Simulation and performance assessment of partitioning tracer tests in heterogeneous aquifers

Characterization of dense non-aqueous phase liquid (DNAPL) in heterogeneous media represents a major challenge in the remediation process due to the complexity of DNAPL distribution in the subsurface. The partitioning interwell tracer test (PITT), which evaluates the relative transport of DNAPL-phase partitioning and conservative (i.e., non-partitioning) tracers, has recently been promoted as a way to quantify entrapped DNAPL mass in source zones. In some cases, the technique has been successfully applied to sites where DNAPL is present primarily at residual saturations. However, the effects of the geologic heterogeneity and DNAPL architecture on the performance and reliability of this technique have not yet been thoroughly examined. A systematic two-dimensional simulation study was conducted to evaluate the influence of DNAPL vertical distribution, well location, and geologic heterogeneity on the performance of the PITT for a total of 60 stochastic aquifer realizations. The aquifer hydraulic conductivity (K) distribution was defined using three different geostatistical parameter values describing the variance of the log K (sigma (super 2) (sub ln k) = [1.22, 0.75, 0.25]). A multiphase flow simulator was used to generate the source zone, allowing the DNAPL to distribute naturally. The tracer test simulations conducted using these various synthetic aquifers were specifically designed to quantify PITT performance for different degrees of heterogeneity. For cases where the aquifer is relatively heterogeneous and the DNAPL architecture is complex, PITT estimation efficiency averaged 48 percent (decreasing to 20 percent in certain scenarios) as a result of bypass flow and the limited tracer accessibility to lower hydraulic conductivity regions. While other studies have suggested the PITT technique may underestimate DNAPL saturation for these conditions, potential errors of this magnitude have not previously been reported. The results of this work underscore the importance of geological heterogeneity on PITT performance.