Microbial degradation of chloroethenes in groundwater systems

The chloroethenes. tetrachloroethene (PCE) and trichloroethene (TCE) are am ong the most common contaminants detected in groundwater systems. As recent ly as 1980, the consensus was that chloroethene compounds were not signific antly biodegradable in groundwater. Consequently, efforts to remediate chlo roethene-contaminated groundwater were limited to largely unsuccessful pump -and-treat attempts. Subsequent investigation revealed that under reducing conditions, aquifer microorganisms can reductively dechlorinate PCE and TCE to the less chlorinated daughter products dichloroethene (DCE) and vinyl c hloride (VC). Although recent laboratory studies conducted with halorespiri ng microorganisms suggest that complete reduction to ethene is possible, in the majority of groundwater systems reductive dechlorination apparently st ops at DCE or VC. However, recent investigations conducted with aquifer and stream-bed sediments have demonstrated that microbial oxidation of these r educed daughter products can be significant under anaerobic redox condition s. The combination of reductive dechlorination of PCE and TCE under anaerob ic conditions followed by anaerobic microbial oxidation of DCE and VC provi des a possible microbial pathway for complete degradation of chloroethene c ontaminants in groundwater systems.