seven-year performance evaluation of a permeable reactive barrier

REMEDIATION Summer 2008

Seven-Year Performance Evaluation of aPermeable Reactive Barrier

Peter Richards

In June and July 2001, the Massachusetts Department of Environmental Protection (MassDEP) in-

stalled a permeable reactive barrier (PRB) to treat a groundwater plume of chlorinated solvents mi-

grating from an electronics manufacturer in Needham, Massachusetts, toward the Town of Welles-

ley’s Rosemary Valley wellfield. The primary contaminant of concern at the site is trichloroethene

(TCE), which at the time had a maximum average concentration of approximately 300 micrograms

per liter directly upgradient of the PRB. The PRB is composed of a mix of granular zero-valent iron

(ZVI) filings and sand with a pure-iron thickness design along its length between 0.5 and 1.7 feet.

The PRB was designed to intercept the entire overburden plume; a previous study had indicated

that the contaminant flux in the bedrock was negligible. Groundwater samples have been collected

from monitoring wells upgradient and downgradient of the PRB on a quarterly basis since instal-

lation of the PRB. Inorganic parameters, such as oxidation/reduction potential, dissolved oxygen,

and pH, are also measured to determine stabilization during the sampling process. Review of the

analytical data indicates that the PRB is significantly reducing TCE concentrations along its length.

However, in two discrete locations, TCE concentrations show little decrease in the downgradient

monitoring wells, particularly in the deep overburden. Data available for review include the organic

and inorganic analytical data, slug test results from nearby bedrock and overburden wells, and

upgradient and downgradient groundwater-level information. These data aid in refining the con-

ceptual site model for the PRB, evaluating its performance, and provide clues as to the reasons for

the PRB’s underperformance in certain locations. Oc 2008 Wiley Periodicals, Inc.

INTRODUCTION

In June and July 2001, the MassDEP installed a permeable reactive barrier within aroadway in Needham, Massachusetts, to treat a plume of chlorinated solvents migratingtoward two public water-supply wells located in the adjacent town of Wellesley,Massachusetts. The solvents originated from an electronics manufacturer locatedapproximately 2,300 feet upgradient of the roadway and 5,200 feet upgradient of thepublic supply wells (Exhibit 1). Chlorinated solvents, primarily trichloroethene (TCE),had migrated past the roadway to within 300 feet of the public supply wells. Prior toinstallation, TCE concentrations in the vicinity of the roadway were as high as 1,100micrograms per liter (μg/L). The objective of the PRB installation was to reduce the TCEconcentration downgradient of the PRB to 5 g/L (the maximum contaminant level, orMCL) or less.

c© 2008 Wiley Periodicals, Inc.Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/rem.20172 63

Seven-Year Performance Evaluation of a Permeable Reactive Barrier

Municipal supply wells are shown in upper left. Note location of PRB along roadway and elementary school

in lower right, downgradient of source.

Exhibit 1. Aerial photo showing source area (in lower right) and the path of the plume

Twenty-one performance-monitoring wells (PM series wells) were installed withinthe sidewalks along Central Avenue (eleven upgradient and ten downgradient) to monitorgroundwater levels and water-quality parameters. Six of the PM wells were installed priorto PRB emplacement in order to collect sufficient data for the PRB design. Several otherwells had previously been installed in the area to determine the plume extent; data fromthese other wells were also used in the PRB design. Groundwater levels have beenmeasured and groundwater samples have been collected from the PM wells on a quarterlybasis since the PRB installation. In addition, quarterly groundwater samples are collectedupgradient of the PRB, in the vicinity of Central Avenue and downgradient of the PRBfrom select wells under a separate sampling program. Sufficient data have been collectedin the seven years since the PRB installation to determine whether the PRB is functioningas designed and to measure long-term trends in performance and groundwater quality.

PRB DESIGN

Groundwater samples have been collected from wells in the vicinity of Central Avenuesince March 1995. The analytical data delineated the central part of the plume and

64 Remediation DOI: 10.1002.rem c© 2008 Wiley Periodicals, Inc.


indicated that the plume had migrated into both the overburden and bedrock aquifers. Aconsultant for the MassDEP reviewed the analytical data in 1999 and found that, given thehydraulic conductivity variation in the bedrock and overburden aquifers, the majority ofthe contaminant flux was in the overburden aquifer.

In January 2000, the MassDEP’s design engineer consultant determined theflow-through thickness of the PRB. The PRB was designed to consist of two sections(Zone A and Zone B) to account for the variability in TCE concentrations across the widthof the plume and in hydraulic conductivity of the sand and gravel aquifer. Zone A wasdesigned to treat the central part of the plume, where contaminant concentrations werehigher, while Zone B was designed to treat the flanks of the plume (Exhibit 2). Zone Aextended from the groundwater table to an approximate depth of 38 feet below grade;Zone B extended from 38 feet to the bedrock surface, at a maximum depth of 55 feet.The design thickness for the PRB was revised upward from previous estimates; the ZoneA revised design thickness was 1.7 feet of pure iron, while the revised design thickness forZone B was 0.5 feet of pure iron.

The design thickness for each zone was based on groundwater velocity calculationsand the residence time required to reduce the TCE concentration to a value less than thedrinking water standard (Exhibit 3). For Zone A, the groundwater velocity was calculatedusing the hydraulic conductivity measured at well PM-7S, the average hydraulic gradientin the vicinity of Central Avenue, and an assumed porosity. The hydraulic conductivityvalue at well PM-7S was chosen as it was the highest hydraulic conductivity valuemeasured in the wells along Central Avenue, thus providing a conservative estimate of thegroundwater velocity. The residence time was calculated using the TCE concentration in

Exhibit 2. Cross-section of PRB, showing locations of upgradient wells, zone

locations, and thickness of ZVI placed in each zone

c© 2008 Wiley Periodicals, Inc. Remediation DOI: 10.1002.rem 65


Exhibit 3. PRB thickness calculations (data from design engineer consultant letter to MassDEP, dated January 7, 2000)

Maximum TCE Residence Hydraulic Groundwater PRBZone Well ID Concentration Time Conductivity Gradient Porosity Velocity Thickness

# of roundsμμg/L sampled (hours) (ft/day) notes (ft/ft) (ft/day) (feet)

A PM-7S 81 1 13 159.6 Max. value 0.0059 0.3 3.14 1.70A CW-19 1,100 14 27 78.8 (1) 0.0059 0.3 1.55 1.76B CW-10S 17 7 6 102 (2) 0.0059 0.3 2.01 0.5B MW-18S 43 12 10 53.9 (3) 0.0059 0.3 1.06 0.45B GT-2I 220 3 17 5.72 (3) 0.0049 0.15 0.19 0.14

(1) Hydraulic conductivity value is from well PM-5M.

(2) Hydraulic conductivity value is mid-range between values measured in wells MW-18S (53.9 ft/day) and PM-7S (159.6 ft/day).

(3) Based on average of rising and falling slug tests in well.

well PM-7S from one round of groundwater sampling in December 1999, shortly afterthe well was installed. The design engineer indicated that the resultant design thickness of1.7 feet would therefore effectively treat TCE concentrations in the vicinity of wellPM-7S as well as much higher TCE concentrations in the vicinity of well CW-19D, whichhad been as high as 1,100 μg/L. This calculation was based on the hydraulic conductivitymeasured at well PM-5M and the elevated TCE concentration at CW-19D.

The design thickness for Zone B was based on the estimated groundwater velocity andTCE concentration at well CW-10S. The groundwater velocity was calculated using theaverage gradient in the vicinity of Central Avenue, an assumed porosity, and a hydraulicconductivity value midway between those measured in wells MW-18S and PM-7S. Thedesign engineer also indicated that the design thickness of 0.5 feet would treat themaximum detected TCE concentrations in samples from wells MW-18S and GT-2I (43and 220 μg/L, respectively) prior to design.

Performance Monitoring

The PRB was installed in the middle of Central Avenue in June and July 2001 by GeoConof Pittsburgh, Pennsylvania. Following installation, the remaining series of PM wells wereinstalled in the upgradient and downgradient sidewalks, and quarterly sampling wasinitiated by the design engineer to monitor the performance of the PRB. The PM series ofwells are located such that each downgradient well has a corresponding well located justupgradient of the PRB; each of these corresponding wells have well screens installed atapproximately the same depth interval to monitor PRB performance. For example, wellPM-3S, located downgradient of the PRB, has its well screen located 9 to 19 feet belowgrade (BG). It is located downgradient of corresponding well PM-2S, which is located justupgradient of the PRB and has its well screen located at the same depth interval(Exhibit 4).

The quarterly sampling is supplemented by sampling conducted in the vicinity ofCentral Avenue by a contractor for the potentially responsible party (PRP).



Even-numbered wells are located in the upgradient sidewalk, while the odd-numbered wells are located in the

downgradient sidewalk.

Exhibit 4. Location of performance-monitoring wells along Central Avenue

Approximately 19 wells downgradient of the PRB were sampled by the contractor eachJuly. Beginning in July 2006, this was reduced to eight wells downgradient of the PRB dueto the consistency of the data.

The design engineer’s quarterly sampling is conducted following the US EPA’s LowStress (Low Flow) Purging and Sampling procedure, as detailed in the manual dated July30, 1996 (revision 2). Measurements of pH, redox potential, dissolved oxygen,temperature, turbidity, and specific conductance are made during the sampling process viaa flow-through cell. Traditionally, groundwater samples are collected from 21 monitoringwells each quarter (PM-1A, PM-1B, PM-2S, PM-2D, PM-3S, PM-3D, PM-4S, PM-4D,PM-5S, PM-5D, PM-6S, PM-6M, PM-6D, PM-7S, PM-7M, PM-7D, PM-8S, PM-8D,PM-9S, PM-9D and PM-10). In addition, wells MW-21S and MW-20D, located south ofthe PRB, are usually sampled semiannually. The groundwater samples are submitted foranalysis of volatile organic compounds (VOCs) via US EPA SW846 method 8260.Groundwater levels are measured during each sampling round from each of these wells; inaddition, groundwater levels are measured in an additional 18 wells, when they areaccessible.

Starting with the January 2006 sampling round, the number of wells to be sampledwas reduced to 12 during alternating quarters due to the consistency of the samplingresults. The smaller set of wells to be sampled included PM-1B, PM-10, PM-6S, PM-6M,



PM-6D, PM-7S, PM-7M, PM-7D, PM-8S, PM-8D, PM-9S, and PM-9D. During theother quarterly sampling rounds (typically occurring in April and October), the full suiteof wells is sampled.

As indicated in the Interstate Technology & Regulatory Council’s Lessons Learneddocument (2005), both organic and inorganic parameters should be measured duringgroundwater sampling rounds to monitor PRB performance. When reviewedcontemporaneously, the inorganic and organic data provide a good indication ofperformance and, if problems are noted, possible reasons why the PRB is not performingas designed. Ideally, when using zero-valent iron (ZVI) as the reactive media, pHincreases as hydrogen ion is consumed in the production of ethene(s), dissolved oxygen(DO) decreases as the oxygen is consumed in the oxidation of the iron, and theoxidation-reduction potential (ORP) becomes strongly negative (less than–200 mV).When functioning properly, as the TCE comes into contact with the ZVI, it quicklydegrades into a series of breakdown products (cis-1,2-dichloroethene and vinyl chloride),which typically ends with the production of ethene.

When reviewed contem-poraneously, the inorganicand organic data providea good indication of per-formance and, if problemsare noted, possible reasonswhy the PRB is not perform-ing as designed.

PRB PERFORMANCE

Exhibit 5 shows the average TCE concentration in the same wells prior to (up to andincluding July 2001 data) and following the PRB installation. The wells shown in theexhibit were included, as there are several years of analytical data for them both beforeand following installation of the PRB. The data indicate a significant reduction in TCEconcentrations in most of the downgradient wells since the PRB was installed. The mostsignificant reductions are noted in wells PM-5D, PM-7S, and CW-22D, which hadaverage TCE concentrations greater than 100 μg/L prior to PRB installation. FollowingPRB installation, the average TCE concentrations in these wells were less than 12 μg/L.Many of the wells listed in the exhibit are located several hundred feet downgradient ofthe PRB, and thus did not have elevated TCE concentrations prior to the PRB installation.

The highest average TCE concentration in the downgradient wells following the PRBinstallation is in samples from well PM-7D, which had an average TCE concentrationgreater than 200 μg/L (Exhibit 6). The next highest average TCE concentrations in thedowngradient wells are in samples from wells PM-9D and PM-7M. Wells PM-7D andPM-7M are located downgradient of wells PM-6D and PM-6M; well PM-9D isdowngradient of well PM-8D (see Exhibit 4). Wells PM-6M and PM-6D have the highestaverage TCE concentrations of any of the upgradient wells. The analytical data indicate amoderate decrease in average TCE concentrations in well pairs PM-6M/PM-7M ofapproximately 245 μg/L to approximately 65 μg/L. The deep well pairsPM-6D/PM-7D and PM-8D/PM-9D indicate almost no change in average TCEconcentrations. However, well pair PM-6S/PM-7S shows a significant reduction in TCEconcentration from approximately 202 μg/L to approximately 6 μg/L; well screens forthese wells are approximately 6 to 16 feet below grade. Similarly, well pairPM-8S/PM-9S also shows a significant reduction in average TCE concentration from104 μg/L to 8 μg/L; these wells also have well screens located in the shallow water tableat 5 to 15 feet BG. The average percent reduction of TCE concentrations in thecorresponding well pairs are graphically shown in Exhibit 7, which demonstrates that fourof the average percent reductions are above 90 percent, mostly in the well pairs screened



Exhibit 5. Average (μg/L) TCE concentrations before and after PRB installation

Pre-PRB Average Post-PRB AverageWell ID TCE Concentration TCE Concentration

PM-1A 7 9.1PM-5D 298 11.7PM-7S 275 6.7PM-7M 167 64.4CW-22D 132 1.4CW-23D 26 0.3CW-32S 5.5 NDCW-33S 70 3.2CW-36D 12.2 0.9CW-37S 15.0 1.1CW-50D 8.0 0.7CW-51S 5.6 0.6CW-72D ND NDCW-78S ND NDCW-84D 0.6 NDCW-85S 0.5 NDCW-89S 2.1 NDCW-90D 2.5 NDCW-91D 4.2 NDCW-92S 4.0 0.4CW-93D ND NDMW-25D 28 7.4MW-26S 76 12.5

in the shallow overburden. The exhibit also shows the inadequate PRB performance atwell locations PM-7D and PM-9D. The averages are based on the results of groundwatersamples collected since October 2001.

To evaluate long-term PRB performance, the field parameter data and analyticalresults were plotted over time for each corresponding well pair. Review of correspondingwell pairs where there is sufficient TCE destruction versus well pairs with much less TCEdestruction reveals interesting trends in both TCE concentration and in the fieldparameters. In each of the figures showing analytical data over time, the solid linesrepresent the upgradient wells while the dashed lines represent the downgradient wells.Exhibit 8 shows TCE concentration in corresponding well pair PM-4 and PM-5 for boththe shallow and deep wells (denoted with an “S” and “D,” respectively); it indicates thatthe PRB is greatly reducing contaminant concentrations to levels at or near the designcriteria of 5 μg/L. It also shows a marked decrease in the upgradient TCE concentrations.Exhibit 9 shows the ORP values in corresponding well pair PM-4 and PM-5 for both theshallow and deep wells. The exhibit shows that the two upgradient wells have very similarORP values over time and also indicates a consistent reduction in ORP from the



Exhibit 6. Average PM comparisons

Upgradient Average TCE Corresponding Average TCEWell Concentration (μμg/L) Downgradient Well Concentration (μμg/L)

PM-2S 64 PM-3S 8PM-2D 93 PM-3D 14

PM-4S 83 PM-5S 3PM-4D 115 PM-5D 11

PM-6S 202 PM-7S 6PM-6M 245 PM-7M 65PM-6D 254 PM-7D 218

PM-8S 104 PM-9S 8PM-8D 89 PM-9D 88

Exhibit 7. Average percent reduction of TCE concentrations

in well pairs

Exhibit 8. TCE concentrations for well pair PM-4 and PM-5 for shallow and deep wells



Exhibit 9. ORP values for well pair PM-4 and PM-5 for shallow and deep wells

Exhibit 10. DO data for well pair PM-4 and PM-5 for shallow and deep wells

upgradient to the downgradient wells in each sampling round. Exhibit 10 shows the DOdata for the same wells; the graph shows that during many of the sampling rounds the DOis reduced to values less than 1 milligram per liter (mg/L). However, during several ofthe sampling rounds, there was no decrease in DO from the upgradient to thedowngradient wells. Indeed, one round showed a marked increase in DO values fromupgradient to downgradient wells. Exhibit 11 shows the pH data for the same wells; thegraph shows a fairly consistent increase in pH values from the upgradient to thedowngradient wells. However, like the DO data, the change from the upgradient to thedowngradient wells is not consistent. Occasional sampling rounds show little to noincrease in pH or a decrease in pH values. Conductivity was not graphed, as road saltapplied in the area makes data interpretation problematic. Of all of the field parametersmeasured, the ORP values show the most consistent change from upgradient todowngradient wells as predicted by PRB theory. The degree of change in ORP values alsocorrelates well with changes in TCE concentrations for the corresponding well pairs.

Review of field parameters and analytical data for corresponding wells where there issignificantly less TCE reduction reveals some interesting indicators of PRB performance



Exhibit 11. pH data for well pair PM-4 and PM-5 for shallow and deep wells

and chemistry. Exhibit 12 shows the TCE concentration in corresponding well pair PM-6and PM-7. At each of these locations, there are three wells: a shallow water table well,one well at moderate depth, and one screened at the bedrock surface, approximately 54feet BG. As in Exhibit 8, the graph shows a steady decrease in upgradient TCEconcentrations, with the three upgradient wells (the PM 6 series) having relativelycomparable TCE concentrations. The downgradient wells (PM 7 series), on the otherhand, show large differences in TCE concentrations, reflecting variations in PRBperformance with depth at this location. The shallow downgradient well has TCEconcentrations at or near the design criteria of 5 μg/L. TCE concentrations in thedowngradient well screened at moderate depth (PM-7M) fluctuate widely, indicatingvariable PRB performance. However, the downgradient well screened at the bedrocksurface (PM-7D) has TCE concentrations very similar to its corresponding upgradientwell, indicating essentially no reduction in TCE at depth in this part of the aquifer.Exhibit 13 shows ORP values over time for the same wells. The ORP values in the threeupgradient wells are remarkably consistent. However, the ORP values in the three

Exhibit 12. DO data for well pair PM-6 and PM-7 for shallow and

deep wells



Exhibit 13. ORP values for well pair PM-6 and PM-7 for shallow and deep wells

corresponding downgradient wells vary greatly. The greatest reduction in ORP values isnoted in the shallow downgradient well (PM-7S). The ORP values in the well screened atmoderate depth shows some decrease relative to its corresponding upgradient well, butthe ORP values in the deep well show little to no change from its correspondingupgradient well. These ORP changes are similar to the changes noted in TCEconcentration in these wells, indicating a good correlation between change in ORP andthe degree of TCE reduction. It should be noted, however, that the contaminant plume inthe shallow overburden in the vicinity of the PM-6 and PM-7 series wells is migratingthrough Zone A (designed with 1.7 feet of ZVI), while the plume in the deep overburdenin this area passes through Zone B of the PRB (designed with 0.5 feet of ZVI) (seeExhibit 2). The graphs of DO and pH for the same wells (not shown) reveal more scatter,indicating less of a correlation in these field parameters with the degree of TCE reduction.

The graph of TCE for corresponding well pair PM-8 and PM-9 (Exhibit 14) indicatesinadequate TCE reduction at depth at this location also. The average TCE reduction forwell pairs PM-8D and PM-9D is 0 percent (Exhibit 7), indicating no contaminant

Exhibit 14. TCE concentrations for well pair PM-8 and PM-9 for shallow and deep wells



treatment observed in these wells, which are screened at a depth of 25 to 35 feet BG. TCEconcentrations in the downgradient shallow well, however, are very low, resulting in anaverage reduction of 92 percent. The graph of ORP for these well pairs (not shown) doesnot reveal the same correlation of ORP change and TCE reduction as at the nearby wellsPM-6 and PM-7.

The elevated TCE concen-trations at depth at loca-tions PM-7D and PM-9Dindicate either a construc-tion- or design-related is-sue affecting PRB perfor-mance at these locations.

The presence of TCE breakdown products cis-1,2-dichloroethene and vinyl chloride(VC) in the downgradient wells can also be used as a measure of PRB performance. Thecis-1,2-dichloroethene isomer has been detected in samples from each of the upgradientPM series wells, with the highest average concentrations detected in samples from thethree PM-6 series wells (approximate average 8 μg/L). It has also been detected insamples from each of the downgradient PM series wells, with the highest averagecis-1,2-dichloroethene concentrations in the downgradient wells observed in samples fromPM-7M, PM-7D, and PM-9D (approximate average 9 μg/L). No vinyl chloride has beendetected in any of the upgradient overburden wells in the 26 sampling rounds conductedfollowing the PRB installation to date. However, vinyl chloride has been detected in fiveof the downgradient PM wells (PM-5S, PM-5D, PM-7S, PM-7M, and PM-7D). Althoughthe maximum average VC concentration is only 4.1 μg/L, observed in samples from wellPM-7S, its consistent absence in upgradient wells and presence in certain downgradientwells indicate that it is the product of incomplete TCE reduction within the PRB,suggesting an inadequate residence time with the ZVI. Interestingly, the presence andconcentration of the breakdown products does not correspond with the percent TCEreduction noted in Exhibit 7. For example, no VC has been detected in downgradientwells PM-9S (shallow) or PM-9D (deep). However, well pair PM-8S/9S had an averageTCE reduction of 92 percent, while well pair PM-8D/9D had an average TCE reductionof 0 percent. One possible explanation for this is that at well pair PM-8S/9S, the processof TCE degradation via the reaction pathway to the production of ethene is complete,while at well pair PM-8D/9D, the TCE degradation process does not occur at all.

PERFORMANCE EVALUATION

The elevated TCE concentrations at depth at locations PM-7D and PM-9D indicate eithera construction- or design-related issue affecting PRB performance at these locations.Conversely, the reductions in TCE concentrations at well pairs PM-6S/PM-7S andPM-8S/PM-9S indicate the PRB is functioning at the shallow depth, near the water table.The consistency of the data indicates that the problem is not related to laboratory orsampling error. A number of explanations are possible to explain the elevated TCEconcentrations at depth downgradient of the PRB, including bedrock contribution to thedeep overburden, higher-than-anticipated permeability values, or TCE concentrations atdepth resulting in inadequate PRB design at these locations (i.e., insufficient design ironthickness), TCE desorbing from soil particles downgradient of the PRB within thesaturated zone, or a construction-related issue (the ZVI was not properly emplaced atdepth to the design thickness or trench failure occurred at depth in discrete areas). In thespring and summer of 2007, the MassDEP conducted additional testing along CentralAvenue to evaluate the possible hypotheses. The hydrogeologic and geochemical dataassisted in refining the conceptual site model and were used, in accordance withOckham’s Razor, to determine the most plausible explanation.



To evaluate the potential for contaminant migration from the bedrock aquifer tocontribute to elevated TCE concentrations in the overburden aquifer, vertical hydraulicgradients were reviewed for bedrock/overburden wells along Central Avenue. There arefour bedrock wells located upgradient of the PRB (shown on Exhibit 2); three of themhave overburden wells located near them providing a well couplet in which to monitorvertical hydraulic gradients between the bedrock and the overburden. Wells MW-22D(bedrock) and MW-23M (overburden) are located adjacent to well PM-4S and PM-4D.Wells GT-1D (bedrock) and GT-2I (overburden) are located adjacent to wells PM-6S,PM-6M, and PM-6D. Wells GT-3D (bedrock) and GT-4I (overburden) are located nearwells PM-8S and PM-8D. Upward vertical hydraulic gradients (from the bedrock to theoverburden) were noted during most of the sampling rounds; the average upwardgradient was less than 0.1 foot/foot.

To evaluate the poten-tial for contaminant mi-gration from the bedrockaquifer to contribute to el-evated TCE concentrationsin the overburden aquifer,vertical hydraulic gradi-ents were reviewed forbedrock/overburden wellsalong Central Avenue.

To further evaluate the hypothesis of a bedrock contribution to the overburdencontaminant plume, permeability testing, using both rising and falling head tests, wasconducted in April 2007 in bedrock wells GT-1D and GT-3D and in the problem area ofwells PM-6, PM-7, PM-8, and PM-9. Groundwater samples were also collected fromthese wells to determine TCE concentrations in both overburden and bedrock wells.Based on the testing, the permeability of the bedrock was found to be between three andfour orders of magnitude less than that in the overburden material. Results of thegroundwater sampling indicated that the bedrock wells in the vicinity of the problem areasat PM-7D and PM-9D contained a maximum of 7 μg/L of TCE, far less than in the deepoverburden wells. Thus, it does not appear plausible that the elevated TCE levels are dueto a bedrock source.

In November 1999, prior to PRB installation, well PM-5D was installeddowngradient of the proposed PRB location in order to collect data for the PRB design.One soil sample from a depth of 25 to 27 feet BG was preserved in methanol andsubmitted for VOC analysis; no VOCs were detected above reporting limits. The soilsample was collected from boring PM-5D as it was believed to be located in the area ofthe plume with the highest TCE concentrations. Boring logs along Central Avenueindicated the presence of medium-grained sands in the aquifer with little to trace silt,consistent with the concept of a buried glacial valley. Based on these data, the elevatedTCE concentrations cannot be attributed to soil desorption downgradient of the PRB.

In order to determine whether the initial PRB design was adequate in the vicinity ofthe deep wells at PM-7D and PM-9D, the design process was revisited using the sameprocess as initially performed prior to the PRB installation. The initial PRB design couldbe flawed if upgradient TCE concentrations or aquifer permeability values are higher thanoriginally anticipated (i.e., insufficient site assessment was conducted prior to the designphase). Exhibit 15 shows the revised design data, calculating the appropriate PRBthickness upgradient of the problem-area wells PM-7 and PM-9. The design wasrecalculated using the hydraulic conductivity data specifically collected at well seriesPM-6, PM-7, PM-8, and PM-9 in the spring of 2007, the average TCE concentration ineach upgradient well from July 2005 through October 2007 and hydraulic gradientsmeasured in each corresponding well pair (i.e., from well PM-6S to PM-7S). The onlyvariable not changed in the postinstallation design calculations was the porosity, whichwas maintained at an assumed value of 0.3 given the nature of the aquifer lithology. Therecalculated PRB thickness for each corresponding well pair, as shown in the right-handcolumn of Exhibit 15, can be compared to that originally determined for Zones A and B.



Exhibit 15. PRB thickness calculations (post installation)

Zone TCE Concentra- Residence Hydraulic Gradient Groundwater PRB(feet ZVI) Well ID tion (μμg/L∗) Time (hours) Conductivity (ft/ft) Porosity Velocity (ft/day) Thickness (ft)

(ft/day)

A PM-6S 100 13 96.8 0.0097 0.3 3.36 1.82(1.7) PM-7S 111.2

104 AverageA PM-6M 163 15.1 82.3 0.0069 0.3 2.01 1.26(1.7) PM-7M 92.2

87.25 AverageB PM-6D 189 15.7 69.2 0.0088 0.3 1.06 0.7(0.5) PM-7D 3.2

36.2 AverageB PM-8S 74 11.7 34.1 0.0018 0.3 0.024 0.12(0.5) PM-9S 46.3

40.2 AverageB PM-8D 58 10.6 45.9 0.0021 0.3 0.36 0.16(0.5) PM-9D 56.6

51.25 Average

The exhibit shows that for corresponding well pairs PM-6M/7M, PM-8S/9S, andPM-8D/9D, the thickness as initially designed (shown in the left-hand column) is greaterthan the redesigned thickness values (shown in the right-hand column). Therefore, ifconstructed as originally designed, the TCE concentrations in those correspondingdowngradient wells should be at or less than the design criteria of 5 μg/L. Conversely,for corresponding well pairs PM-6S/7S and PM-6D/7D, the recalculated design thicknessis greater than that initially calculated and installed within the PRB, suggesting that thereis insufficient ZVI at these locations to fully remediate the TCE plume to the MCL.

However, comparison of the revised design data with actual PRB performance yieldsseveral inconsistencies. For example, the revised design PRB thickness for correspondingwell pair PM-6S/7S implies that there is insufficient ZVI to fully remediate the TCEplume at that shallow water table location. However, Exhibit 7 shows that there is, onaverage, 92 percent TCE reduction at this location. Conversely, the revised design PRBthickness for corresponding well pair PM-8D/9D suggests that there is sufficient ZVI totreat the contaminant plume at that location, implying that the TCE levels indowngradient well PM-9D should be below the design criteria. Exhibit 7, however,shows an average of 0 percent TCE reduction for well pair PM-8D/9D. At well pairPM-6D/7D, the revised design thickness is 0.7 feet; the initial PRB design called for 0.5feet at this location. Exhibit 7 shows an average of 13 percent TCE reduction for this wellpair. If a thickness of 0.5 feet of ZVI was installed within the PRB in the vicinity of wellsPM-6D/7D as designed, then there should be more TCE reduction than observed.Likewise, there should be more TCE reduction observed in corresponding well pairPM-8D/9D if a thickness of 0.5 feet of ZVI was installed within that area of the PRB.



These data suggest that although the PRB may not have been initially designed withsufficient ZVI in some places, the flaws in PRB performance noted at two locations atdepth cannot completely be attributed to the design.

The pH, density, andviscosity of the slurry weremonitored daily duringconstruction to ensurethat it did not prematurelydegrade.

Given that the explanations noted above cannot sufficiently explain the empiricaldata, the problems noted with PRB performance at depth must, at least in part, beattributable to construction. The PRB was installed to the bedrock surface, which in thevicinity of deep well PM-6D is approximately 55 feet BG. During the PRB construction, aboulder was encountered at the bedrock surface in the section of the PRB downgradientof well PM-6D. The boulder could not be removed after numerous attempts and was leftin place. It should also be noted that the biopolymer slurry used to keep the trench open“failed” during construction of the PRB at two locations, including in the vicinity of wellPM-8S/8D. The pH, density, and viscosity of the slurry were monitored daily duringconstruction to ensure that it did not prematurely degrade. Microbial activity breaksdown the slurry, reducing its viscosity. When the viscosity of the slurry “failed” (i.e., wasunacceptably low), an “enhanced” slurry with elevated pH was added to increase theviscosity and reduce the biological activity. Thus, enhanced slurry was added to the trenchduring construction of the PRB in the vicinity of wells PM-8S/8D; the trench wasexcavated to the bedrock surface and backfilled with the appropriate ZVI mix as quickly aspossible to preclude trench failure.

To determine whether the ZVI mix was emplaced uniformly during PRBconstruction and note the potential presence of failed sections of the trench, fourfour-inch diameter borings were installed within the ZVI mixture following completion ofthe PRB installation using a roto-sonic drill rig. In addition, small-diameter microwellswere installed in each borehole during installation to collect groundwater samples withinthe PRB and conduct permeability tests. Fortuitously, one of the borings was installedwithin the trench just downgradient of well series PM-8 to a depth of 35 feet, whileanother one was installed just downgradient of well series PM-6 to a depth of 54 feet (i.e.,within sections of the PRB where problems were subsequently noted). Examination of theZVI material removed from the well casing at each borehole location did not reveal thepresence of native material (i.e., there was no indication of trench failure at theselocations). However, it is possible that relatively small-scale trench failures could haveoccurred between the borehole locations without being observed or that the ZVI mix wasnot uniformly emplaced at depth. Given the presence of the large cobbles and the slurryfailure in the problematic sections of the PRB, a construction-related explanation for theflaws in PRB performance remains the most plausible hypothesis. From this viewpoint, itdoes not matter what the reactive media is within the PRB (ZVI, organic mulch, etc.), asthe difficulty lies in emplacing that media at the proper depth and at the proper thickness.

CONCLUSIONS

In June and July 2001, the MassDEP installed a PRB within a roadway in Needham,Massachusetts, to treat a plume of TCE emanating from an electronics manufacturer.Since then, groundwater samples have been collected quarterly from a series of 21monitoring wells located upgradient and downgradient of the PRB to monitor itsperformance. Evaluation of the inorganic and organic analytical data since installationindicates that the PRB is destroying most of the TCE except in two problematic areas,



located at depth near the bedrock surface. A mass flux analysis of the PRB’s performance,conducted by discretizing the PRB into numerous sections and evaluating TCEconcentrations and groundwater flux in each section, indicates that on average the PRB isdestroying approximately 80 percent of the TCE plume. Evaluation of the site conceptualmodel suggests a number of explanations for the presence of elevated TCE concentrationsdowngradient of the PRB. Through a review of the data gathered to date, the mostplausible explanation is trench failure occurring at depth during installation of the PRB,leading to inadequate placement of the proper thickness of ZVI. Analysis of the initialdesign parameters suggests that although there may be an insufficient ZVI thickness insome areas, there should be greater TCE reduction than currently observed if the PRBwas constructed specifically as designed.

ACKNOWLEDGMENTS

The assistance of Patricia Donahue (MassDEP) during the preparation and review of thisdocument is gratefully acknowledged.

REFERENCES

Interstate Technology & Regulatory Council (ITRC). (2005). Permeable reactive barriers: Lessons learned/new

directions. PRB-4. Washington, DC: Author.

U.S. Environmental Protection Agency (US EPA). (1996). Low stress (low flow) purging and sampling

procedure for the collection of ground water samples from monitoring wells, SOP # GW 0001. Revision

2. Region 1.

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.


seven-year performance evaluation of a permeable reactive barrier

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