Non-aqueous phase liquid dissolution in heterogeneous systems: Mechanisms and a local equilibrium modeling approach

Experiments quantifying rates of non-aqueous phase liquid (NAPL) dissolution from heterogeneous media are presented and compared with model simulations. This work specifically addresses the overall dissolution of NAPL entrapped in a coarse sand lens at a high saturation. To explore the mechanisms governing dissolution rates, mathematical models describing the hydrodynamics of flow through the heterogeneous system were developed and coupled with a mass balance equation and a local equilibrium assumption (LEA) to quantify interphase mass transfer processes. Variations in the effective permeabilities as a function of NAPL saturation and the intrinsic permeabilities of the sands were employed to characterize the hydrodynamic aspects of flow through the heterogeneous system. Relative: to errors generated by the ill-defined aqueous phase relative permeabilities at high NAPL saturations, the model incorporating the system hydrodynamics as the sole rate-limiting process provided a reasonable first estimate of effluent concentrations. With the representative elemental volume defined here, rate-limited dissolution becomes important for low-NAPL saturations (S-n < similar to 0.05-0.15) causing tailing in the observed dissolution data and deviations between these data and the LEA model.