seven-year performance evaluation of a permeable reactive barrier

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  • 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-

    leys 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 PRBs 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 (g/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.

  • REMEDIATION Summer 2008

    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 MassDEPs 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, zonelocations, and thickness of ZVI placed in each zone

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

  • Seven-Year Performance Evaluation of a Permeable Reactive Barrier

    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 roundsg/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 g/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 g/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).

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

  • REMEDIATION Summer 2008

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