Characterization of residual NAPL in the vadose zone using gas-phase partitioning tracers

Abstract

The purpose of the field experiment conducted at City of Tucson Fire Station Number 10 (Fire Station) was to conduct a gas-phase tracer test in the vadose zone in order to help determine the nature and extent of non-aqueous phase liquid (NAPL) contamination at the Thomas 0. Price Service Center (TPSC) Fire Station site. Current methods for characterization of NAPL saturation, such as soil-gas analysis and core sampling provide data only at discrete points and can be limited in their effectiveness. Gas-phase partitioning tracers can sample a larger volume of the vadose zone, allowing the location and amount ofresidual NAPL to be determined more accurately. A gas-phase partitioning tracer experiment was conducted at the Fire Station to evaluate the use of partitioning tracers in determining NAPL saturation in the vadose zone. Well R-032A was used as the injection well and well R-047A was used as the extraction well. The tracer test yielded a global retardation of 1.23, which indicates NAPL presence. A swept volume of approximately 770 m3 was contacted by the tracer and approximately 2.8m3 volumetric NAPL content was measured. However, a majority of the retention of the partitioning tracer is associated with recovery from early travel times, seen in the first peak of the breakthrough curve. Analysis of only the first peak yields a retardation factor of 1.26, a swept volume of 260 m3, and a total volumetric NAPL content of 1.lm3.