Scientific Investigations Report 2006-5056
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5056
Contaminant degradation is the transformation of a chemical compound (the parent compound) into one or more other compounds (daughter products). Biodegradation reactions are mediated by subsurface microorganisms, whereas abiotic degradation reactions are not. Over the past two decades, numerous field and laboratory studies have shown that microbes indigenous to ground-water systems can degrade chloroethenes (see Bradley, 2003 for a historical review). The most relevant degradation mechanisms for TCE, cis-DCE, and VC are summarized here.
Abiotic degradation of chloroethenes occurs slowly under conditions commonly found in aquifers (Vogel, 1994). Although considerable variability exists, reported half-lives for abiotic degradation of TCE and DCE are as long as 108 years (Jeffers and others, 1989). Abiotic degradation half-lives for TCE can be as short as minutes or a few days in systems amended with an abundance of zero-valent iron (Gillham and O’Hannesin, 1994), but abundant zero-valent iron is uncommon in natural settings.
Biodegradation mechanisms for chloroethenes can be categorized into two groups—reductive dechlorination and microbial oxidation (Bradley, 2003). The occurrence and efficiency of chloroethene biodegradation are sensitive to the redox conditions of ground water (table 2). Reductive dechlorination mechanisms are most favorable under strongly reducing redox conditions (sulfate reduction and methanogenesis), and are most efficient for biodegradation of the highly chlorinated compounds PCE and TCE. Reductive dechlorination of the parent compounds PCE and TCE to form the daughter products cis-DCE and VC commonly occurs in anaerobic ground water. The potential for reductive dechlorination of VC to form ethane and ethene is less common and requires strongly reducing redox conditions.
In contrast, microbial oxidation mechanisms are most favorable under aerobic or mildly reducing redox conditions (manganese and iron reduction), and are most efficient for biodegradation of the less-chlorinated compounds cis-DCE and VC. Widespread occurrence of microbial oxidation mechanisms is not as well documented as that of reductive dechlorination, in large part because the oxidation daughter products carbon dioxide and chloride are not uniquely diagnostic to chloroethene biodegradation. The potential for microbial oxidation mechanisms under aerobic conditions is substantial where an anaerobic contaminant plume encounters aerobic water (such as at a discharge point to surface water). Likewise, the potential for microbial oxidation of VC and cis DCE under anaerobic conditions is substantial in contaminant plumes where redox conditions are only mildly reducing.
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