QUANTITATIVE-EVALUATION OF THE ENHANCEMENT OF NAPL-POOL DISSOLUTION BY FLUSHING AND BIODEGRADATION

Nonaqueous-phase liquids (NAPLs) commonly are trapped in the subsurfac e as pools. Often, subsurface remediation of NAPL pools is dependent u pon the physicochemical process of dissolution, which can be enhanced by flushing and in situ biodegradation, both of which decrease the sol ute concentration, which increases the dissolution driving force. The solution to the advection-dispersion-reaction equation for a two-dimen sional domain with one-dimensional flow past a NAPL pool is used to de velop quantitative tools and criteria for assessing the effects of flu shing and biodegradation on NAPL-pool dissolution for quasi-steady-sta te conditions. The effectiveness of flushing alone for enhancing the d issolution flux depends on the relative magnitude of mechanical disper sion and molecular diffusion, with the greatest effect for average por e water velocities sufficiently large so that transverse mechanical di spersion is much greater than molecular diffusion. The analysis of flu shing and biodegradation together identifies that biodegradation incre ases the dissolution flux from the pool only when Da2 (the ratio of th e 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.