Problem: A shopping center property consisting of several retail stores, including a dry cleaners, was suspected of containing subsurface contamination. Releases/losses of dry cleaning fluid (tetrachloroethylene or PCE) were believed to have occurred during daily dry cleaning operations. The areal and vertical extents of the impacted area due to these releases were unknown.
Approach: A soil vapor survey was conducted on the shopping center property, including inside the dry cleaners and other shops, to determine the areal (horizontal) extent of chlorinated hydrocarbons such as PCE and various daughter products including TCE (trichloroethylene), DCE (dichloroethylene), vinyl chloride, ethene and ethane.
Results: The soil vapor data and contoured plume maps indicated the presence of subsurface hydrocarbon contamination over the majority of the survey area. The highest concentrations of PCE were measured in vapor samples collected in the vicinity of the dry cleaners (source area). The data show a well-documented anaerobic degradation sequence where PCE —>TCE —>DCE —>Vinyl Chloride —>Ethene —>Ethane. The plume maps for PCE and the anaerobic degradation (daughter) products are similar in geometry and areal extent and indicate migration of contaminants downgradient to the east of the source area. The soil vapor data (methane and ethane) also indicate the presence of a leak in a natural gas line located at the northwest corner of the survey area. Drilling/sampling and monitor well installations will be required to delineate the vertical extent of the subsurface chlorinated hydrocarbon contamination. The soil vapor plume maps will be used as guides for the placement of the borings/monitor wells.
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