AbstractResults are presented that demonstrate the in‐situ biotransformation of vinyl chloride (VC), trans‐l,2‐dichloroethylene (t‐DCE), cis‐l,2‐dichloroethylene (c‐DCE), and trichloroethylene (TCE) by an enhanced population of methane‐utilizing (methanotrophic) bacteria. Biostimulation was accomplished by introducing dissolved methane and oxygen into a shallow, confined aquifer, to encourage the growth of the native methanotrophic bacteria. Biotransformation of the target compounds ensued immediately after the commencement of methane utilization, and reached steady‐state values within three weeks. The approximate extents of transformation achieved in the two meter biostimulated zone were as follows: VC, 95; t‐DCE, 90; c‐DCE, 50; and TCE, 20. The biotransformation of VC and t‐DCE was observed to be competitively inhibited by methane. Cyclic variations in methane concentration caused by the alternate pulse injection of dissolved methane into the test zone caused oscillations of the aqueous concentrations of VC and t‐DCE. When formate and methanol were substituted for methane as alternative electron donors, inhibition ceased (no oscillations), and concentrations were reduced to levels achieved during periods when no methane was present, confirming the inhibition by methane. Higher transformation rates were achieved temporarily, i.e., for several days, through the addition of formate or methanol. When electron donor addition was terminated, the concentration of target compounds rapidly increased, indicating that the transformation promptly ceased. Although these experiments indicated that methane competitively inhibits transformation rates, this competition is a second‐order effect: methane as substrate for growth was also required for transformation of VC, t‐DCE, c
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