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

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<ul><li><p>cEnviroMetal Technologies, Inc., 745 Bridge St. W., Suite 7, Waterloo, ON, Canada, N2V 2G6d</p><p>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</p><p>Journal of Contaminant Hydrology 78 (2005) 313325</p><p> 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</p><p>Received 27 May 2004; received in revised form 31 May 2005; accepted 31 May 2005</p><p>Abstract</p><p>Stable carbon isotopic analysis, in combination with compositional analysis, was used to evaluate</p><p>the performance of an iron permeable reactive barrier (PRB) for the remediation of ground water</p><p>contaminated with trichloroethene (TCE) at Spill Site 7 (SS7), F.E. Warren Air Force Base,</p><p>Wyoming. Compositional data indicated that although the PRB appeared to be reducing TCE to</p><p>concentrations below treatment goals within and immediately downgradient of the PRB,</p><p>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</p><p>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</p><p>suggests that the TCE observed at these wells is desorbing from local aquifer materials and was</p><p>present before the PRB was installed. In contrast, three other downgradient wells show significantlyMonitoring trichloroethene remediation at an iron</p><p>permeable reactive barrier using stable carbon</p><p>isotopic analysis</p><p>Nancy VanStone a,b, Andrzej Przepiora c, John Vogan c,</p><p>Georges Lacrampe-Couloume a, Brian Powers d, Ernesto Perez e,</p><p>Scott Mabury b, Barbara Sherwood Lollar a,*</p><p>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/$ -</p><p>doi:10.1016/j.</p><p>* Correspon</p><p>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.</p><p>jconhyd.2005.05.013</p><p>ress: (B. Sherwood Lollar).</p></li><li><p>expected for the products of the reductive dechlorination of TCE. Since concentrations of both TCE</p><p>and degradation products drop to below detection limit in wells within the PRB and directly below it,</p><p>these downgradient chlorinated hydrocarbon concentrations are attributed to desorption from local</p><p>aquifer material. The carbon isotope values indicate that this dissolved contaminant is subject to local</p><p>degradation, likely due to in situ microbial activity.</p><p>D 2005 Elsevier B.V. All rights reserved.</p><p>Keywords: Carbon isotopes; Groundwater remediation; Permeable reactive barrier; Reductive dechlorination;</p><p>Trichloroethene; Zero-valent iron</p><p>1. Introduction</p><p>Permeable reactive barriers (PRBs) constructed of elemental iron have emerged as an</p><p>effective passive remediation method for ground water contaminated with a range of</p><p>contaminants, mainly chlorinated hydrocarbons (CHCs) (e.g. Agrawal and Tratnyek,</p><p>1996; Alowitz and Scherer, 2002; Gillham and OHannesin, 1994; Hozalski et al., 2001;</p><p>Nam and Tratnyek, 2000). A PRB is an in situ engineered zone of reactive material placed</p><p>across the path of contaminated ground water. The major advance of PRBs over other</p><p>ground water remediation approaches is the lack of above ground structures, low operation</p><p>and maintenance cost and the enhanced remediation efficiency, particularly compared with</p><p>pump-and-treat systems. There are currently more than 90 PRBs installed at CHC</p><p>contaminated sites (OHannesin, 2003). Many studies have investigated the pathways,</p><p>mechanisms, kinetics and longevity of CHC degradation using elemental iron (see Scherer</p><p>et al., 2000; Tratnyek, 2002).</p><p>Assessment of iron PRB performance is based on monitoring CHC concentrations,</p><p>along with pH and Eh, and major inorganic constituents, in ground water well transects</p><p>across the PRB. Often, iron PRBs are installed within existing contaminant plumes and</p><p>therefore elevated concentrations of contaminants are observed downgradient of PRBs for</p><p>some time after the system has been installed, depending on the extent of initial</p><p>contamination, ground water flow rates, desorption rates and type of the aquifer material.</p><p>The persistence of CHCs downgradient of iron PRBs is expected because significant</p><p>volumes of ground water may be needed for the remnant contaminants to be desorbed and</p><p>flushed from downgradient aquifer materials (Heneman et al., 2001; Powell et al., 1998). It</p><p>is difficult, however, to evaluate PRB performance in situations where elevated</p><p>contaminant concentrations downgradient of the PRB remain for longer than expected</p><p>periods (Powell et al., 1998). This could arise for three reasons: (1) incomplete degradation</p><p>within the PRB, (2) hydraulic bypass underneath or around the PRB, and (3) slower than</p><p>expected desorption of remnant contaminants trapped within aquifer materials. Distin-</p><p>guishing between these processes is important for the assessment of PRB performance and</p><p>consequently developing new techniques that help resolve these processes is of</p><p>considerable interest.</p><p>Fractionation of stable carbon isotopes has been observed for chlorinated ethenes</p><p>N. VanStone et al. / Journal of Contaminant Hydrology 78 (2005) 313325314during processes such as biodegradation (Bloom et al., 2000; Hunkeler et al., 1999;</p><p>Sherwood Lollar et al., 2001; Slater et al., 2001), oxidation by permanganate (Hunkeler et</p></li><li><p>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</p><p>fractionation and has been shown to be a powerful tool for identifying in situ</p><p>biodegradation in the field for contaminants such as perchloroethene (PCE) and</p><p>trichloroethene (TCE) (Hunkeler et al., 1999; Sherwood Lollar et al., 2001; Song et al.,</p><p>2002; Vieth et al., 2003). Several studies have shown large and reproducible carbon</p><p>isotopic fractionation during the degradation of chlorinated ethenes on electrolytic and cast</p><p>iron (Bill et al., 2001; Dayan et al., 1999; Slater et al., 2002; VanStone et al., 2004), but</p><p>these observations have not been applied in the field to date. The objective of this study</p><p>was to investigate the use of carbon stable isotopic analysis to help evaluate PRB</p><p>performance at a field installation for the remediation of TCE. Routine performance</p><p>monitoring of the PRB consisted of quantification of TCE and its products upgradient,</p><p>within and downgradient of the iron PRB. In addition, carbon CSIAwas carried out in the</p><p>vicinity of the PRB to investigate the source of elevated chlorinated ethene concentrations</p><p>observed downgradient of the installation.</p><p>2. Site background</p><p>A PRB constructed of iron filings was installed in 1999 at the F.E. Warren Air Force</p><p>Base, Wyoming, Spill Site 7 (SS7) to remediate ground water contaminated with TCE.</p><p>The TCE originates from a defunct on-site liquid oxygen manufacturing facility. A grease</p><p>trap at the plant was the source of organic solvents (primarily TCE) to a surface drainage</p><p>ditch, resulting in migration of the solvents through the vadose zone to local ground water</p><p>(Heneman et al., 2001). Construction of the PRB was completed in October 1999. A</p><p>schematic of the SS7 PRB is shown in Fig. 1, with the direction of ground water flow</p><p>indicated. The PRB is 173 m long, has a flow through thickness of 1.2 m and a saturated</p><p>depth of 4.6 m below the historic low ground water level, with a low-permeability clay cap</p><p>installed on the PRB to minimize ground water flow over the top of the treatment, 2003; Poulson and Naraoka, 2002), and reductive dechlorination on zero-valent iron</p><p>(Bill et al., 2001; Dayan et al., 1999; Slater et al., 2002; VanStone et al., 2004). Carbon</p><p>isotopic fractionation results from differences in the rates of reaction for 13C- and 12C-</p><p>bearing molecules. The differences in reaction rates for the different isotopes are due to</p><p>mass-dependent differences in activation energies for the respective reactions (Fry, 1971).</p><p>In general, for most chlorinated ethenes and aromatic compounds, the lighter isotope (12C)</p><p>reacts faster than the heavier isotope (13C), leading to fractionation, and enrichment of the</p><p>heavy isotope in the remaining reactant as the reaction proceeds. Likewise, this leads to</p><p>enrichment in the light isotope with respect to the parent compound in the products of the</p><p>reaction. Laboratory experiments have shown that carbon isotopic fractionation for</p><p>chlorinated ethenes is not significant (i.e. b0.5x) for non-degradative processes such asdissolution, vaporization or adsorption under equilibrium conditions (Dempster et al.,</p><p>1997; Harrington et al., 1999; Poulson and Drever, 1999; Slater et al., 1999, 2000),</p><p>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</p><p>to the bottom of the contaminated shallow aquifer. The PRB is located within 20 to 60 m</p></li><li><p>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</p><p>along the line of PRB installation, the amount of iron in the PRB was varied by using a</p><p>different proportion of iron/sand in each of the three sections: Section 1 with 100% iron,</p><p>Section 2 with 25% iron/75% sand mix, and Section 3 with 38% iron/62% sand mix.</p><p>After the PRB was installed in October 1999, the system was left undisturbed for 6</p><p>months to allow the ground water conditions to equilibrate before implementing a</p><p>performance monitoring program (Heneman et al., 2001). The monitoring wells discussed</p><p>in this study are shown in Fig. 1 and represent only a portion of the wells at the site. These</p><p>monitoring wells are made of PVC and have 1.5 m screens. There are a series of</p><p>monitoring wells at the centre of each segment of the treatment PRB (Transects 1, 2 and</p><p>3), which are shown in expanded view in Fig. 1. In Transect 1, wells 101A and 101B are</p><p>located within a meter of the upgradient side of the PRB at depths of 8.5 and 4.5 m,</p><p>respectively, with well 101A located about 1 m below the PRB and 101B located within</p><p>Fig. 1. Schematic Map of SS7 PRB. Gray-filled circles indicate the position of monitoring wells that contained</p><p>concentrations of contaminants above the detection limits (d.l.) for isotopic analysis, and black-filled circles</p><p>indicate wells that did not contain adequate concentrations for CSIA. The treatment PRB is approximately 173 m</p><p>long. Base map is to scale (1 cm=10 m). Hatched lines show expanded view of transects 1, 2 and 3. Section 1 is</p><p>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</p><p>the centre of each main section (Transects 1 to 3). See text for detailed description of each monitoring well. Wells</p><p>indicated (*) are at a depth at least 1 m below the PRB.</p></li><li><p>projects (OHannesin and Gillham, 1998).Performance monitoring of the SS7 PRB revealed a decrease in CHC concentration</p><p>across the PRB from upgradient to non-detectable values within the PRB and immediately</p><p>downgradient (Heneman et al., 2001). In contrast, in wells further downgradient of the</p><p>PRB (e.g. wells 186, 700B, 707B and 708 located 9 to 12 m from the PRB) concentrations</p><p>of both TCE and cDCE were comparable to upgradient concentrations and were suggested</p><p>to be due to desorption of TCE from aquifer materials downgradient of the PRB (Heneman</p><p>et al., 2001). Corresponding increases in pH and decreases in Eh, carbonate alkalinity,</p><p>Ca2+, and Mg2+ were noted within the PRB and immediately downgradient in comparison</p><p>to upgradient wells (Heneman et al., 2001). These trends are expected due to reducing</p><p>conditions created by iron corrosion and are consistent with those noted at other PRB sites</p><p>(OHannesin and Gillham, 1998). The values of iron-sensitive parameters (i.e. Eh and pH)</p><p>and constituents (Ca2+, alkalinity) measured in wells 700B, 707B, and 173B were similar</p><p>to the upgradient values. It was unclear whether the persistent high concentrations of TCE</p><p>and cDCE in the downgradient wells were due to incomplete degradation of TCE within</p><p>the PRB, or to continued desorption from the aquifer material (Heneman et al., 2001).</p><p>Based on hydraulic heads measured at each monitoring well, ground water flow paths in</p><p>the vicinity of the PRB were not significantly altered from pre-PRB ground water flow</p><p>records (Heneman et al., 2001), ruling out hydraulic bypass of the PRB as the source of</p><p>elevated downgradient concentrations of CHCs.</p><p>3. Methods</p><p>Samples for concentrations of chlorinated hydrocarbons and carbon isotopic analysis</p><p>were taken in June 2002 (Table 1). Four 40 mL samples from each well were preserved</p><p>with several drops of undiluted HCl to inhibit microbial activity, and stored without</p><p>headspace. Additional water samples were taken from each well for analysis of TCE,</p><p>cDCE and VC concentrations by EPA method 502.1. The detection limit for this analysis</p><p>is 0.05 ug/L. All samples for carbon CSIA were packed on ice and shipped to the Stable</p><p>Isotope Laboratory at the University of Toronto, Canada, and analyzed within 1 month of</p><p>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</p><p>PRB. Wells 103A and 103B are located approximately 1 m downgradient of the PRB at</p><p>depths of 8.5 and 4.5 m, respectively, with 103A located about 1 m below the PRB and</p><p>103B located within the ground water plume intercepted by the PRB. Well 700B is located</p><p>9 m downgradient from the PRB. The wells for the other 2 transects (Transect 2 and 3) are</p><p>similarly located with respect to the PRB.</p><p>Periodic sampling of the groundwater monitoring wells has been conducted since April</p><p>2000 for concentrations of CHCs (TCE, cDCE, trans-dichloroethene and VC) using</p><p>standard EPA method 502.1, and several inorganic indicators of water quality (Heneman et</p><p>al., 2001). There were consistent decreases in Eh, sulfate, calcium and magnesium</p><p>concentrations and increases in pH consistent with those documented at other PRB</p><p>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</p><p>mL water samples were transferred to 160 mL glass bottles with 40 g of NaCl to facilitate</p></li><li><p>Table 1</p><p>Concentrations and stable carbon isotopic compositions of VC, cDCE and TCE from monitoring wells at</p><p>F.E.Warren AFB spill site 7 iron-filings PRB</p><p>Well TCE cDCE VC</p><p>(Ag/L) d13C (x) (Ag/L) d13C (x) (Ag/L) d13C (...</p></li></ul>