Offshore Passive Soil Vapor Survey at a Contaminated Coastal Site

A high-resolution passive soil vapor survey was recently conducted at a coastal military base in southern California where a chlorinated and aromatic hydrocarbon plume threatens San Diego Bay. An existing sampling grid was extended offshore the entire length of the contaminated site. PETREX sampling tubes were emplaced conventionally onshore and were emplaced offshore in sandy bay-bottom sediments by divers working in 3 to 17 m (10 to 55 ft) of water. Results of mass spectrometric analysis of passive vapor samples were consistent with submarine ground water discharge measurements made previously with seepage meters, yet also produced one unexpected result: evidence of in situ degradation. Tetrachloroethene (PCE), widespread onshore, was not detected offshore. Trichloroethene (TCE), the primary contaminant onshore, was detected in only three points offshore. Dichloroethene (DCE), present only at trace levels onshore, was the dominant constituent measured in the offshore plume. Results were confirmed by laboratory analyses of sediment samples. The study demonstrated the utility of extending the passive soil vapor technique offshore in up to 17 m (55 ft) of sea water in a heavily trafficked waterway with a strong tidal current.