Reductive Dechlorination of the Vinyl Chloride Surrogate Chlorofluoroethene in TCE-Contaminated Groundwater ELLIOT ENNIS, † RALPH REED, ‡ MARK DOLAN, § LEWIS SEMPRINI, § JONATHAN ISTOK, § AND JENNIFER FIELD, ‡ Department of Chemistry, Department of Environmental and Molecular Toxicology, and Department of Civil, Construction, and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331 At many trichloroethene (TCE)-contaminated field sites, microbial transformation of TCE results in the accumulation of vinyl chloride (VC), a known carcinogen and neurotoxin. Quantitative tools are needed to determine the in situ rates of VC transformation to ethene in contaminated groundwater. For this study, E-/Z-chlorofluoroethene (E-/Z-CFE) was evaluated as a surrogate for VC in laboratory microcosm and field push-pull tests. Single-well push- pull tests were conducted at a TCE-contaminated field site by injecting E-/Z-CFE and monitoring for the formation of fluoroethene (FE) over a period of up to 80 days. The rates for VC transformation to ethene and E-CFE transformation to FE were within a factor of 2.7 for laboratory microcosm systems and all preferentially transformed E-CFE over Z-CFE. In the field, the in situ rates of FE production from injected E-CFE ranged from 0.0018 to 1.15 μM/day, while the in situ rates of E-CFE disappearance ranged from 0.17 to 0.99 μM/day. No significant Z-CFE transformation was observed in field tests, which indicated preferential utilization of E-CFE over Z-CFE under in situ field conditions. The results of this study indicate E-CFE as a potential surrogate for estimating the in situ rates of VC transformation. Introduction Vinyl chloride (VC) is used by the plastics industry to produce poly(vinyl chloride) (PVC) and copolymers. In 1995, the U.S. production of VC was 6.8 billion kilograms, placing it among the top 50 high volume chemicals produced in the U.S. (1). VC contamination of groundwater can occur during ac- cidental release or improper disposal associated with PVC manufacturing. In addition, accumulation of VC, a known carcinogen (2), is sometimes observed during reductive dehalogenation of TCE and/or PCE (3). The transformation of VC to ethene under anaerobic conditions is considered to be one of the more difficult steps in TCE and/or PCE mineralization, especially at sites where Dehalococcoides organisms, which are organisms that are known to transform DCE and VC to ethene, are not present (4). Incomplete transformation of TCE under anaerobic conditions may result when the last transformation step is cometabolic in nature and because Ks values for VC are higher relative to the higher chlorinated compounds (5-7). Currently, it is difficult to quantify the in situ rate of VC transformation to ethene in contaminated aquifers. Con- ventional methods for measuring rates of VC transformation to ethene include (a) temporal and spatial monitoring of VC, ethene, ethane, and electron donors and acceptors and (b) laboratory microcosm tests. The temporal and spatial monitoring approach is problematic for chlorinated solvents and their transformation products because changes in concentration due to nonbiological processes such as ad- vection, dispersion, and sorption can obscure changes due to biological processes (8-10). Laboratory microcosms provide qualitative evidence that dechlorinating populations are present, but the rates obtained do not necessarily reflect in situ conditions. Although stable carbon isotope analysis of chlorinated ethenes (natural abundance) in groundwater has been used to estimate the extent of TCE degradation (10-13), stable carbon isotopes for quantifying in situ rates has only recently been investigated in the vadose zone (14) and under controlled hydraulic conditions in the saturated zone (15). Single-well push-pull tests can provide quantitative information on transformation rates for in situ processes including reductive dechlorination (16-18), denitrification (18, 19), radionuclide reduction (20), anaerobic aromatic hydrocarbon degradation (21), and aerobic cometabolism (22). A single-well push-pull test consists of injecting or pushing a test solution containing a reactive tracer (reactant) into an existing monitoring well and pulling samples of the test solution/groundwater mixture from the same well over time. Reactions are detected by the loss of injected reactant, the in situ formation of a reaction product, or both. The length of the extraction phase is tailored in accordance with the objectives of the test. For example, fast reactions such as aerobic respiration and denitrification can be quantified when the extraction phase immediately follows injection and consists of continuously withdrawing at least 3 times the injected volume over a period of hours (19). In the case where the extraction phase occurs over a period of hours, trans- formation rates can be obtained for reactants whose reactions are rapid enough to occur over the time scale of the test (23). Alternatively, rates can be obtained for slower reactions such as reductive dechlorination (17, 18, 23), anaerobic alkyl- benzene transformation (21), and uranium or technicium reduction (20, 24), if the extraction phase consists of periodic sampling over an extended period of time (e.g., days to months). For slow reactions, the volume of injected test solution may need to be increased for field sites characterized by higher groundwater velocities. In situ rates for chlorinated solvents and their transformation products are com- puted from measured reactant and product concentra- tions by applying a forced mass balance technique developed by Hageman et al. (16) that compensates for the effects of dilution and sorption as a result of transport processes. To detect microbial transformations in contaminated groundwater that contains high and variable concentrations of contaminants and their transformation products, it is desirable to include labeled contaminants or other surrogates in the injected test solution. For example, deuterium-labeled forms of aromatic hydrocarbons (e.g., toluene-d 5) and a fluorine-labeled form of TCE, trichlorofluoroethene (TCFE), were used to detect anaerobic transformations of petroleum * Corresponding author e-mail: Jennifer.Field@orst.edu. † Department of Chemistry. ‡ Department of Environmental and Molecular Toxicology. § Department of Civil, Construction, and Environmental Engi- neering. Environ. Sci. Technol. 2005, 39, 6777-6785 10.1021/es048640f CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6777 Published on Web 07/30/2005