Abstract. 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, ratelimited dissolution becomes important for low-NAPL saturations (Sn • •0.05-0.15) causing tailing in the observed dissolution data and deviations between these data and the LEA model. IntroductionA significant amount of theoretical and laboratory research has been conducted to study the transport and fate of nonaqueous phase liquids (NAPLs) in subsurface systems. Much of this research has utilized homogeneous porous media, overlooking the potentially significant effects of geological heterogeneities. It is expected that both microscale and macroscale heterogeneities will affect NAPL entrapment and dissolution processes. The high saturations expected for nonwetting NAPLs in zones defined by larger sand grains will reduce the effective permeabilities in these regions, changing the overall patterns of aqueous phase flow and subsequent NAPL dissolution rates.The work reported here evaluates processes controlling the long-term dissolution of NAPL entrapped at high saturations in heterogeneous porous media composed of a coarse sand lens surrounded by a finer medium. It is hypothesized that water in the direct vicinity of NAPL is equilibrated with the organic phase and that lower concentrations observed at locations down gradient of NAPL sources can be attributed to dilution effects. This assumption implies that NAPL dissolution processes in heterogeneous media can be predicted based on knowledge of the system hydrodynamics and thermodynamic equilibrium relationships. To test this hypothesis, experimental data from both column and two-dimensional sand tank experiments were collected for a simple heterogeneous system. Effluent concentration data were then compared to mathematical models describing the system hydrodynamics and dilution as the sole rate-limiting step.•Now at Malcolm Pirnie, Inc., Newport News, Virginia. . These efforts have confirmed the dependence of mass transfer rates on aqueous phase velocity, NAPL saturation, and...
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