Effects of flow bypassing and nonuniform NAPL distribution on the mass transfer characteristics of NAPL dissolution

Ground water concentrations of nonaqueous phase liquid (NAPL) are usually less than their aqueous solubility; this can be attributed to irregular NAPL distribution, nonuniform flow patterns, dilution effects, and rate-limited mass transfer between the NAPL and aqueous phases. This paper uses two-domain and parallel column models to demonstrate the effects of nonuniform NAPL distribution and flow bypassing on apparent mass transfer kinetics for NAPL dissolution. The hypothesis of this study is that much of the apparent nonequilibrium dissolution can be explained by the effects of heterogeneity in NAPL distribution and in porous media properties. Models incorporating two-domain concepts to represent heterogeneity are able to reproduce the timescale dependency of mass transfer coefficients which has often been observed but is inconsistent with mass transfer theory. Parallel column models, with equilibrium partitioning assumed in each column, are able to reproduce the concentration drop-off and tailing observed in published column studies of NAPL dissolution. The parallel column model is also able to reproduce the experimentally observed phenomenon of mass transfer zones which lengthen with time and distance traveled. The results of this study support the hypothesis that much of the apparent nonequilibrium mass transfer kinetics of NAPL dissolution can be described by heterogeneity in NAPL distribution and in porous media properties.