construction of a permeable reactive barrier in a residential neighborhood

65

REMEDIATION Autumn 2002

In June 2001, the Massachusetts Department of Environmental Protection (DEP) installed a

permeable reactive barrier (PRB) within a roadway in Needham, Massachusetts, to treat a plume

of chlorinated solvents migrating toward two public water-supply wells located in the adjacent

town of Wellesley, Massachusetts. The solvents originated from an electronics manufacturer

located approximately 2,300 feet upgradient of the roadway and 5,200 feet upgradient of the

public supply wells. Chlorinated solvents, primarily trichloroethene (TCE), had migrated past the

roadway to within 300 feet of the public supply wells. Two contaminant transport models prepared

by the DEP’s design contractor and the EPA indicated that the plume would reach the well field

if no response actions were taken. To mitigate the future impact to the municipal well field, the

DEP decided to install a PRB composed of zero-valent granular iron across the path of the plume

along Central Avenue in Needham. Though several dozen PRBs have been installed at sites

worldwide and the technology is no longer considered innovative, the application of the

technology in a roadway that receives 17,000 vehicles per day within a residential neighborhood

is unique and presented difficulties not typically associated with PRB installations. The Needham

PRB was also one of the first zero-valent iron PRBs installed using the slurry trench method to treat

chlorinated compounds. © 2002 Wiley Periodicals, Inc.

INTRODUCTION

In 1985, chlorinated solvent contamination was discovered in groundwater during aroutine real estate investigation at Crescent Road in Needham, Massachusetts, across thestreet from property owned by Microwave Development Laboratory Inc. (MDL)(Exhibit 1).The chlorinated solvents, primarily trichloroethlene (TCE), were tracedback to the MDL property.Though the exact source of the release has not beenidentified, it is believed to have been dry wells and floor drains in the buildings on theMDL property. Chlorinated solvents have not been identified in groundwaterupgradient of the facility, which is primarily residential.

In March 1985, the DEP identified MDL as a potentially responsible party (PRP)for the chlorinated solvent contamination emanating from its property. Investigations inthe late 1980s and early 1990s indicated that the plume had migrated from the sourcearea(s) through glacial till deposits down a steep hill and underneath an elementaryschool, at the base of the hill. Given the potential for TCE and its breakdown products,particularly vinyl chloride, to volatilize and accumulate within the Hillside ElementarySchool, the DEP installed a sub-slab depressurization system (SSDS) within the schooland eight nearby houses in 1989.With the mitigation of the indoor air issues at theschool, the DEP focused on determining the extent of the solvent plume. Additional

© 2002 Wiley Periodicals, Inc.Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/rem.10046

Peter Richards

Construction of a Permeable ReactiveBarrier in a Residential Neighborhood

hydrogeologic investigations in the early and mid-1990s indicated that the plume hadmigrated from the dense glacial tills on the hillside beneath wetlands and RosemaryBrook to a buried glacial valley flanked by competent meta-rhyolite.The RosemaryBrook watershed consists of a bedrock trough with glacial till deposits on the higherelevations and slopes and directly overlying the bedrock surface. Glacio-fluvial deposits,consisting of fine to medium sand with lenses of coarse sand and gravel, overlie the thinbasal till layer along the axis of the valley.The greatest thickness of stratified driftdeposits lie within the buried valley, underneath the existing wetlands.The investigationsnoted that the plume migrated beneath Central Avenue, then turned sharply, followingthe axis of the buried valley to a point upgradient of the T. F. Coughlin well and twocaisson wells referred to as the Wellesley Avenue well; combined, these wells have anaverage pumping rate of approximately 1,050 gallons per minute (EPA, 2001).TheWellesley Water Department maintains four supply wells within Rosemary Brook valley,which account for approximately 52 percent of the town’s water supply (Duggan,1996).The remaining two wells in Rosemary Brook valley are 4,200 feet downgradientof the T.F. Coughlin and Wellesley Avenue wells.

With engineering assistance from Harding ESE, in 1997 the DEP initiated a two-stage feasibility study evaluating three technologies to control the solvent plume and

Construction of a Permeable Reactive Barrier in a Residential Neighborhood

© 2002 Wiley Periodicals, Inc.66

Exhibit 1. Aerial photo shows source area and the path of the plume. Municipal supply wells

are shown in upper left, just northwest of the impoundment area. Note location of PRB along

roadway and elementary school, downgradient of source.

prevent it from impacting Wellesley’s supply wells.The DEP evaluated in-situ airsparging, permeable reactive barriers (PRBs), and air stripping at the wellhead inWellesley.The evaluation was based on technical feasibility, ease ofimplementation/installation, and cost of implementation and operation. All threetechnologies were deemed to be effective in plume control, though the PRB wasdetermined to be the least expensive of the alternatives. In May 1997,TCE was detectedin a monitoring well approximately 300 feet from the T. F. Coughlin well.

A follow-up feasibility report was completed in March 1999, which incorporatedthe results of additional investigations in the vicinity of Central Avenue.The follow-upfeasibility report investigated 11 different installation methods for PRBs, includingslurry trench, augering, deep soil mixing, driven I-beam, and hydraulic fracturing,concluding that the slurry trench, hydraulic fracturing, and driven or vibrated I-beamtechniques were the most applicable (IT Group, 1999).

Given the residential nature of the area, the abundant wetlands between the sourceand the supply wells, the path of the solvent plume, and the type of constructionequipment necessary for PRB installations, the DEP was limited to choosing onelocation for the PRB: Central Avenue, a main commuter route between the westernsuburbs of Boston and Route 128. Statistics supplied by the town of Needham DPWindicated that the roadway receives over 17,000 vehicles per day, with a maximum ofapproximately 1,200 vehicles per hour during the morning and afternoon rush hours. Itis also a main route for school buses transporting children to and from the twoelementary schools near the site.

In August 1998, the DEP began a modeling effort to evaluate whether a PRBinstalled across the path of the plume within Central Avenue would preclude any futureimpact to the two supply wells. By this time, the solvent plume had migratedapproximately 2,500 feet downgradient of Central Avenue, leaving a large part of theplume that would not be remediated by any future PRB.The model’s main objective wasto determine the impact, if any, of this section of the plume to the two municipal supplywells, given the installation of a PRB within Central Avenue. A numerical model wasconstructed using the U.S. Geological Survey MODFLOW program; contaminanttransport was simulated using MT3D. Initial model results were reviewed by staff at theEPA’s R. S. Kerr Modeling Center in Ada, Oklahoma. EPA staff subsequently completedtheir own numerical fate and transport model in February 2001 using MODFLOW andMT3D.The models indicated that, subsequent to PRB installation, the section of theplume downgradient of Central Avenue would not impact the wells due to dispersion anddilution. If no remedial measures were taken, the models indicated that the TCEconcentration in the Wellesley Avenue supply well would increase to a levelapproximating the drinking water standard of 5 micrograms per liter; the models showedno impact to the T.F. Coughlin well. Given the inherent uncertainties in groundwatermodels, a desire to remediate the aquifer upgradient of the well field, and the potentialimpact to the Wellesley Avenue supply well, the DEP proceeded with PRB installation.

DESIGN AND OBJECTIVES

The DEP has collected groundwater samples from wells within Central Avenue since 1995.The highest average TCE concentration in monitoring wells within Central Avenue prior toPRB installation was approximately 300 micrograms per liter (µg/l); the maximum TCEconcentration detected in the wells was 1,100 µg/l, noted in November 1996.The highest


© 2002 Wiley Periodicals, Inc. 67

The model’s main objectivewas to determine theimpact, if any, of thissection of the plume to thetwo municipal supply wells,given the installation of aPRB within Central Avenue.

average TCE concentration in the source area is approximately 27,383 µg/l (Cygnus GroupInc., 2001). Based upon the proximity of the well field to Central Avenue, the objective ofthe PRB was to reduce the contaminant levels to the drinking water standard for TCE (5µg/l).To meet this objective, the PRB was designed to be installed within Central Avenueacross the width of the plume, which was determined from the collection and analysis ofgroundwater samples from the four pre-existing bedrock and 20 pre-existing overburdenwells located within Central Avenue.Accordingly, the PRB design length was 550 feet,installed from a point in Central Avenue approximately 20 feet north of the culvert overRosemary Brook northward to a point just south of the driveway to a residential property.The PRB design depth was from the seasonal high water table to the bedrock surface, whichvaries from approximately 35 to 60 feet below grade, based on the soil-boring program.Early in the design process, the decision was made not to target remediation within thebedrock, given the relatively low permeability of the bedrock, the low TCE concentrationsin the bedrock aquifer, and the technical difficulty associated with placing the PRB withinthe bedrock along the length of the trench.

The amount of zero-valent iron that the contaminated groundwater must passthrough to reduce TCE to the design parameter (5 µg/l) is based on the groundwatervelocity and the required residence time. Column testing completed at the University ofWaterloo using site groundwater indicated that a residence time of approximately 20hours would reduce the TCE concentration of the influent water (spiked to a TCEconcentration of approximately 500 µg/l) to the design objective of 5 µg/l, based on aTCE half-life of three hours. Groundwater for the column test was collected from themonitor well with the highest average TCE concentration.To convert the residence timeto a PRB thickness, the DEP evaluated permeability test data and calculated hydraulicgradients for wells in Central Avenue. Permeability testing within the overburden wellsalong Central Avenue indicated a range of hydraulic conductivity values, from 0.75 to160 feet per day. Given the range in both permeability values and TCE concentrationsacross the width of the plume, the PRB was designed with two different zones of ironthickness. On the flanks of the plume and near the bedrock surface at the core of theplume, a PRB thickness of 6 inches of pure zero-valent iron would be sufficient toachieve the required residence time (zone B). In the middle of the plume, where theTCE concentrations were higher, a PRB thickness of 1.7 feet of pure zero-valent ironwas required (zone A). Since it is not feasible to excavate a trench 0.5 feet wide to adepth of 60 feet, the bid specifications allowed the zero-valent iron to be mixed withsand in the proper ratios to backfill the trench yet also achieve the proper pure-ironthickness for each zone.The bid specifications indicated that the minimum volume ofiron permitted in the sand/iron mix was 20 percent, yielding a maximum allowabletrench width of 2.5 feet in zone B.To minimize the quantity of excavated soils to beremoved from the site and the potential for damage to underground utilities, thecontractor eventually proposed a uniform trench width of 2 feet.Thus, the zone Adesign mix consisted of 85 percent iron and 15 percent sand by volume, while the zoneB design mix consisted of 25 percent iron and 75 percent sand by volume.

BID SPECIFICATIONS AND PROTEST

Following the hydrogeologic investigations and design process, the DEP prepared adetailed bid package.Technical assistance in the bid preparation process was garnered



The PRB . . . wasdetermined from thecollection and analysis ofgroundwater samplesfrom the four pre-existingbedrock and 20 pre-existing overburden wellslocated within CentralAvenue.

from EPA manuals, previous bid specifications for PRBs installed in southern NewHampshire, and EnviroMetal Technologies Inc. (ETI), a Canadian firm with a patent onthe use of the PRB technology.The bid specifications indicated that the PRB was to beinstalled using the biopolymer (slurry) trench method.This particular method usesbiopolymer slurry to maintain trench sidewall stability during excavation. Once thetrench is excavated to the desired depth, the sand/iron mix is installed via a tremie pipeto the bottom of the excavation.The biopolymer slurry eventually degrades, yieldingend products of CO2 and H2O.

To accommodate concerns regarding the closure of a busy commuter roadway forseveral months during the PRB installation, the bid specifications were preparedallowing the bidder to submit two bids for the project: one with Central Avenuecompletely closed in the vicinity of the proposed PRB and the second with one lane ofCentral Avenue remaining open during PRB installation. Invoicing for the project wasbroken into a series of seven payments.To facilitate PRB installation, the paymentsection of the bid specifications was written such that the first payment to the contractorwould be made only after 25 percent of the PRB was installed.The second, third, andfourth payments would be made after the installation of 50 percent, 75 percent, and 100percent of the PRB, respectively. Further payments would be made following thecompletion of additional milestones (site restoration, etc.).The bid specifications werereleased on April 26, 2000. Approximately 25 copies of the bid specifications werereleased before the bid period closed on June 23, 2000, and the bids were publicly read,at which point the DEP had received two bid submittals.

The first bidder (firm A) submitted bids for both options: complete closure ofCentral Avenue and one lane remaining open. Firm A’s costs for the two options were$2,212,667 and $2,758,280, respectively.The second bidder, GeoCon, submitted a bidsolely for the complete closure of Central Avenue for $2,889,832. Following the receiptof additional information from both bidders to supplement their bid packages andsubsequent bid review, the DEP tentatively awarded the project to GeoCon in August2000. Firm A subsequently filed an application for preliminary injunction with SuffolkSuperior Court in September 2000, contending that it was the lowest responsible andeligible bidder for the project. In October 2000 Suffolk Superior Court denied theinjunction, allowing the DEP to award the contract to GeoCon (the higher bidder).

During the bid protest, DEP staff spoke to EPA personnel who indicated thatGeoCon had used the preservative Troysan 142® in a previous PRB installation insouthern New Hampshire.Troysan 142®, also known as Dazomet® (tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione), was used to limit bacterial growth in thebiopolymer slurry during the PRB installation and prevent the trench from collapsingprior to backfill emplacement.Troysan 142® is an organic biocide and is commonly usedas a soil fumigant and in paints to prevent mildew growth (National Institute forOccupational Safety and Health, 2000). Given the location of the proposed PRB withina highly transmissive aquifer upgradient of two municipal supply wells, DEP riskassessment staff evaluated Troysan 142® in October 2000 for its potential impact tohuman health. Breakdown products of Troysan 142® include methyl isothiocyanate(MITC), formaldehyde, hydrogen sulfide, and methylamine; formaldehyde is listed bythe EPA as a probable human carcinogen. MITC is fairly toxic if swallowed and isconsidered very highly toxic to fish and aquatic invertebrates (EPA, 2002). Ultimately,little information was available regarding Troysan 142’s® longevity and degradation rates



Given the location of theproposed PRB within ahighly transmissive aquiferupgradient of twomunicipal supply wells,DEP risk assessment staffevaluated Troysan 142®. . .for its potential impact tohuman health.

in the subsurface and fate and transport data for its breakdown products.Thus, inDecember 2000 the DEP requested that Troysan 142® not be used in this application.

Following the bid award, the DEP conducted a bench-scale treatability test todetermine whether the iron, sand, and biopolymer products GeoCon proposed for usein the project would effectively degrade the chlorinated plume to the design objectives.The biopolymer was mixed using potable water collected from the town of Needham;the sand/iron mix was poured through the biopolymer into the column to simulate theinstallation in the field. Site groundwater was then pumped through the column at roomtemperature at approximately the same velocity as that at the site and water sampleswere collected from the column every 4 to 14 pore volumes to establish degradationrates. Using a sand/iron mix that was 68 percent iron by volume, the half-life (timerequired for the initial TCE concentration to decrease by half) was approximately onehour.This value is adjusted to three hours to account for the lower groundwatertemperatures noted in the field. No biofouling of the iron was observed during thebench-test, which had been a concern of the DEP.

While the column test was being conducted, the DEP filed a Notice of Intent withthe local Conservation Commission in order to excavate soils within the 100-foot bufferzone of a vegetated wetlands (i.e., the roadway).The Conservation Commissionexpressed concerns regarding possible reduced permeability of the barrier and thepotential for flooding of local basements, siltation of the storm drain system andwetlands from exposed or excavated soils, long-term monitoring and performancerequirements for PRBs, damage to utilities during PRB installation, whether dewateringwould be conducted, and the potential breakdown products of the biopolymer. Afterseveral meetings, an Order of Conditions and approval of the final design plans weregranted by the Conservation Commission on April 19, 2001. During these discussionsGeoCon applied for a street-opening permit from the local Department of Public Works(DPW), which was granted on May 4, 2001.

The DEP also continued its public outreach program by holding meetings with localresidents to inform them of the extent of the work and the potential impact to theneighborhood.The DEP had held public meetings for the MDL project as activitiesdictated since the early 1990s; several public meetings were held prior to and during theinstallation of the SSDS at the Hillside Elementary School. A local citizen’s interestgroup, the Hillside Advisory Committee, was formed to address parents’ and teachers’concerns regarding the contaminant plume beneath the school. In March 2001, afterGeoCon was selected as the contractor, the DEP held a public meeting at the school todiscuss the details of the project and introduce the residents to the contractor. Severalconcerns were raised by local residents at the meeting concerning their health andsafety.These issues included machinery noise, dust, exposure to diesel emissions fromconstruction machinery, volatilization of TCE and other chlorinated solvents fromexcavated soils and groundwater, the presence of trucks hauling contaminated soils onlocal roads, the expected duration of the project, access to the residences in theimmediate area by the local police and fire departments, site security, and the threat toreal estate valuations. Residents also noted that the work was being conducted in onetown while the project benefited the adjacent town, with no apparent benefit toNeedham and its residents.The health and safety concerns reflected the public’sperception of the risks posed by hazardous-waste sites and their cleanup and weregreatly magnified by the fact that hazardous-waste cleanup activities were proposed



. . . the DEP held a publicmeeting at the school todiscuss the details of theproject and introduce theresidents to thecontractor.

within a residential neighborhood, within 30 feet of occupied dwellings (see Exhibit 2).Since most PRB installations occur on industrial or commercial property without thepublic’s knowledge, such concerns typically do not arise. Aside from the technical issuesof design and installation associated with the project, the construction of a PRB within aresidential neighborhood, upgradient of a public water supply, and the public’sperception of the project’s risks made this PRB installation unique.

The DEP noted at the meeting that many of these issues had been addressed in thebid specifications, including requirements to (1) periodically monitor noise levels with anoise meter during the project, (2) continuously monitor volatile organic compound(VOC) levels by the trench with a photoionization detector, (3) monitor and, ifnecessary, employ dust control techniques, (4) maintain police details at the site duringworking hours and security guards at the site during off-hours, (5) cover the trench withsteel plates during off-hours, and (6) obtain a letter from a landfill operator prior to soilexcavation stating that they would accept the soil from this project. However, given thelevel of concern raised at the meeting, the DEP, GeoCon, and the local board of healthinstituted additional controls and conducted additional activities to address the issuesraised at the meeting. On behalf of the DEP, Harding ESE completed a risk assessmentfor the volatilization of chlorinated solvents from the site soils and groundwater.The riskassessment had a number of conservative assumptions, including (1) a trench width of 4feet, as compared with the actual width of 2 feet; (2) a groundwater TCE concentrationof 630 micrograms per liter, the maximum concentration noted in any of the existingwells over several previous years; (3) the immediate volatization of all VOCs from thesoil and groundwater at the time of excavation; (4) that the residents would be home 24hours per day for the entire five-week estimated construction duration; and (5) that the



Exhibit 2. Location of PRB within roadway immediately adjacent to houses. Note size of

excavator.

contaminated air from the excavation area flows directly to the adjacent residentialproperties with no dilution or dispersion.The risk assessment assumed a TCEconcentration in the soil of 250 micrograms per kilogram; no TCE had been detected insoil samples from Central Avenue, so the method detection limit for TCE was chosen asthe TCE concentration in the soil.The risk assessment results indicated that the healthrisks posed by the project were well below the risk levels posed in the MassachusettsContingency Plan, the state regulations that govern the assessment and cleanup ofhazardous waste sites.The cumulative receptor noncancer risk was an order ofmagnitude less than the applicable DEP limit, while the cumulative receptor cancer riskwas over two orders of magnitude less than the DEP’s limit. Once completed, the riskassessment was provided to the local board of health for an independent review.

The DEP developed a plan to monitor air quality at the site using a variety oftechniques.The process began with the collection and analysis of a background airquality sample via summa canister following EPA Method TO-14 to establish baseline airquality.This particular method tests for a wide variety of VOCs at very low detectionlimits. Additional air-quality samples for TO-14 analysis were proposed for collectionduring the first day of trench excavation and while excavating in the area with thehighest anticipated soil contaminant levels. Air quality during construction wasmonitored continuously for total VOCs with a photoionization detector (PID) anddatalogger; action levels were established in the health and safety plan to determinewhen measures would be taken to reduce VOC levels in the air, if necessary.Thesemeasures included covering soil piles, containing liquids within frac tanks, andequipment shutdown. Air quality was also monitored continuously for carbonmonoxide, hydrogen sulfide, lower explosive limit, and dust. Finally, the DEP alsocollected four air-quality samples each day for analysis on site in a gas chromatograph,which provided much lower detection limits than the PID and could identify specificVOCs.The DEP also coordinated with GeoCon for the installation of a diesel-emissionsretrofit on the excavator, which greatly reduced diesel emissions and also reduced theamount of noise it produced.The DEP began talks with two residents who had specialconcerns regarding the construction activities; for example, one of the residents sufferedfrom epilepsy. His condition was aggravated by loud noise, such as from constructionequipment.These residents moved temporarily to a local hotel while the excavation wasconducted in front of their houses. GeoCon also monitored noise levels with a decibelmeter prior to road closure to obtain information on background noise levels.

Once construction began, the local board of health coordinated biweekly meetingswith the DEP, the contractor, and residents to discuss project status and constructiondifficulties and to foster informal communication channels between the affectedresidents and the contractors on the site.

PRB INSTALLATION

Following issuance of the required permits from the Conservation Commission and theDPW, Central Avenue was closed to vehicle travel within the construction area on May14, 2001. Excavation with a 100-ton, 400-horsepower Linkbelt 7400 excavator beganon June 5, after installing silt fence and other siltation control devices, mobilizingequipment to the site and assembling it, relocating some overhead utilities, installinginclinometers in the roadway to monitor potential utility movement and damage, and



relocating the storm drain line out of the path of the proposed PRB. Excavation andinstallation of the PRB was completed in sections or panels, with panel lengths varyingfrom 30 to 60 feet.The PRB was completed in 12 panels with a constant width of 2feet. At the time of installation, it was the deepest zero-valent iron PRB installed usingthe biopolymer trench method.

Due to space constraints, the sand and iron were mixed off-site, trucked to the siteeach day, and placed in a temporary storage pile.The sand and iron were mixed in atwo-hopper pugmill dedicated to the project for the duration of the PRB installation.The pugmill was calibrated for the two sand/iron mix ratios required for the PRB (forZone A and Zone B) to within a 1 percent tolerance at the beginning of the project.Thepugmill had been steam-cleaned and rebuilt just prior to the project, alleviatingconcerns of possible contamination from asphalt mixing.The iron was supplied byPeerless Metal Powders and Abrasives of Detroit, Michigan.

The biopolymer was mixed on-site in a batch plant and pumped to the trench via 4-inch high-density polyethylene (HDPE) piping (Exhibit 3).The biopolymer consisted ofguar gum, a galactomannan polymer that develops a viscosity of 3,800 to 4,500centipoise (cP) after two hours of hydration (personal communication, RantecCorporation). It typically has a low shear gel strength, meaning that it will not suspendsolids, though it can be increased with the addition of cross-linkers. Guar gum isdeveloped from the endosperm of Cyamopsis tetragonolobus, a legume grown mainly in



Exhibit 3. Batch plant for mixing biopolymer slurry.

India and Pakistan, and is used widely as a food additive and in industrial applications (G.M. Associates, 2000). It is broken down from a polysaccharide to oligosaccharides andmonosaccharides via bacteria and other microbes, which use the guar gum as a foodsource, reducing the biopolymer’s viscosity. Adding lime and soda ash to the biopolymerraises its pH, thus creating unfavorable conditions for bacterial growth and extendingthe biopolymer’s use. Conversely, the addition of an enzyme to the biopolymerfacilitates its degradation.The biopolymer was supplied by Rantec Corporation ofRanchester,Wyoming.

Excavation progressed in each section until bedrock was reached.The excavatorbucket was then scraped along the trench bottom to confirm competent bedrock; thevibrations could be felt at the ground surface.The trench depth was measured andrecorded every ten feet along the trench using a weighted tape.These measurementswere compared with the trench profile developed from the previously installed bedrockwells. Stop panels were installed at each end of the section to prevent trench soils fromadjacent sections from flowing into the excavated section.The stop panels were 2-foot-wide I-beams installed to the bedrock surface. Six-inch-diameter PVC wells were theninstalled every 30 feet in the PRB for use in subsequent PRB development.

Once the stop panels and development wells had been installed in the excavatedsection, a 2-foot-wide steel tremie tube was lowered into the section through thebiopolymer slurry to the bottom of the trench.The bottom ten feet of the tremie tubehad holes cut in it to allow the sand/iron backfill to flow out of it.The sand/iron mixwas then dumped into the tremie tube via a front-end loader; the tremie tube was fittedwith a hopper to facilitate backfilling.While the front-end loader backfilled one sectionwith sand/iron mix, the excavator would begin excavating the adjacent panel.Excavation and backfilling continued in this leap-frog manner until PRB installation wascompleted on July 7, 2001. Approximately 1,479 tons of zero-valent iron were used toconstruct the PRB, which, when completed, was 535 feet long and reached a maximumdepth of 54.5 feet.

Slurry pH, density, and viscosity were monitored several times each day to makesure it wasn’t prematurely degrading; the trench failed on two occasions when theviscosity was too low. After each trench failure, GeoCon began excavation in adjacentpanels and returned to the failed panel only after the biopolymer had completelydegraded or been pumped out into an on-site storage tank.

Excavated soils were dumped from the excavator into a dump truck andtransported to the southern end of the site to a temporary storage area for dewatering.As per the Order of Conditions from the local Conservation Commission, the storagearea was constructed of jersey barriers and a plastic liner to prevent water and soilsfrom entering the storm drain system. After the soils had sufficiently dried they wereloaded and transported to a landfill in Fall River, Massachusetts, under a materialshipping record for use as daily cover.

During the PRB installation, samples of the iron/sand mix were collected each dayfrom the on-site storage pile immediately prior to installation and from the PRB trenchimmediately following installation.The post-installation samples were collected atvarying depths via the excavator bucket.The samples were observed for evidence ofsand/iron segregation and analyzed for iron content. No segregation was observed inthese post-installation samples. Indeed, even though the mix had been installed throughthe biopolymer slurry, the samples obtained from the excavator bucket were dry; no



Slurry pH, density, andviscosity were monitoredseveral times each day tomake sure it wasn’tprematurely degrading . . .

slurry was observed within the mix.This alleviated the DEP and design engineer’s initialconcern that the biopolymer would possibly coat the iron particles during theinstallation process and potentially reduce the reduction efficiency of the iron.

After all of the 12 panels had been excavated and backfilled with sand/iron mix,LEB-H enzyme breaker was added to each of the 6-inch recirculation wells to promotedegradation of the residual slurry.Water and residual slurry was then recirculatedthrough the trench by pumping out of each well into adjacent recirculation wells.Theviscosity of the recirculation water was monitored during the process; afterapproximately 3.8 pore volumes of the PRB trench had been pumped, the trench fluidsyielded a viscosity similar to that of groundwater.

During PRB development four boreholes were advanced within the PRB using arotosonic drill rig. Core samples of the iron/sand mix were obtained at varying depthsto determine whether the mix had segregated during placement and whether anyresidual slurry remained in the PRB.The samples indicated that no segregation hadoccurred and did not indicate the presence of any residual slurry. Nine 1-inch-diametermonitor wells were installed as couplet wells or triplet wells within the boreholes tofacilitate future groundwater sample collection.Twenty-one wells (nine upgradient, tendowngradient, and one at each end of the PRB) were also installed in the upgradient anddowngradient sidewalks for use in evaluating the PRB performance.The road reopenedon August 24, 2001, following well installations and road paving.

RESULTS

Following PRB installation and site restoration, the DEP initiated a quarterly samplingprogram in October 2001 to assess the PRB’s performance.Three rounds of quarterlygroundwater samples were collected from the upgradient and downgradientperformance monitoring wells to date (October 2001, January and April 2002). Inaddition, groundwater samples from some of the pre-existing wells were also collectedto determine whether the plume’s path changed since PRB installation.The wells withinthe PRB are not accessed during the sampling rounds, as they are located within theroadway. Groundwater levels are collected at 41 wells to assess hydraulic gradients andflow direction to compare with pre-PRB conditions. Groundwater samples are collectedfollowing the EPA’s Region I Low Stress (LowFlow) Purging and Sampling proceduresmanual (EPA Region 1, 1996). Field parameters, including pH, redox potential,dissolved oxygen, temperature, turbidity, and specific conductance, are measured using aflow-through cell at the time of sample collection, as recommended in the EPA’s PRBtraining manual (EPA, 1999).The samples are then sent to a laboratory and analyzed forvolatile organic compounds following EPA Method 8260B.

Review of pre- and post-installation hydraulic gradients of upgradient anddowngradient monitor well pairs do not indicate any loss in permeability across thePRB. Similarly,VOC analysis of groundwater samples from wells located on the flanks ofthe PRB indicate that the plume’s path has not fluctuated since installation, implying thatthe PRB’s permeability is at least as great as the native soils.

Field parameters indicate that dissolved-oxygen levels drop sharply and reducingconditions are prevalent downgradient of the PRB. For example, dissolved-oxygenvalues noted during the April 2002 sampling round dropped from an average of 2.5mg/l in the upgradient wells to 0.0 mg/l in the downgradient wells. Redox potentials



Review of pre- and post-installation hydraulicgradients of upgradientand downgradient monitorwell pairs do not indicateany loss in permeabilityacross the PRB.

dropped from an average of 59 millivolts in the upgradient wells to -25 millivolts in thedowngradient wells. During the October 2001 round, the pH increased from an averageof 6.3 in the upgradient wells to 7.5 in the downgradient wells.These trends ininorganic groundwater chemistry are typical of PRBs (EPA, 1999).

Groundwater analytical results for some of the wells installed within the sidewalksupgradient and downgradient of the PRB are shown in Exhibit 4.The even-numberedwells are located in the upgradient sidewalk while the odd-numbered wells are located inthe downgradient sidewalk.The table lists the results for the upgradient and downgradientwell pairs; for example, well 3S is directly downgradient of well 2S. It should be notedthat vinyl chloride has not been detected in any of the groundwater samples.

The results indicate a significant decrease in VOC concentrations in thedowngradient wells relative to the upgradient wells. For example, many relatively highTCE concentrations noted in upgradient wells are reduced to nondetect levels in thecorresponding downgradient well in at least two of the three sampling rounds (wellpairs 2S/3S, 4S/5S, 4D/5D, and 6S/7S). Other well pairs have shown large reductionin TCE concentrations across the PRB but have not achieved the drinking water standardin the corresponding downgradient well in at least two of the three sampling rounds(2D/3D, 6M/7M).Well pairs 6D/7D and 8S/9S have shown significant TCE decreasesfrom upgradient well to downgradient well in some rounds and increases in TCEconcentration in other rounds.The average TCE reductions for the first three samplingrounds are 73 percent, 83 percent, and 60 percent; results from other PRBs indicatethat large decreases in VOC concentrations in downgradient wells are typically notobserved within the first year (ETI, personal communication).

Concentrations of cis-1,2-dichloroethene (cis-1,2-DCE) have increased from theupgradient well to the corresponding downgradient well in at least one sampling round



Exhibit 4. VOC analytical results for well pairs in upgradient and downgradient sidewalks.

in well pairs 2S/3S, 2D/3D, 4D/5D, and 6D/7D. Possible explanations for the increasein the cis-1,2-dichloroethene concentration from upgradient to downgradient wellsinclude the following: (1) the TCE has not been completely dechlorinated to theexpected end product (ethene) as it passes through the PRB, causing an increase in thecis-1,2-DCE concentration downgradient of the PRB; (2) high cis-1,2-DCEconcentrations in groundwater remain in the aquifer between the PRB and thedowngradient monitor wells; and (3) strong reducing conditions exist downgradient ofthe PRB, causing dechlorination of TCE sorbed to soil particles downgradient of thePRB and a resultant increase in cis-1,2-DCE concentration in groundwater (HardingESE, 2002). Given the limited number of sample rounds, it is unclear which of theseprocesses is causing these fluctuations in cis-1,2-DCE concentrations.

Analysis of the data is problematic at this point, given the recent installation of thePRB and the temporal and spatial variation in TCE concentrations noted in wells alongCentral Avenue prior to PRB installation. During the three sampling rounds,TCEconcentrations in the upgradient wells varied from 36 ppb to 700 ppb, which is notunusual when compared with previous data. Several additional rounds of samples willhave to be collected to establish the long-term performance of the PRB during bothdrought and high-water conditions and with widely varying influent TCE concentrations.

CONCLUSIONS

The installation of a PRB in June 2001 represented the culmination of several years ofplanning and design.Though there were several unexpected issues that arose during boththe planning and installation processes, factors that contributed greatly to the relativeease of implementation included the preparation of a well-written bid document, the



Exhibit 4. (continued) VOC analytical results for well pairs in upgradient and downgradient

sidewalks.

use of an experienced contractor, and on-going community outreach with town officialsand residents. Long-term groundwater monitoring will be conducted to determinewhether VOC concentrations above design standards persist in downgradient wells and,if so, whether they indicate potential voids within the PRB or represent influentgroundwater concentrations that exceed the PRB’s design criteria.

ACKNOWLEDGMENTS

Representatives from many different entities assisted the DEP’s Bureau of Waste SiteCleanup in this project, including the EPA’s Center for Subsurface Modeling andSupport; EnviroMetal Technologies Inc.; Harding ESE; and DEP’s Office of Researchand Standards and Office of General Counsel.The towns of Wellesley and Needham alsoprovided a great deal of assistance, especially the Needham Board of Health and theWellesley Water and Sewer Department.The assistance of Stephen Johnson (DEP) andPatrick Hurley (DEP) during the preparation and review of this document is gratefullyacknowledged.

REFERENCES

Cygnus Group Inc. (2001). Final Phase II comprehensive site assessment report revisions. Southborough,

MA: Author.

Duggan, J. (1996). Report on the delineation of Zone II and other protective zones for Wellesley’s

municipal wells in Rosemary Brook Aquifer. Wellesley, MA: Wellesley Department of Public Works.

EnviroMetal Technologies Inc. (2000). Bench-scale treatability tests in support of a permeable reactive

barrier design for the Microwave Development Laboratories Inc. site, Needham, Massachusetts.

Waterloo, Ontario: Author.

Environmental Protection Agency (EPA). (1999). In-situ permeable reactive barriers: Application and

deployment (EPA542/B-99/001). Washington, DC: Author.

EPA. (2001). Groundwater flow and contaminant transport modeling report, Microwave Development Lab,

Needham, Massachusetts. Ada, OK: Center for Subsurface Modeling Support, National Risk

Management Research Laboratory.

EPA. (2002). Methyl isothiocyanate (MITC)-Wood preservative. Office of Pesticide Programs. Retrieved from

http://www.epa.gov/pesticides/citizens/methylf.htm

EPA Region 1. (1996). Low stress (low flow) purging and sampling procedure for the collection of ground

water samples from monitoring wells (SOP #GW 0001, Revision 2).

GeoCon Inc. (2000). Technical bid submittal for PRB installation, Needham, MA (MADEP BWSC-2000-001).

Pittsburgh, PA: Author.

G. M. Associates. (2000). The history of guar. Retrieved from http://www.gempolym.com/guarhistory.htm

Harding ESE. (1999). Contaminant fate and transport modeling assessment, Stage II focused feasibility

study, Microwave Development Laboratories, Inc. site, RTN 3-0386, Needham, Massachusetts.

Wakefield, MA: Author.

Harding ESE. (2000). Permeable reactive wall flow-through thickness. Wakefield, MA: Author.

Harding ESE. (2001). Risk assessment for construction at Central Avenue. Wakefield, MA: Author.



Harding ESE. (2002). 1st quarter post-construction groundwater sampling and water level gauging report.


Harding ESE (2002). 2nd quarter post-construction groundwater sampling and water level gauging report.


IT Group. (1999). Summary report and technology evaluation. Norwood, MA: Author.

Massachusetts Department of Environmental Protection. (2000). Instructions to bidders, contract,

agreement, specifications and bonds for permeable reactive barrier installation, Microwave

Development Laboratories, Inc. site, Needham, Massachusetts, RTN 3-0386. Boston, MA: Bureau of

Waste Site Cleanup, Division of Contract Procurement and Management.

National Institute for Occupational Safety and Health. (2000). Registry of toxic effects of chemical

substances: 2H-1,3,5-Thiadiazine-2-thione, tetrahydro-3,5-dimethyl-. Retrieved from

http://www.cdc.gov/niosh/rtecs/xi2ab980.html

Peter Richards has worked as an environmental analyst at the Massachusetts Department of

Environmental Protection’s office in Wilmington, Massachusetts, since 1998. Prior to that he worked for

private environmental consulting firms for approximately ten years.



construction of a permeable reactive barrier in a residential neighborhood

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