Pore-scale modeling of dissolution from variably distributed nonaqueous phase liquid blobs

Contamination of groundwater by nonaqueous phase liquids (NAPLs) is widely recognized as a serious environmental problem. Predicting the dissolution, fate, and transport of these organic chemicals in the subsurface is challenging because geological heterogeneity exists at numerous scales. To better understand heterogeneity at the pore scale, we use the lattice Boltzmann (LB) method to simulate water flow and solute transport from distributed NAPL blobs in a two-dimensional porous media. The LB method approximates the momentum and mass transport equations at the pore scale, easily incorporating complex boundary conditions of the porous media. The effects of NAPL blob configuration and Peclet number (Pe) on steady state mass transfer are studied at 7% and 15% NAPL saturation. We find that the solute flux out of the simulated system decreases substantially as the transverse length over which NAPL blobs are distributed decreases; for example, the solute flux is reduced by a factor of 2 by confining the NAPL blobs to only half of the transverse length. Values of Sherwood numbers determined from our simulations are slightly less than values determined from previously published mass transfer correlations. Our results indicate that pore-scale NAPL configuration significantly affects mass transfer and that correlations should be modified to account for it. We find that the dimensionless mass transfer coefficient increases with Pe for the values used in our simulations, where the rate of increase decreases with increasing Pe. We observe that much of the variability in computed mass transfer coefficients is accounted for by differences in the NAPL-water interfacial area at high Pe. However, at lower Pe, variability remains due to NAPL configuration.