Field evaluation of interfacial and partitioning tracers for characterization of effective NAPL-water contact areas

The use of surface-active tracers for measuring the interfacial area between nonaqueous phase liquids (NAPLs) and water was evaluated in a hydraulically isolated test cell installed in a surficial aquifer located at Hill Air Force Base (AFB), Utah. Interfacial tracers mere developed at the University of Florida for quantifying immiscible fluid-fluid interfaces (air-water or NAPL-water) in porous media. Sodium dodecyl benzene sulfonate (SDBS) was used as the interfacial tracer to measure the effective NAPL-water interfacial area (a(Nw)), while 2,2-dimethyl-3-pentanol (DMP) was used as the partitioning tracer to estimate the NAPL saturation (S-N). The observed retardation of SDBS and DMP when compared to a nonreactive tracer (bromide or methanol) at eight multilevel sampling locations and one extraction well, was used to quantify the a(Nw) and S-N values averaged over the interval between the injection wells and each monitoring point. The NAPL morphology index, defined here as H-N = a(Nw)/phi S-N (phi = porosity), varied significantly within the test cell. In locations where the magnitude of H-N was large, the NAPL was assumed to be more or less uniformly spread, providing good contact with the mobile fluid, In contrast, regions with low H-N values were assumed to have NAPL that was more heterogeneously distributed as isolated patches providing poor contact with the mobile fluid. The index H-N, provided by the combined use of interfacial and partitioning tracers, has important implications for NAPL source region remediation employing in situ flushing technologies.