Abstract1,1,1‐Trichloroethane was readily degraded in samples from an alluvial aquifer under both methanogenic and sulfate reducing conditions. No lag period was noted prior to disappearance of the parent compound. Products detected in the reaction mixtures included 1,1‐dichloroethane, chloroethane, 1,1‐dichloroethene, acetic acid and carbon dioxide. No other chlorinated metabolites were detected. Biological reductive dehalogenation was responsible for the metabolism of 1,1,1‐trichloroethane to 1,1‐dichloroethane and to chloroethane. Abiotic transformations resulted in 1,1‐dichloroethene and acetic acid. However, biological mechanisms also appeared to contribute to the formation of acetic acid, which was subsequently mineralized to carbon dioxide. Pseudofirst‐order rate constants estimated for the biotransformation of 1,1,1‐trichloroethane were in the range of 0.0034 day−1to 0.015 day−1, whereas the rate constant for abiotic degradation was approximately 0.0006 day−1. Biotransformation of the compound was not observed in aquifer samples under aerobic or de
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