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Elucidation of the Mechanisms and Environmental Relevance of cis-Dichloroethene and Vinyl Chloride Biodegradation
Evan Cox | GeoSyntec Consultants
Through two decades of laboratory and field research, it has been generally believed that cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) can further degrade through a variety of mechanisms, including anaerobic reductive dechlorination to ethene and/or ethane; anaerobic oxidation to carbon dioxide (CO2) under iron- and/or manganese-reducing conditions; aerobic oxidation and co-oxidation to CO2; and reduction through a variety of metal and metal-oxide surface-catalyzed reactions. Knowledge of the various cDCE and VC degradation processes has led to the development and deployment of monitored natural attenuation (MNA) and enhanced in situ bioremediation (EISB) remedies at a significant number of Department of Defense (DoD) sites impacted by chlorinated solvents. However, it has proven difficult to confidently and reliably quantify several of the degradation processes (mainly the oxidation and abiotic reactions) at field scale using common investigation techniques, and as such, MNA and EISB cases invoking these reactions as explanations for apparent cDCE and VC mass loss often appear speculative. The objective of this project was to develop information and tools to help DoD remediation project managers (RPMs) and remediation practitioners better understand and quantify the frequency of occurrence, relative contribution, and overall environmental relevance of the various cDCE and VC degradation processes in field settings.
This project included three research modules: (1) elucidation of the aerobic oxidation pathway for cDCE; (2) isolation of organisms and elucidation of the anaerobic oxidation pathway(s) for cDCE and VC under iron- and/or manganese-reducing conditions; and (3) improvement in the understanding of isotopic fractionation signatures related to cDCE and VC reductive dechlorination, and assessment of the isotopic signatures associated with ethene degradation. At SERDP’s request, a fourth module was added to support the Remediation Technology Development Forum’s Source Area BioREmediation (SABRE) project.
The key results of this project can be summarized as follows:
- JS666 remains the only isolated organism known to mediate aerobic oxidation of cDCE to CO2, and DNA-based molecular biological tools (MBTs) exist to track its presence and fate during EISB bioaugmentation projects
- Significant advancements were made in understanding the pathway, mechanisms, and enzymes associated with aerobic oxidation of cDCE in JS666
- Anaerobic oxidation of cDCE and/or VC under iron- or manganese-reducing conditions could not be confirmed, despite substantial efforts with materials from many sites
- Suspected anaerobic oxidation of VC may in fact be aerobic oxidation to CO2 at extremely low levels of oxygen in the subsurface
- Compound-specific isotope fractionation of carbon occurs in both anaerobic and aerobic microbial degradation of ethane, allowing the use of this analytical tool to assess ethene degradation as a possible means to explain poor ethene mass balance in EISB and MNA projects.