Investigation of Vapor Intrusion from an Urban Gasoline Plume in Washington DC Reveals Anomalous Behavior of Tetrachloroethylene (PCE)

In 2007 and 2008, a substantial vapor intrusion study was completed to investigate dissolved contamination from a gasoline release extending approximately 1400 feet under a neighborhood in Washington DC.  The primary contaminants of concern were BTEX and MTBE, although PCE was known to be an associated contaminant of unknown provenance.  Over three calendar quarters, we collected 128 indoor air samples from 115 homes, 39 samples of ambient outdoor air, 378 samples from sub-slab vapor monitoring ports in 106 homes, and 151 samples of subsurface vapors from outdoor vapor monitoring ports. All vapor samples were collected in 6-liter Summa canisters, and analyzed for volatile organic compounds via USEPA method TO-15 (modified). 

For the petroleum-related and chlorinated compounds detected, concentrations of most contaminants were higher in indoor air than in ambient air or subsurface vapor samples.  An exception to this observation was PCE for which the maximum sub-slab to indoor air concentration ratio exceeded 1 in 72% of the homes. The maximum concentrations of PCE detected in indoor air and sub-slab vapors were 42.3 ug/m³ and 929  ug/m³, respectively. 

Sub-slab PCE concentrations showed a strong geographic correlation with groundwater concentrations.  Data from nested wells, as well as geoprobe samples from the current study, however, indicated that the highest PCE concentrations were in deeper groundwater and not the shallow groundwater in contact with the vadose zone.  Screening level analyses using the Johnson and Ettinger model with site-specific parameters suggest that the observed concentrations in shallow groundwater are insufficient to generate the observed vapor concentrations.  Similarly, calculations show that the shallow groundwater concentrations were insufficient to account for the maximum sub-slab concentrations, even if no attenuation was considered.  These results suggest either the presence of significant non-attenuating vapor migration pathways or problems in assessing the actual concentrations of PCE in shallow groundwater.