Carbon Isotopic Enrichment: A Tool for Monitoring Degradation of Chlorinated Ethenes on Iron PRBs and Mineralization by Microbial Consortia

Monitoring contaminant degradation provides important information regarding the success of permeable reactive barriers (PRBs) constructed of Fe(0) iron for the remediation of chlorinated ethenes (e.g. TCE or PCE). Compositional data may be ambiguous in some cases due to the number of different reaction mechanisms by which contaminants may be degraded and due to other processes by which contaminants are reduced in concentration in aqueous samples (such as adsorption and dilution due to transport). Furthermore, indigenous microbial consortia may be capable of degrading chlorinated ethenes. Analytical methods capable of delineating the relative contribution of these different processes would be key to evaluating the performance of PRBs. Carbon isotopic enrichment has been used to identify degradation of chlorinated ethenes by microbial consortia under field conditions. Similarly, carbon isotopic enrichment has been noted during degradation of chlorinated ethenes on Fe(0) iron in bench-scale experiments. The isotopic enrichment observed during both biotic and abiotic degradative processes can be described using Rayleigh enrichment factors. The extent of isotopic enrichment during biodegradation has been found to decrease with increasing extent of chlorination, with enrichment factors ranging from -5.5 permil for PCE to -21 permil for vinyl chloride (VC) (Slater et al. 2001). Enrichment factors for chlorinated ethene degradation on Fe(0) iron vary according to both the extent of chlorination and the nature of the iron. For example, on one iron type, enrichment factors ranged from -5.7 permil for PCE to -16.6 permil for VC while for another iron type enrichment factors were independent of the degree of chlorination of the compound. In some cases, therefore, it may be possible to differentiate between the effects of microbial degradation versus abiotic degradation in groundwater treated by iron PRBs. Such information may allow optimization of conditions to promote concurrent use of both abiotic and biotic remediation technologies. Slater et al. (2001) Env. Sci. Technol. 35:901-907.