Vadose Zone Characterization at a Contaminated Field Site Using Partitioning Interwell Tracer Technology

The partitioning interwell tracer test (PITT) has been demonstrated to be effective in determining volume and spatial distribution of residual nonaqueous phase liquids (NAPLs) in the saturated subsurface. Use of PITT technology in the vadose zone has been limited to this point. Two PITTs were completed in the vadose zone of a NAPL contaminated site at Kirtland Air Force Base in Albuquerque, NM. The PITTs were completed before and after a thermally enhanced soil vapor extraction remediation effort, to provide performance assessment. Methane was used as a nonpartitioning tracer. Four perfluorocarbons (perfluoromethylcyclohexane, perfluoro-1,3-dimethylcyclohexane, perfluoro-1,3,5-trimethylcyclohexane, and perfluorodecalin) were utilized as air−NAPL partitioning tracers. Difluoromethane was used as an air−water partitioning tracer to estimate water saturation. The complex light NAPL (LNAPL) consisted of hydrocarbon fuels ranging from approximately 12−70 carbons in size. Laboratory experimental data and thermodynamic modeling were used to characterize compositional changes in the NAPL due to the remediation effort. About 260 kg of NAPL were removed from the subsurface by the remediation effort. The PITT assessment of the remediation was compared favorably to the results from soil borings taken at the site. The PITT has been shown to be a useful technology for both NAPL characterization and remedial performance assessment in the vadose zone.