Quantitative evaluation of flushing and biodegradation for enhancing in situ dissolution of nonaqueous-phase liquids

Flushing and in situ biodegradation can enhance the dissolution rate for NAPL contamination by decreasing the solute concentration, which increases the dissolution driving force. NAPL interphase transfer and how flushing and in situ bioremediation enhance it are evaluated using a simple NAPL contamination scenario: one-dimensional flow through a saturated, homogeneous, isotropic medium containing a residual saturation of uniformly distributed, immobilized, single-component NAPL blobs. A mathematical model of the system is developed using a one-dimensional advection-dispersion reaction (ADR) equation incorporating a first-order interphase mass-transfer relationship and first-order biodegradation kinetics. The analysis is simplified by assuming quasi-steady-state conditions in which NAPL blob size and saturation are constant. Using dimensionless groups that describe the relative rates in the system and analytical solutions to the ADR, criteria are delineated for when equilibrium and nonequilibrium exist and when flushing and biodegradation can effectively enhance NAPL dissolution in the system. These analyses are performed for flushing alone and for flushing and biodegradation in conjunction. The results demonstrate that flushing is effective for enhancing dissolution when the flow rate gives solute concentrations that are neither zero nor the solubility limit throughout the domain. In situ biodegradation can accelerate the dissolution rate when the biodegradation rate becomes large, compared to the mass-transfer rate, and as long as neither the advection rate nor the biodegradation rate is so great that the solute concentration is zero throughout the domain.