ProtectionAgency), and David Bird (State of Utah). Insights providedby these individuals and those of Clay Carpenter and JodyWaugh (S. M. Stoller Corporation) and Tim Bartlett’s (MFG,Inc.) many years of careful observation and analysis of thegroundwater system were essential to this study. Threeanonymous reviewers graciously provided thoughtful cri-tiques that improved the manuscript. The EnvironmentalSciences Laborary is operated by the S. M. Stoller Corp. forthe U.S. Department of Energy Grand Junction Office underDOE Contract DE-AC13-02GJ79491.

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Received for review September 27, 2002. Revised manuscriptreceived February 14, 2003. Accepted March 5, 2003.

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