Monitoring Biodegradation of Ethene and Bioremediation of Chlorinated Ethenes at a Contaminated Site Using Compound-Specific Isotope Analysis (CSIA)

Abstract

Chlorinated ethenes are commonly found in contaminated groundwater. Remediation strategies focus on transformation processes that will ultimately lead to nontoxic products. A major concern with these strategies is the possibility of incomplete dechlorination and accumulation of toxic daughter products (cis-1,2-dichloroethene (cDCE), vinyl chloride (VC)). Ethene mass balance can be used as a direct indicator to assess the effectiveness of dechlorination. However, the microbial processes that affect ethene are not well characterized and poor mass balance may reflect biotransformation of ethene rather than incomplete dechlorination. Microbial degradation of ethene is commonly observed in aerobic systems but fewer cases have been reported in anaerobic systems. Limited information is available on the isotope enrichment factors associated with these processes. Using compound-specific isotope analysis (CSIA) we determined the enrichment factors associated with microbial degradation of ethene in anaerobic microcosms (ε = -6.7‰ ± 0.4‰, and -4.0‰ ± 0.8‰) from cultures collected from the Twin Lakes wetland area at the Savannah River site in Georgia (United States), and in aerobic microcosms (ε = -3.0‰ ± 0.3‰) from Mycobacterium sp. strain JS60. Under anaerobic and aerobic conditions, CSIA can be used to determine whether biotransformation of ethene is occurring in addition to biodegradation of the chlorinated ethenes. Using δ(13)C values determined for ethene and for chlorinated ethenes at a contaminated field site undergoing bioremediation, this study demonstrates how CSIA of ethene can be used to reduce uncertainty and risk at a site by distinguishing between actual mass balance deficits during reductive dechlorination and apparent lack of mass balance that is related to biotransformation of ethene.

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

“…Presently, there is no way to predict whether this reaction is occurring other than microcosm studies. It is possible that anaerobic ethene oxidation will have a stable isotope fractionation signature as does reduction to ethane (Mundle et al, 2012). It is also possible that the unique bacterial phylotype we have found associated with this reaction can serve as a biomarker for it, but considerably more study will be needed to support or refute this hypothesis.…”

“…Ethene produced by reductive dechlorination of chloroethenes in anaerobic zones of contaminated sites is often stable, but under some circumstances can be reduced to ethane, a process possibly associated with methanogenesis (de Bruin et al, 1992;Koene-Cottaar and Schraa, 1998;Mundle et al, 2012). The reasons why ethane is produced by some samples and not others are not understood.…”

“…At some of these sites, it is possible that oxygen is below the level of detection but high enough to allow for aerobic VC oxidation. Use of Compound-Specific Isotope Analysis can be used to reduce uncertainty about the fate of biotransformed ethenes (Mundle et al, 2012), but cannot fully explain the observed VC loss at contaminated sites. To accurately assess the flux of VC at sites where VC is observed to be decreasing without concomitant ethene formation, detection of known VC-oxidizing phylotypes like Mycobacterium spp.…”

“…Another possible explanation for poor mass balances on chlorinated ethenes seen in the field is reduction of ethene to ethane (de Bruin et al, 1992;Koene-Cottaar and Schraa, 1998;Mundle et al, 2012), which is not as frequently monitored as ethene. Ethene reduction is a Edwards and colleagues (Mundle et al, 2012) measured the δ 13 C enrichment factor for reduction of ethene to ethane, using anaerobic microcosms developed at Clemson University with soil and groundwater from the Twin Lakes wetland at the Savannah River Site (SRS).…”

“…Ethene reduction is a Edwards and colleagues (Mundle et al, 2012) measured the δ 13 C enrichment factor for reduction of ethene to ethane, using anaerobic microcosms developed at Clemson University with soil and groundwater from the Twin Lakes wetland at the Savannah River Site (SRS). In the process of enriching the microcosms for their ethene reduction capability, Johnson (2009) (a M.S.…”