THREE-PHASE PARTITIONING TRACER TESTS (PTTS) FOR GROUND-WATER SYSTEMS: THEORY AND FIELD RESULTS

Partitioning tracer tests (PTT) are most commonly designed to quantify NAPL saturation below the water table where two-phase tracer partitioning (i.e., water-NAPL) is assumed. In this application, a suite of dissolved tracers is swept across the NAPL source zone, and the transport times of the partitioning tracers provide direct information regarding NAPL volume in the sweep zone. If trapped air is present within the sweep zone, tracers with large Henry’s Law constants (such as dissolved gases) will also partitioning into the immobile gas phase. Using a series of tracer mass-balance equations, we present a solution for quantifying both NAPL saturation and trapped air for a three-phase ground-water system (mobile water, immobile NAPL, immobile gas). The ability to estimate trapped air in addition to NAPL saturation may be particularly beneficial for assessment of some remediation strategies. For example, significant gas-phase CO2 may be generated during chemical oxidation treatments, and the ability to quantify this reaction product may improve contaminant mass balance estimation. During the summer of 2002, a series of field PTTs were conducted at the Naval Amphibious Base Little Creek (Virginia Beach, VA) to assess performance of a remediation effort. The tracer suite included both water-NAPL partitioning tracers (aliphatic alcohols), and water-NAPL-gas partitioning tracers (dissolved helium and neon). The three-phase partitioning tracers provided both a reasonable estimate of trapped air, and a NAPL saturation value that closely agreed with the results of the two-phase partitioning tracers.