Chlororespiration is a key component of remediation at many chloroethene-contaminated sites. In some instances, limited accumulation of reductive dechlorination daughter products may suggest 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, non-conservative 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 non-diagnostic mineralization products. An ongoing technical debate over the potential for mineralization of dichloroethene (DCE) and vinyl chloride (VC) to carbon dioxide in the complete absence of diatomic oxygen has largely obscured a practical and critically important question: is the potential for microbial DCE/VC mineralization significant at dissolved oxygen (DO) concentrations below the current field standard (DO < 0.1-0.5 mg/L) for nominally anoxic conditions?
The objective of this project was to assess microbial degradation of DCE and VC to carbon dioxide as a component of chloroethene bioremediation.
This project assessed the kinetics of microbial degradation of DCE and VC to carbon dioxide. Kinetic characteristics of anoxic microbial mineralization of DCE and VC in groundwater and surface water sediments were evaluated along with the response of anoxic chloroethene oxidation processes to changes in substrate concentration. In addition, the impact of dissolved hydrogen concentrations on the pathways and final products of microbial degradation of VC were determined via microcosm treatments incorporating varying hydrogen concentrations. Finally, the potential for microbial degradation of ethene and ethane under anoxic conditions was evaluated to assess the validity of using ethene and ethane accumulation as an indicator of the extent of anoxic VC degradation.
Mineralization of 14C-radiolabeled VC and cis-DCE under hypoxic (initial DO concentrations about 0.1 mg/L) and nominally anoxic (DO minimum detection limit = 0.01 mg/L) conditions was examined in chloroethene-exposed sediments from two groundwater and two surface water sites. The results show significant VC and DCE mineralization under hypoxic conditions. All sample treatments exhibited pseudo-first-order kinetics for DCE and VC mineralization over an extended range of substrate concentrations. First-order rates for VC mineralization were approximately one to two orders of magnitude higher in hypoxic groundwater sediment treatments and at least three times higher in hypoxic surface-water sediment treatments than in the respective anoxic treatments. For VC, oxygen-linked processes accounted for 65 to 85% of mineralization at DO concentrations below 0.1 mg/L and 14CO2 was the only degradation product observed in VC treatments under hypoxic conditions. Because the lower detection limit for DO concentrations measured in the field is typically 0.1 to 0.5 mg/L, these results indicate that oxygen-linked VC and DCE biodegradation can be significant under field conditions that appear anoxic. Furthermore, because rates of VC mineralization exceeded rates of DCE mineralization under hypoxic conditions, DCE accumulation without concomitant accumulation of VC may not be evidence of a DCE degradative stall in chloroethene plumes. Significantly, mineralization of VC above the level that could reasonably be attributed to residual DO contamination also was observed in several nominally anoxic (DO minimum detection limit = 0.01 mg/L) microcosm treatments.
A modified redox framework for assessing the role and potential importance of oxygen to chloroethene biodegradation was formulated based on the results. This investigation provides a better understanding of the mechanisms of, the environmental relevance of, and the conditions conducive to microbial degradation of DCE and VC to products not typically associated with reductive dechlorination.
DCE and VC biodegradation at low concentrations of dissolved oxygen appears to be a key contaminant attenuation mechanism, and failure to recognize this process can have important consequences. Existing protocols for assessing in situ biodegradation of chloroethene contaminants focus on chlororespiration. Accordingly, the accumulations of reductive dechlorination products like DCE, VC, ethene, and, by extension, ethane are prima facie evidence of ongoing reductive dechlorination biodegradation. Aerobic microbial processes can degrade all of these compounds to carbon dioxide. Consequently, unless the role of oxygen is recognized, the associated lack of reductive dechlorination products will be interpreted incorrectly as evidence of incomplete degradation, a so-called degradative stall. Mischaracterization of contaminant biodegradation processes can lead to expensive and ineffective remedial actions.