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A field experiment was conducted in which carbon tetrachloride (CT) was found to transform to chloroform (CF) and carbon disulfide (CS2) in a ratio of about 2:1. Subsequent laboratory work was undertaken to better define the conditions required for this product ratio and to investigate its use as a means of distinguishing biologically driven and abiotic transformations in aquifers. The field experiment was conducted to assess the efficacy of a bioremediation scheme for treating CT in groundwater. The test section of the aquifer was taken to sulfate reducing conditions by the periodic addition of acetate from a nutrient injection wall (NIW). Under these conditions, CT was observed to transform completely producing primarily CF and CS2 at a ratio of approximately 2:1. In laboratory columns designed to mimic the field conditions, low-input concentrations of CT (<70 μg/L) resulted in complete dechlorination of the molecule, while higher input concentrations (>400 μg/L) led to the production of CF and CS2 in the same ratio as observed in the field. It was determined that amorphous iron sulfide had precipitated on the sand grains during the column experiments. Stertile laboratory batch experiments were conducted to test CT reactivity with various iron sulfide solids. A narrow ratio of 2:1 ± 0.4 CF to CS2 was only observed for CT reacting with freshly precipitated, amorphous FeS in near pH neutral solutions. The 2:1 ratio may be a useful tool for distinguishing abiotic transformations from biodegradation in sulfate reducing environments where FeS is actively precipitated.