Influence of porous media heterogeneity on nonaqueous phase liquid dissolution fingering and upscaled mass transfer

Abstract

The utility of existing models for describing upscaled mass transfer from nonaqueous phase liquid (NAPL) were examined when preferential dissolution pathways form in NAPL‐contaminated zones that extend over the scale of decimeters. Laboratory experiments were conducted in two well‐characterized, heterogeneous packings. Using data from these experiments and simulations, existing methods for upscaling the mass transfer rate coefficient for NAPL dissolution based on dissolution front length growth (LDF), aquifer heterogeneity and spatial moments of NAPL distribution, and the ganglia‐to‐pool ratio (GTP) were evaluated along with an equilibrium stream tube (EST) model for predicting contaminant flux. When the correlation length of permeability perpendicular to the mean water flow direction was 6.0 cm, greater than the scale of dissolution fingers, only 4.8% of the NAPL resided in pools. Dissolution fingers formed in this experiment, and the LDF, GTP, and EST models resulted in similar predictions of effluent concentrations, with root‐mean‐square errors (RMSEs) between 0.035 and 0.079 and the LDF‐heterogeneous model best. When the correlation scale was smaller (1.0 cm), 66.7% of the NAPL was in pools, and preferential dissolution pathways were dominated by channeling, preferential dissolution caused by spatial variations in aqueous phase permeability, and NAPL saturation. For this experiment the EST and GTP models performed well, with RMSEs of 0.055 and 0.103, respectively. Dissolution fingering was important when the permeability correlation length was sufficiently large that dissolution finger formation was not disrupted and NAPL pools were not dominant.