FIELD TRAPPING OF SUBSURFACE VAPOR-PHASE PETROLEUM-HYDROCARBONS

Soil gas samples from intact soil cores were collected on adsorbents a t a field site, then thermally desorbed and analyzed by laboratory gas chromatography (GC). Vertical concentration profiles of predominant v apor phase petroleum hydrocarbons under ambient conditions were obtain ed for the zone directly above the capillary fringe. Water and residua l phase weathered aviation gasoline were present in this region of the profile. The sampling, trapping, and GC methodology was effective in most respects. Reproducibility, trapping, and desorption efficiency we re generally satisfactory, and different sorbent tubes gave similar re sults. A minor shortcoming of the method occurred with the most volati le compound, 2,3-dimethylbutane, which was poorly retained during seve ral weeks of storage time and was also poorly desorbed. Vapor phase co ncentrations of predominant hydrocarbon compounds all increased with d epth at one sampling location. At a more highly contaminated location, concentrations of highly volatile compounds increased with depth whil e concentrations of less volatile compounds remained constant or decre ased, possibly indicating distillation effects. Scatter in the data wa s attributed to heterogeneities in water and residual phase distributi on.