A soil vapor survey conducted in 1992 on a chemical facility indicated the presence of a leak in a six inch vinyl chloride (VC) pipeline located on the facility. The pipeline was purged and taken out of service following the confirmation of the release. In 1997 the state regulatory agency requested another soil vapor survey be conducted to determine the areal extent of the VC contamination. The results of this survey were to be utilized to determine the configuration of the vapor extraction system planned for the remediation of the impacted area.

Approach: A soil vapor survey was conducted in 1997 on the facility in the vicinity of the pipeline release to delineate the areal extent of the VC and possible degradation (daughter) products. Plume maps were constructed for VC, ethene (ethylene) and ethane to demonstrate the areal extent of the constituents.

Results: Although vinyl chloride soil vapor concentrations measured in 1997 were relatively high, the concentrations were lower than those measured in 1992. In addition, high concentrations of ethene and ethane were measured in the soil vapor samples, strongly suggesting anaerobic dechlorination of the VC had occurred in the survey area. These data indicated natural attenuation of the VC was occurring in the study area and the possible introduction of oxygen into the subsurface environment (resulting from the installation/operation of a vapor extraction system) could diminish and/or shut down the anaerobic degradation processes at work. The regulatory agency did not require an active remediation system (vapor extraction) to be installed; yearly monitoring of the area was ordered.

Soil vapor surveys were subsequently conducted each year from 1998 to 2001. The analytical data and resulting plume maps indicate the continued natural attenuation of vinyl chloride and generation of anaerobic daughter products (ethene and ethane).

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