A dual-domain approach is developed for calculating the advective and dispersive mass flux of chemical leaving one or more sub-grid-block-scale DNAPL pool(s) in an integral finite difference numerical simulation. The contaminated zone in a simulation is divided into two fractions: one that contains DNAPL pools and one in which DNAPL is not present. With a dual-permeability formulation the two regions may have the same or different flow properties depending on the conceptual model for pool formation, and fluids can freely flow through both domains. The local dispersive flux of contaminant away from the DNAPL pool is calculated using a well-known analytical solution for steady state advection and dispersion. This expression is integrated analytically to give a first-order mass transfer relationship for the total dissolution rate inside a grid block as a function of the horizontal water velocity, the transverse dispersivity, the DNAPL solubility, and the dissolved concentration in the media not containing the DNAPL. The calculated interphase mass transfer rate using a single dual-domain integral finite difference grid block is shown to match experimental data and an analytical solution for DNAPL pool dissolution.