Cosolvent-Enhanced Remediation of Residual Dense Nonaqueous Phase Liquids: Experimental Investigation

Abstract

The removal of denser than water nonaqueous phase liquids (DNAPLs) trapped at residual saturation is an important problem at many contaminated groundwater sites. Because pump-and-treat technologies have been ineffective in removing DNAPLs, alternative strategies have been suggested, one of which is enhancing the mobilization and dissolution of DNAPLs by flushing with a cosolvent. Tetrachloroethylene (PCE)/methanol/water systems were studied to evaluate the effect of methanol on the remediation of PCE-contaminated porous media. Experimental measurements of interfacial tension, equilibrium phase composition, and phase density at various methanol/water fractions were combined with other published properties to characterize these systems. In methanol flushing experiments, PCE mobilization, non-equilibrium PCE dissolution, and flow bypassing were all observed. The results demonstrate that (a) small-scale heterogeneities may lead to locally high residual DNAPL saturations that are more easily mobilized than DNAPL residuals in homogeneous media ; (b) mass transfer rate coefficients for PCE/methanol/water systems can be predicted to within 30% using an existing correlation developed for systems with similar NAPL emplacement procedures ; and (c) flow bypassing, due to nonuniform distributions of DNAPL residual or dissolution fingering, can occur in even small-scale experiments.