The partitioning tracer test (PTT) is a promising new characterization technique for locating and quantifying subsurface nonaqueous phase liquid (NAPL); however, the potential significance of several complicating factors needs further investigation. For example, NAPL saturation calculated from a PTT is based upon the observed retardation of the partitioning tracer(s) and the tracer’s NAPL-water partition coefficient. For most sites, NAPL composition varies spatially and temporally (especially after remediation efforts); however, tracer partition coefficients are typically determined in laboratory batch tests using laboratory-grade NAPL, or site NAPL collected at discrete locations and times. Consequently, the partition coefficients used may be inappropriate, yielding an inaccurate calculation of NAPL saturation. Recent work has shown that partition coefficients of some alcohol tracers are very sensitive to NAPL composition. Because there are notable advantages for dissolved gas tracers, including excellent analytical sensitivity, Divine (2000) investigated the use of dissolved helium and neon as NAPL partitioning tracers in a series of batch and column-scale PTTs. For these experiments, pure trichloroethene (TCE) was used for the NAPL. We are currently measuring the robustness (i.e., sensitivity) of the NAPL-water partition coefficients for these gases for various mole fractions of TCE and tetrachloroethene (PCE). We are also investigating the effect of residual cyclodextrin (remediation fluid) on partition coefficients. Based on preliminary results, the partition coefficients for helium and neon appear to be more robust than many common alcohol tracers. The results of these experiments and the general significance of partitioning tracer robustness on PTT accuracy and interpretation are presented.

Divine, C. E. 2000. The Applicability of Dissolved Helium and Neon as Dense Nonaqueous Phase Liquid Partitioning Tracers. M.S. Thesis, Colorado State University, Fort Collins, CO.