Modeling field-scale dense nonaqueous phase liquid dissolution kinetics in heterogeneous aquifers

Abstract

This study investigates field-scale DNAPL dissolution kinetics using high-resolution numerical simulations of DNAPL releases and dissolved phase transport. A percolation model is employed to simulate the distribution of TCE within 10 x 10 x 10 m source zones with spatially heterogeneous aquifer properties following a release event. Distributed aquifer properties and DNAPL saturations are utilized to simulate coupled groundwater flow and long-term dissolved phase transport. Grid-scale dissolution rates are computed based on published bench-scale relationships. Effective field-scale mass transfer coefficients are computed from simulated TCE fluxes at the downstream source zone boundary. Heterogeneity in groundwater velocity and DNAPL distributions leads to field-scale mass transfer coefficients that are much lower than laboratory-scale values. Field-scale mass transfer coefficients are observed to vary in direct proportion to the mean groundwater velocity, in contrast to laboratory studies that indicate proportionality with velocity to a power of ∼0.7. Computed field-scale mass transfer coefficients vary approximately in proportion to relative DNAPL mass raised to an empirical depletion exponent, which is <1 for laterally extensive DNAPL lenses and >1 for more randomly oriented residual DNAPL regions. The former DNAPL geometries exhibit slow reductions in source concentration and contaminant flux with time as mass depletion proceeds. The latter DNAPL geometries exhibit significant and steady declines in source concentration and contaminant flux with time as depletion occurs.