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Remediation and Control Technologies
Geochemical and Isotope Study of Trichloroethene Degradation in a Zero-Valent Iron Permeable Reactive Barrier: A Twenty-Two-Year Performance Evaluation Richard Thomas Wilkin, Tony Lee, Molly Sexton, Steven D. Acree, Robert Puls, David W. Blowes, Christopher Kalinowski, Jennifer Tilton, and Leilani Woods Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 09 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018
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Environmental Science & Technology
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Geochemical and Isotope Study of Trichloroethene Degradation in a Zero-Valent Iron Permeable
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Reactive Barrier: A Twenty-Two-Year Performance Evaluation
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Richard T. Wilkina, Tony R. Leea, Molly R. Sextona, Steven D. Acreea, Robert W. Pulsb, David W. Blowesc,
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Christopher Kalinowskid, Jennifer M. Tiltond, and Leilani L. Woodse
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aU.S.
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Watershed, and Ecosystem Restoration Division, 919 Kerr Research Drive, Ada, OK 74820, United
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States; bPulsEnvironmental Consulting, Hilton Head, SC, 29926, United States; cDepartment of Earth and
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Environmental Sciences, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1; dArcadis U.S., Inc.,
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801 Corporate Center Drive, Suite 300 Raleigh NC, 27607; eU.S. Coast Guard Base Elizabeth City, 1664
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Environmental Protection Agency, National Risk Management Research Laboratory, Groundwater,
Weeksville Road, Bldg 981, Elizabeth City, NC, 27909, United States
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Abstract: This study provides a twenty-two-year record of in situ degradation of chlorinated organic
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compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of
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trichloroethene (TCE) entering the PRB were as high as 10,670 µg/L. Treatment efficiency ranged from
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81 to >99% and TCE concentrations from C4
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compounds, and possibly CO2(aq) and methane. Abiotic patterns of TCE degradation were indicated by
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compound-specific stable isotope data and the distribution of degradation products. δ13C values of
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methane within and down-gradient of the PRB varied widely from -94‰ to -16‰; these values cover
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most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for
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inorganic carbon in zero-valent iron PRBs that competes with carbonate mineralization and this process
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is important for understanding pore-space clogging and longevity of iron-based PRBs. The carbon
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isotope signatures of methane and inorganic carbon were consistent with open-system behavior and
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22% molar conversion of CO2(aq) to methane.
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Introduction Contamination of groundwater with chlorinated solvents is a common problem at hazardous
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waste sites. In the United States, nearly 75 percent of the Superfund sites with groundwater remedies
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selected between 2012 and 2014 addressed contamination by halogenated (primarily chlorinated)
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hydrocarbons.1 Over the past decade, the increased use of in situ groundwater treatment technologies
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(29% in 2004 to 53% in 2014) correlates with an overall decrease in the selection of aboveground pump-
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and-treat remedies (35% in 2004 to 17% in 2014).1 These trends reflect acceptance and reliance on the
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innovative groundwater remediation technologies for site cleanup that were developed and first
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implemented during the 1980s and 1990s, including permeable reactive barriers (PRBs), in situ chemical
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oxidation, in situ chemical reduction, bioremediation, and thermal treatment. The continued application
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of these technologies will benefit from long-term performance data to improve system designs and
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guide the selection of remedial measures that best match site-specific hydrogeochemical conditions.
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The seminal study by Gillham and O'Hannesin2 on the degradation of chlorinated hydrocarbons
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by zero-valent iron (ZVI) kicked off a period of intense bench-scale analysis to understand parent-
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daughter relationships, rates, and mechanisms of reaction between ZVI and chlorinated methanes,
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ethanes, and ethenes.3-16 Parallel studies explored the degradation potential of reactive iron minerals
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that precipitate within ZVI PRBs.17-20 In ZVI systems, chlorinated ethene removal proceeds by a
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combination of reductive β-elimination reactions (producing chloroacetylene and breakdown products)
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and sequential hydrogenolysis reactions (producing cis-dichloroethene, vinyl chloride, and breakdown
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products).14 Recent studies developed and applied compound specific stable carbon isotope analysis to
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differentiate biotic and abiotic dechlorination processes and to evaluate the overall efficiency of
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contaminant degradation in field applications.21-33
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The first field demonstration of ZVI technology showed effective chlorinated solvent treatment
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over an initial five-year period of performance.34 Warner et al.35 reported on the hydraulic and chemical
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performance of the first commercial PRB constructed in the U.S. with granular iron metal after 10 years
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of operation. Phillips et al.36 provided a 10-year performance evaluation of Europe’s oldest
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commercially-installed granular iron PRB located in Northern Ireland. These studies showed effective
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treatment of trichloroethylene (TCE) over a decade, although processes such as mineralization and gas
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generation within ZVI PRBs were identified as factors that could reduce the longevity of these
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engineered systems for groundwater cleanup.
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The Elizabeth City ZVI PRB is one of the earliest full-scale PRBs in the United States. The PRB was
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constructed in 1996 to treat contaminated groundwater (TCE, hexavalent chromium) originating from a
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metal plating shop.37,38 Previous reports focused on mineralization, hydraulic performance, and
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treatment efficiency of the Elizabeth City PRB, mainly with respect to the removal of hexavalent
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chromium from groundwater.39-41 The current study examines a twenty-two-year history of TCE
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treatment, including analysis of daughter products and compound specific stable isotope data for
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chlorinated compounds, methane, and inorganic carbon. This study provides the longest available
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record of treatment performance of chlorinated compounds by ZVI and the first analysis of carbon and
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hydrogen stable isotope ratios in methane produced from a ZVI system.
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Materials and Methods
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Field Site - The site is located on the southern bank of the Pasquotank River, about 5 km southeast of
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Elizabeth City, North Carolina. In June 1996, a 46 m long, 7.3 m deep, and 0.6 m wide PRB (continuous
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wall configuration) of ZVI (Peerless Metal Powders, Inc.) was installed approximately 30 m from the
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Pasquotank River37,38 (Figure 1; see Supporting Information). The PRB was designed to remediate
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hexavalent chromium-contaminated groundwater and portions of a larger overlapping plume of volatile
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chlorinated organic compounds, mainly TCE with lesser amounts of cis-dichloroethene (cis-DCE) and
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vinyl chloride (VC). Subsurface sampling points discussed here include monitoring wells screened in the
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aquifer at positions up-gradient (MW48) and down-gradient (MW46, MW47, MW49, MW50, and
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MW52) of the PRB, as well as transects consisting of multi-level well clusters (ML series; 15-cm screens)
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placed at depths ranging from 2 to 7 m below ground surface (BGS) at locations up-gradient, within, and
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down-gradient of the PRB (Figure 1; Table S1).
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Groundwater Sampling - Groundwater samples were collected from 1996 to 2018 using peristaltic
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(Alexis) or submersible centrifugal (Grundfos) pumps. Prior to sample collection, groundwater was
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pumped through a flow-cell containing calibrated electrodes to measure pH, oxidation-reduction
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potential, specific conductance, and dissolved oxygen. Samples for volatile organic compounds (VOCs)
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were collected with no headspace into 43-mL amber glass vials containing an acid preservative (pH
