Reinterpreting the Importance of Oxygen‐Based Biodegradation in Chloroethene‐Contaminated Groundwater

Abstract

Chlororespiration is common in shallow aquifer systems under conditions nominally identified as anoxic. Consequently, chlororespiration is a key component of remediation at many chloroethene‐contaminated sites. In some instances, limited accumulation of reductive dechlorination daughter products is interpreted as evidence that natural attenuation is not adequate for site remediation. This conclusion is justified when evidence for parent compound (tetrachloroethene, PCE, or trichloroethene, TCE) degradation is lacking. For many chloroethene‐contaminated shallow aquifer systems, however, nonconservative losses of the parent compounds are clear but the mass balance between parent compound attenuation and accumulation of reductive dechlorination daughter products is incomplete. Incomplete mass balance indicates a failure to account for important contaminant attenuation mechanisms and is consistent with contaminant degradation to nondiagnostic mineralization products like CO2. While anoxic mineralization of chloroethene compounds has been proposed previously, recent results suggest that oxygen‐based mineralization of chloroethenes also can be significant at dissolved oxygen concentrations below the currently accepted field standard for nominally anoxic conditions. Thus, reassessment of the role and potential importance of low concentrations of oxygen in chloroethene biodegradation are needed, because mischaracterization of operant biodegradation processes can lead to expensive and ineffective remedial actions. A modified interpretive framework is provided for assessing the potential for chloroethene biodegradation under different redox conditions and the probable role of oxygen in chloroethene biodegradation.

Compound‐Specific Isotope Analyses to Assess TCE Biodegradation in a Fractured Dolomitic Aquifer

“…High concentrations of certain organic contaminants, including TCE, may substantially inhibit the microbial activity necessary to generate more reducing groundwater environments. The observation that more reducing environments are sampled by wells further from the center of the contaminant mass is consistent with other research (e.g., Butler and Barker 1996;Chapelle and Bradley 2003;Bradley 2011).…”

Microbial Mineralization of Dichloroethene and Vinyl Chloride under Hypoxic Conditions

“…Clearly, improved methods for assessment of the role and potential importance of oxygen in chloroethene biodegradation are needed, because mischaracterization of operant biodegradation processes can lead to expensive and ineffective remedial actions. A modified redox assessment framework, which addresses the potential contribution of aerobic mineralization processes to in situ chloroethene attenuation, is presented by Bradley (2011).…”

Characterization of Microbes Capable of Using Vinyl Chloride and Ethene as Sole Carbon and Energy Sources by Anaerobic Oxidation

“…strain from presumably anaerobic groundwater highlights the fact that groundwater contamination plumes are dynamic, and our understandings of the biogeochemical cycles within are not fully understood. Studies which track VC in shallow aquifers show VC mineralization under hypoxic conditions (Bradley, 2011), though this is the first study to identify an organism that may be responsible for these in situ observations. Further studies must be performed to determine whether these strains of Mycobacterium spp.…”

“…There are several possible explanations for the lack of mass balances at field sites for chlorinated ethenes, including: 1) ethene produced from reductive dechlorination is oxidized to CO 2 as soon as it is produced; 2) VC is directly oxidized to CO 2 ; 3) VC is oxidized to acetate which is further metabolized to CO 2 and CH 4 ; or 4) oxygen diffuses into the system at below detection limits, and is continually consumed (Bradley, 2000;2011;Bradley and Chapelle, 2000;Gossett, 2010). Production of 14 C-acetate, 14 CO 2 and 14 CH 4 , from 14 C-labeled VC has been observed in microcosms (Bradley and Chapelle, 1999a;2000).…”