Application of gas-phase partitioning tracer tests to characterize immiscible-liquid contamination in the vadose zone beneath a fuel depot

Gas-phase partitioning tracer tests were conducted at a fuel depot to evaluate the utility of the partitioning tracer method for characterizing organic immiscible-liquid contamination in the vadose zone. One test was conducted within the boundaries of an existing fuel dispensing island and former underground fuel tank facility. Core sampling indicated that the vadose zone at this location is heavily contaminated throughout its vertical extent by fuel-related hydrocarbons. A tracer test was conducted after 30 mo of operation of a soil vapor extraction (SVE) system. Analysis of hydrocarbon concentrations in the SVE effluent indicates that approximately 355 000 L of hydrocarbons were recovered during the 30-mo operation period. Comparing this value to the initial volume of hydrocarbons present, estimated to be approximately 454 000 L based on core data, produces an estimate of 99 000 (64 000-134 000) L of hydrocarbons remaining within the area influenced by the SVE system. Extrapolation of the tracer test results (S_n = 0.44%) to the SVE-impacted zone produces an estimate of 107 000 (59 000-155 000) L of hydrocarbon present. The two values and associated approximate uncertainty ranges are relatively similar. The second test was conducted approximately 200 m from the former tank facility. Analysis of borehole material collected during well drilling at this location indicates minimal hydrocarbon contamination of the vadose zone, except in the vicinity of the water table (approximately 30 m below ground surface [bgs]), on which floating free product is found. Analysis of the tracer test results produced a hydrocarbon saturation value of 0.37%, which is significantly smaller than the initial value of approximately 1.9% (i.e., before SVE operation) estimated based on core data for the former tank facility location. The lower level of vadose-zone contamination suggests that the source of immiscible-liquid contamination found at the second location may be related to lateral migration of floating free product along the water table from the up-gradient tank facility, rather than vertical migration from the surface above. However, additional studies would be required to more fully evaluate this hypothesis. The results of this work illustrate the utility of the gas-phase partitioning tracer method for characterizing immiscible-liquid contamination in the vadose zone.