Effect of mass transfer rate limitation on detection of nonaqueous phase liquids with partitioning tracer tests

Partitioning interwell tracer tests (PITTs) are a relatively new technique to quantify the amount of nonaqueous phase liquid (NAPL) within saturated porous media. PITTs have been used to determine domain-average NAPL saturations as well as the spatial distribution of the NAPL. While these tracer tests work well when the NAPL is distributed uniformly throughout the domain, if NAPL is located nonuniformly, either as millimeter-scale ganglia or pools that are centimeter-scale and larger, the flow paths of the injected tracer solution may bypass NAPL-contaminated zones. In this case, the transfer of tracer mass from the main flow paths to the NAPL may be slow, resulting in extensive tailing of tracer breakthrough curves and underestimation of NAPL mass. In this work, the influence of nonuniform NAPL distribution and local-scale mass transfer resistance as well as co-tracer interactions and partitioning nonlinearities on the accuracy of measured NAPL saturations using PITTs was investigated. Tracer partitioning was examined in static batch systems using 2,3-dimethyl-2-butanol and 1-hexanol as partitioning tracers and trichloroethylene as the NAPL. Laboratory column experiments were conducted with these same tracers to explore the influence of tracer partition coefficient and injected tracer mass on NAPL measurement when the NAPL was distributed nonuniformly. Nonlinear tracer partitioning associated with co-tracer effects resulted in underestimation of NAPL mass. Mass transfer effects were pronounced in column experiments with small NAPL pools resulting in extensive tailing of the tracer breakthrough curves. NAPL measurement errors decreased with decreasing tracer partition coefficient and increasing injected tracer mass. Extrapolating breakthrough curves exponentially reduced but did not eliminate systematic errors in NAPL measurement. The results from the static batch partitioning tests and dynamic column experiments provide insight into the influence of mass transfer limitations and tracer partitioning on NAPL detection.