Influence of dense nonaqueous phase liquid pool morphology on the performance of partitioning tracer tests: Evaluation of the equilibrium assumption

Abstract

Partitioning tracer techniques, initially developed in the petroleum industry, are promoted as a way to determine entrapped dense nonaqueous phase liquid (DNAPL) mass in source zones. A suite of chemical tracers, some of which partition into the DNAPL phase, is injected into the source zone to estimate the entrapped saturation of contaminant. Current application of the technique can determine DNAPL at residual saturation but does not provide reliable mass estimates of DNAPL in high‐saturation areas (e.g., pools) that may occur in some source zones. To evaluate this technique's limitations in characterizing source zones with complex entrapment architectures that may contain pools, we investigated estimation errors associated with well‐characterized single‐isolated DNAPL pools. X‐ray attenuation was used to determine the spatial distribution of saturation within the pool under different total DNAPL mass entrapment while the DNAPL was subjected to tracer tests. Model analysis used showed that the partitioning tracer breakthrough simulations based on equilibrium partition coefficient (Kp, from equilibrium batch tests) fit the experimental data poorly, suggesting nonequilibrium partitioning that is controlled by the water flow velocity through the NAPL entrapment zone. In order to relax the requirement of partitioning to be at equilibrium, we investigated the use of “effective partition coefficients” (Kpe). A comparison of the saturation estimates using method of moments and inverse modeling with both equilibrium and effective partition coefficients was conducted. Results demonstrated that the incorporation of nonequilibrium behavior in the analysis can improve DNAPL mass estimation by tracers in high‐saturation zones.