Quantitative Evaluation of the Enhancement of NAPL-Pool Dissolution by Flushing and Biodegradation

Nonaqueous-phase liquids (NAPLs) commonly are trapped in the subsurface as pools. Often, subsurface remediation of NAPL pools is dependent upon the physicochemical process of dissolution, which can be enhanced by flushing and in situ biodegradation, both of which decrease the solute concentration, which increases the dissolution driving force. The solution to the advection–dispersion–reaction equation for a two-dimensional domain with onedimensional flow past a NAPL pool is used to develop quantitative tools and criteria for assessing the effects of flushing and biodegradation on NAPL-pool dissolution for quasi-steady-state conditions. The effectiveness of flushing alone for enhancing the dissolution flux depends on the relative magnitude of mechanical dispersion and molecular diffusion, with the greatest effect for average pore water velocities sufficiently large so that transverse mechanical dispersion is much greater than molecular diffusion. The analysis of flushing and biodegradation together identifies that biodegradation increases the dissolution flux from the pool only when Da₂ (the ratio of the biodegradation rate to the advection rate) is greater than 0.1. When Da₂ > 0.1, many different combinations of biodegradation and flushing can be used to obtain a desired dissolution flux.