Mass transfer rate limitation effects on partitioning tracer tests

Partitioning tracer tests are often used to quantify the amount of non-aqueous phase liquid (NAPL) present in a porous medium. Results from such tests are usually interpreted by using models that assume local equilibrium exists between the NAPL and the aqueous phase or by using a moment method to solve for the fraction of the pore space occupied by the NAPL. We investigated the transport of partitioning alcohol tracers through heterogeneous sand porous medium systems containing a stationary trichloroethylene (TCE) phase for a range of aqueous phase velocities. NAPL saturations were quantified using a non-destructive X-ray absorption methodology. Experimental results were simulated using a numerical solution to an advective–dispersive model in which mass transfer between the NAPL and the aqueous phase was approximated using a dual-resistance approach consisting of a boundary layer mass transfer resistance and a diffusional resistance within the NAPL. The numerical model agreed well with the experimental data, and moment analysis yielded reasonable estimates of actual NAPL saturations.