THE INFLUENCE OF MASS-TRANSFER CHARACTERISTICS AND POROUS-MEDIA HETEROGENEITY ON NONAQUEOUS PHASE DISSOLUTION

A two-dimensional multiphase flow and species transport model was deve loped and applied to the case of nonaqueous phase liquid (NAPL) emplac ement and dissolution in both homogeneous and heterogeneous porous med ia systems. Simulations were performed to observe dissolution rate var iations and the degree of NAPL-aqueous phase nonequilibrium as a funct ion of two aqueous phase velocities and five forms of the NAPL-aqueous phase mass transfer formulation. An integrated form of the Damkohler number was introduced to analyze the degree of NAPL-aqueous phase none quilibrium. Mass removal rates for homogeneous media were insensitive to the form df the NAPL-aqueous phase mass transfer formulation, yield ing results similar to a local equilibrium approach for all but one ma ss transfer formulation. This latter formulation was most sensitive to NAPL saturation and yielded significant nonequilibrium behavior, whic h was manifested as a decrease in NAPL dissolution rates as the NAPL v olume fraction decreased. Variations in mass elution rates between hom ogeneous and heterogeneous media were observed, with more significant variations found for variances in porous media properties than for hor izontal correlation lengths. In heterogeneous media, decreases in diss olution rates were attributed to the existence-of relatively immobile regions of NAPL with saturations greater than the residual saturation of the media, so-called NAPL pools. These results illustrate the impor tance of the statistical characteristics of heterogeneous porous media on NAPL distribution and dissolution processes.