Monitoring trichloroethene remediation at an iron permeable reactive barrier using stable carbon isotopic analysis

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  • cEnviroMetal Technologies, Inc., 745 Bridge St. W., Suite 7, Waterloo, ON, Canada, N2V 2G6d

    more enriched d13C values compared to the upgradient mean. In addition, d13C values for thedegradation products of TCE, cis-dichloroethene and vinyl chloride, show fractionation patterns

    Journal of Contaminant Hydrology 78 (2005) 313325

    www.elsevier.com/locate/jconhydURS Corporation, 8181 East Tufts Ave., Denver, CO, 80237, United StateseAFCEE/ ERD-FEW, 300 Vesle Drive, Building 367, F.E. Warren AFB, WY, 82005, United States

    Received 27 May 2004; received in revised form 31 May 2005; accepted 31 May 2005

    Abstract

    Stable carbon isotopic analysis, in combination with compositional analysis, was used to evaluate

    the performance of an iron permeable reactive barrier (PRB) for the remediation of ground water

    contaminated with trichloroethene (TCE) at Spill Site 7 (SS7), F.E. Warren Air Force Base,

    Wyoming. Compositional data indicated that although the PRB appeared to be reducing TCE to

    concentrations below treatment goals within and immediately downgradient of the PRB,

    concentrations remained higher than expected at wells further downgradient (i.e. N9 m) of thePRB. At two wells downgradient of the PRB, TCE concentrations were comparable to upgradient

    values, and d13C values of TCE at these wells were not significantly different than upgradient values.Since the process of sorption/desorption does not significantly fractionate carbon isotope values, this

    suggests that the TCE observed at these wells is desorbing from local aquifer materials and was

    present before the PRB was installed. In contrast, three other downgradient wells show significantlyMonitoring trichloroethene remediation at an iron

    permeable reactive barrier using stable carbon

    isotopic analysis

    Nancy VanStone a,b, Andrzej Przepiora c, John Vogan c,

    Georges Lacrampe-Couloume a, Brian Powers d, Ernesto Perez e,

    Scott Mabury b, Barbara Sherwood Lollar a,*

    aStable Isotope Laboratory, University of Toronto, 22 Russell St., Toronto, ON, Canada, M5S 3B1bDepartment of Chemistry, University of Toronto, 80 St. George St., Toronto, ON, Canada, M5S 3H60169-7722/$ -

    doi:10.1016/j.

    * Correspon

    E-mail addding author. Tel.: +1 416 978 0770; fax: +1 416 978 3938.see front matter D 2005 Elsevier B.V. All rights reserved.

    jconhyd.2005.05.013

    ress: bslollar@chem.utoronto.ca (B. Sherwood Lollar).

  • expected for the products of the reductive dechlorination of TCE. Since concentrations of both TCE

    and degradation products drop to below detection limit in wells within the PRB and directly below it,

    these downgradient chlorinated hydrocarbon concentrations are attributed to desorption from local

    aquifer material. The carbon isotope values indicate that this dissolved contaminant is subject to local

    degradation, likely due to in situ microbial activity.

    D 2005 Elsevier B.V. All rights reserved.

    Keywords: Carbon isotopes; Groundwater remediation; Permeable reactive barrier; Reductive dechlorination;

    Trichloroethene; Zero-valent iron

    1. Introduction

    Permeable reactive barriers (PRBs) constructed of elemental iron have emerged as an

    effective passive remediation method for ground water contaminated with a range of

    contaminants, mainly chlorinated hydrocarbons (CHCs) (e.g. Agrawal and Tratnyek,

    1996; Alowitz and Scherer, 2002; Gillham and OHannesin, 1994; Hozalski et al., 2001;

    Nam and Tratnyek, 2000). A PRB is an in situ engineered zone of reactive material placed

    across the path of contaminated ground water. The major advance of PRBs over other

    ground water remediation approaches is the lack of above ground structures, low operation

    and maintenance cost and the enhanced remediation efficiency, particularly compared with

    pump-and-treat systems. There are currently more than 90 PRBs installed at CHC

    contaminated sites (OHannesin, 2003). Many studies have investigated the pathways,

    mechanisms, kinetics and longevity of CHC degradation using elemental iron (see Scherer

    et al., 2000; Tratnyek, 2002).

    Assessment of iron PRB performance is based on monitoring CHC concentrations,

    along with pH and Eh, and major inorganic constituents, in ground water well transects

    across the PRB. Often, iron PRBs are installed within existing contaminant plumes and

    therefore elevated concentrations of contaminants are observed downgradient of PRBs for

    some time after the system has been installed, depending on the extent of initial

    contamination, ground water flow rates, desorption rates and type of the aquifer material.

    The persistence of CHCs downgradient of iron PRBs is expected because significant

    volumes of ground water may be needed for the remnant contaminants to be desorbed and

    flushed from downgradient aquifer materials (Heneman et al., 2001; Powell et al., 1998). It

    is difficult, however, to evaluate PRB performance in situations where elevated

    contaminant concentrations downgradient of the PRB remain for longer than expected

    periods (Powell et al., 1998). This could arise for three reasons: (1) incomplete degradation

    within the PRB, (2) hydraulic bypass underneath or around the PRB, and (3) slower than

    expected desorption of remnant contaminants trapped within aquifer materials. Distin-

    guishing between these processes is important for the assessment of PRB performance and

    consequently developing new techniques that help resolve these processes is of

    considerable interest.

    Fractionation of stable carbon isotopes has been observed for chlorinated ethenes

    N. VanStone et al. / Journal of Contaminant Hydrology 78 (2005) 313325314during processes such as biodegradation (Bloom et al., 2000; Hunkeler et al., 1999;

    Sherwood Lollar et al., 2001; Slater et al., 2001), oxidation by permanganate (Hunkeler et

  • whereas carbon isotopic fractionation associated with degradative processes is on the orderof tens of permil (x).Carbon compound specific isotopic analysis (CSIA) is used to measure carbon isotope

    fractionation and has been shown to be a powerful tool for identifying in situ

    biodegradation in the field for contaminants such as perchloroethene (PCE) and

    trichloroethene (TCE) (Hunkeler et al., 1999; Sherwood Lollar et al., 2001; Song et al.,

    2002; Vieth et al., 2003). Several studies have shown large and reproducible carbon

    isotopic fractionation during the degradation of chlorinated ethenes on electrolytic and cast

    iron (Bill et al., 2001; Dayan et al., 1999; Slater et al., 2002; VanStone et al., 2004), but

    these observations have not been applied in the field to date. The objective of this study

    was to investigate the use of carbon stable isotopic analysis to help evaluate PRB

    performance at a field installation for the remediation of TCE. Routine performance

    monitoring of the PRB consisted of quantification of TCE and its products upgradient,

    within and downgradient of the iron PRB. In addition, carbon CSIAwas carried out in the

    vicinity of the PRB to investigate the source of elevated chlorinated ethene concentrations

    observed downgradient of the installation.

    2. Site background

    A PRB constructed of iron filings was installed in 1999 at the F.E. Warren Air Force

    Base, Wyoming, Spill Site 7 (SS7) to remediate ground water contaminated with TCE.

    The TCE originates from a defunct on-site liquid oxygen manufacturing facility. A grease

    trap at the plant was the source of organic solvents (primarily TCE) to a surface drainage

    ditch, resulting in migration of the solvents through the vadose zone to local ground water

    (Heneman et al., 2001). Construction of the PRB was completed in October 1999. A

    schematic of the SS7 PRB is shown in Fig. 1, with the direction of ground water flow

    indicated. The PRB is 173 m long, has a flow through thickness of 1.2 m and a saturated

    depth of 4.6 m below the historic low ground water level, with a low-permeability clay cap

    installed on the PRB to minimize ground water flow over the top of the treatment PRB.al., 2003; Poulson and Naraoka, 2002), and reductive dechlorination on zero-valent iron

    (Bill et al., 2001; Dayan et al., 1999; Slater et al., 2002; VanStone et al., 2004). Carbon

    isotopic fractionation results from differences in the rates of reaction for 13C- and 12C-

    bearing molecules. The differences in reaction rates for the different isotopes are due to

    mass-dependent differences in activation energies for the respective reactions (Fry, 1971).

    In general, for most chlorinated ethenes and aromatic compounds, the lighter isotope (12C)

    reacts faster than the heavier isotope (13C), leading to fractionation, and enrichment of the

    heavy isotope in the remaining reactant as the reaction proceeds. Likewise, this leads to

    enrichment in the light isotope with respect to the parent compound in the products of the

    reaction. Laboratory experiments have shown that carbon isotopic fractionation for

    chlorinated ethenes is not significant (i.e. b0.5x) for non-degradative processes such asdissolution, vaporization or adsorption under equilibrium conditions (Dempster et al.,

    1997; Harrington et al., 1999; Poulson and Drever, 1999; Slater et al., 1999, 2000),

    N. VanStone et al. / Journal of Contaminant Hydrology 78 (2005) 313325 315The PRB was installed at the site as an interim remediation measure and it does not extend

    to the bottom of the contaminated shallow aquifer. The PRB is located within 20 to 60 m

  • N. VanStone et al. / Journal of Contaminant Hydrology 78 (2005) 313325316of a perennial creek. The SS7 PRB contains approximately 1.6106 kg of iron filings. Toaccommodate the variations in ground water velocity and CHC concentration expected

    along the line of PRB installation, the amount of iron in the PRB was varied by using a

    different proportion of iron/sand in each of the three sections: Section 1 with 100% iron,

    Section 2 with 25% iron/75% sand mix, and Section 3 with 38% iron/62% sand mix.

    After the PRB was installed in October 1999, the system was left undisturbed for 6

    months to allow the ground water conditions to equilibrate before implementing a

    performance monitoring program (Heneman et al., 2001). The monitoring wells discussed

    in this study are shown in Fig. 1 and represent only a portion of the wells at the site. These

    monitoring wells are made of PVC and have 1.5 m screens. There are a series of

    monitoring wells at the centre of each segment of the treatment PRB (Transects 1, 2 and

    3), which are shown in expanded view in Fig. 1. In Transect 1, wells 101A and 101B are

    located within a meter of the upgradient side of the PRB at depths of 8.5 and 4.5 m,

    respectively, with well 101A located about 1 m below the PRB and 101B located within

    Fig. 1. Schematic Map of SS7 PRB. Gray-filled circles indicate the position of monitoring wells that contained

    concentrations of contaminants above the detection limits (d.l.) for isotopic analysis, and black-filled circles

    indicate wells that did not contain adequate concentrations for CSIA. The treatment PRB is approximately 173 m

    long. Base map is to scale (1 cm=10 m). Hatched lines show expanded view of transects 1, 2 and 3. Section 1 is

    1.2 m thick, Section 2 is 0.3 m thick and Section 3 is 0.5 m thick. Expanded views show sampling wells located at

    the centre of each main section (Transects 1 to 3). See text for detailed description of each monitoring well. Wells

    indicated (*) are at a depth at least 1 m below the PRB.

  • projects (OHannesin and Gillham, 1998).Performance monitoring of the SS7 PRB revealed a decrease in CHC concentration

    across the PRB from upgradient to non-detectable values within the PRB and immediately

    downgradient (Heneman et al., 2001). In contrast, in wells further downgradient of the

    PRB (e.g. wells 186, 700B, 707B and 708 located 9 to 12 m from the PRB) concentrations

    of both TCE and cDCE were comparable to upgradient concentrations and were suggested

    to be due to desorption of TCE from aquifer materials downgradient of the PRB (Heneman

    et al., 2001). Corresponding increases in pH and decreases in Eh, carbonate alkalinity,

    Ca2+, and Mg2+ were noted within the PRB and immediately downgradient in comparison

    to upgradient wells (Heneman et al., 2001). These trends are expected due to reducing

    conditions created by iron corrosion and are consistent with those noted at other PRB sites

    (OHannesin and Gillham, 1998). The values of iron-sensitive parameters (i.e. Eh and pH)

    and constituents (Ca2+, alkalinity) measured in wells 700B, 707B, and 173B were similar

    to the upgradient values. It was unclear whether the persistent high concentrations of TCE

    and cDCE in the downgradient wells were due to incomplete degradation of TCE within

    the PRB, or to continued desorption from the aquifer material (Heneman et al., 2001).

    Based on hydraulic heads measured at each monitoring well, ground water flow paths in

    the vicinity of the PRB were not significantly altered from pre-PRB ground water flow

    records (Heneman et al., 2001), ruling out hydraulic bypass of the PRB as the source of

    elevated downgradient concentrations of CHCs.

    3. Methods

    Samples for concentrations of chlorinated hydrocarbons and carbon isotopic analysis

    were taken in June 2002 (Table 1). Four 40 mL samples from each well were preserved

    with several drops of undiluted HCl to inhibit microbial activity, and stored without

    headspace. Additional water samples were taken from each well for analysis of TCE,

    cDCE and VC concentrations by EPA method 502.1. The detection limit for this analysis

    is 0.05 ug/L. All samples for carbon CSIA were packed on ice and shipped to the Stable

    Isotope Laboratory at the University of Toronto, Canada, and analyzed within 1 month of

    sampling. Detection limits for isotopic analysis varies from compound to compound but isthe ground water plume transected by the PRB. Well 102 is located in the middle of the

    PRB. Wells 103A and 103B are located approximately 1 m downgradient of the PRB at

    depths of 8.5 and 4.5 m, respectively, with 103A located about 1 m below the PRB and

    103B located within the ground water plume intercepted by the PRB. Well 700B is located

    9 m downgradient from the PRB. The wells for the other 2 transects (Transect 2 and 3) are

    similarly located with respect to the PRB.

    Periodic sampling of the groundwater monitoring wells has been conducted since April

    2000 for concentrations of CHCs (TCE, cDCE, trans-dichloroethene and VC) using

    standard EPA method 502.1, and several inorganic indicators of water quality (Heneman et

    al., 2001). There were consistent decreases in Eh, sulfate, calcium and magnesium

    concentrations and increases in pH consistent with those documented at other PRB

    N. VanStone et al. / Journal of Contaminant Hydrology 78 (2005) 313325 317between 10 and 30 ug/L for TCE, cDCE and VC. One day prior to analysis, 2 of each 40

    mL water samples were transferred to 160 mL glass bottles with 40 g of NaCl to facilitate

  • Table 1

    Concentrations and stable carbon isotopic compositions of VC, cDCE and TCE from monitoring wells at

    F.E.Warren AFB spill site 7 iron-filings PRB

    Well TCE cDCE VC

    (Ag/L) d13C (x) (Ag/L) d13C (x) (Ag/L) d13C (...

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