Isotopic Fractionation of Tetrachloroethene Undergoing Biodegradation Supported by Endogenous Decay

A long term column study of the anaerobic biodegradation and associated isotopic fractionation of tetrachloroethene and transformation products was conducted. The column, initially fed with tetrachloroethene, ethanol, and yeast extract, was operated for one year with input of dissolved tetrachloroethene only to examine reductive dehalogenation and isotopic fractionation under endogenous decay conditions. A one-dimensional, inverse, multispecies, reactive transport model was employed to estimate the degradation rate constants and enrichment factors of the chlorinated ethenes at different times and under different column conditions. The inverse method combined a genetic algorithm with a gradient-based method for efficient parameter estimation. The trichloroethene and cis-1,2-dichloroethene first-order degradation rate constants decreased with time as endogenous decay progressed in the absence of ethanol and yeast extract. Temporal trends in tetrachloroethene degradation rate constants were not significant. The enrichment factors did not change significantly with time, biodegradation rates, or readdition of ethanol at the end of the study. This is encouraging with respect to using isotope enrichment factors in reactive transport models to estimate the extent of biodegradation in the field.