A two-year natural attenuation monitoring program was completed in 1998 at a dry cleaning facility that formerly operated as a gasoline service station. The dry cleaning facility utilizes tetrachloroethene (PCE) as the cleaning solvent. Groundwater samples were collected from a network of nine monitoring wells and four piezometers on a semi-annual basis using low-flow sampling techniques for the analysis of volatile organic compounds, dissolved gasses, and geochemical parameters. The monitoring program results indicated that the downgradient plumes of dissolved PCE and daughter products had either remained stable or receded, and the upgradient plume of dissolved benzene had receded. Highly reducing conditions favoring complete reductive dechlorination did not exist within the plume of chlorinated hydrocarbons. Dissolved methane, oxidation-reduction potential, and microbiological analyses suggested that the degradation of cis-1,2-dichloroethene (cis-1,2-DCE) and vinyl chloride (VC), the daughter products of trichloroethene (TCE), may be mediated by methane-oxidizing bacteria.
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