Measurement of Gas-Accessible NAPL Saturation in Soil Using Gaseous Tracers

In this laboratory study, a new experimental method involving the use of a set of four gaseous tracers, was developed for measuring the NAPL saturation directly accessible to the mobile gas as well as the total NAPL saturation in unsaturated sand. One tracer with low water solubility (n-pentane) was used as the tracer that partitions into NAPL directly accessible to the mobile gas, and another (chloroform) tracer with moderate water solubility and NAPL-partitioning, was selected for detecting total NAPL saturation. Helium and difluoromethane were used as the nonreactive and water-partitioning tracers, respectively. A saturated hydrocarbon, n-decane, was used as NAPL. Column experiments were conducted at two water saturations (Sw = 0.68 and 0.16). The total NAPL saturation and NAPL saturation not directly accessible to the mobile gas were also successfully measured using the combined results of tracer experiments. At Sw = 0.68, only 28% of the total NAPL was detected by n-pentane, whereas 87% of the total NAPL was accessible to n-pentane at Sw = 0.16, implying more NAPL was accessible to the mobile gas phase at lower water saturation.