| Abstract
| - Stable carbon isotope analysis of chlorinated ethenes andethene was performed at a site contaminated withtrichloroethene (TCE), a dense non-aqueous phase liquid(DNAPL). The site is located in fractured bedrock and hadvariable groundwater hydraulic gradients during thestudy due to a local excavation project. Previous attemptsto biostimulate a pilot treatment area at the site resultedin the production of cis-1,2-dichloroethene (cis-DCE), the firstproduct of reductive dechlorination of TCE. Cis-DCEconcentrations accumulated however, and there was noappreciable production of the breakdown productsfrom further reductive dechlorination, vinyl chloride (VC)and ethene (ETH). Consequently, the pilot treatment areawas bioaugmented with a culture of KB-1, a natural microbialconsortium known to completely reduce TCE to nontoxicETH. Due to ongoing dissolution of TCE from DNAPL in thefractured bedrock, and to variable hydraulic gradients,concentration profiles of dissolved TCE and its degradationproducts cis-DCE, VC, and ETH could not convincinglyconfirm biodegradation of the chlorinated ethenes. Isotopicanalysis of cis-DCE and VC, however, demonstrated thatbiodegradation was occurring in the pilot treatment area.The isotope values of cis-DCE and VC became significantlymore enriched in 13C over the last two sampling dates(in one well from −17.6‰ to −12.8‰ and from −22.5‰ to−18.2‰ for cis-DCE and VC, respectively). Quantificationof the extent of biodegradation in the pilot treatment areausing the Rayleigh model indicated that, depending onthe well, between 21.3% and 40.7% of the decrease in cis-DCE and between 15.2% and 36.7% of the decrease inVC concentrations can be attributed to the effects ofbiodegradation during this time period. Within each well,the isotope profile of TCE remained relatively constant dueto the continuous input of undegraded TCE due toDNAPL dissolution.
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