We studied the dissolution of nonaqueous phase liquids (NAPLs) at residual saturation in two‐dimensional systems using physical and numerical experiments. In all cases, preferential dissolution pathways developed in the NAPL‐contaminated regions. A permeability feedback mechanism led to the formation of centimeter‐scale dissolution fingers in homogeneous porous media, while fingers up to 1 order of magnitude wider formed in media with heterogeneous distributions of intrinsic permeability k, when σ2(lnk) was large. The structure of the interface between the NAPL‐contaminated and NAPL‐free regions was examined as dissolution progressed. In all cases the interface was a self‐affine fractal. The scaling relationship of Family and Vicsek [1985] fitted the interface data from the numerical simulations reasonably well.

Despite recent advances in the understanding of non-aqueous phase liquid (NAPL)-aqueous mass transfer, there is still a relatively poor understanding of the process at the laboratory scale and especially at the field-scale. In this work, we briefly summarize some of these limitations while focusing on heterogeneous, field-scale problems. We conclude that media heterogeneity plays a significant role in determining the morphology of the NAPL residual distribution, and this in turn plays a significant role in determining mass transfer from the NAPL residual. We review the factors affecting the NAPL residual morphology and use a high-resolution compositional modelling approach to simulate the dissolution process. Flowfield complexities affecting NAPL-aqueous phase mass transfer are demonstrated based upon simulations performed using this model.

Aquifers contaminated with dense nonaqueous phase liquids (DNAPLs) are extremely difficult to remediate with standard pump-and-treat methods. Enhanced remediation methods, such as flushing with cosolvent or surfactant solutions, promise to reduce remediation times but result in complex physicochemical systems for which we still lack both fundamental understanding and reliable process-based models. The overall objective of this work was to observe and quantify various physicochemical transport processes acting during the removal of a typical residual DNAPL (tetrachloroethylene, PCE) by solutions containing a nonionic surfactant (Triton X-100). To achieve this goal, we measured the phase behavior of the water/PCE/Triton X-100 system in batch systems and performed a set of glass bead column experiments to investigate residual PCE removal mechanisms, nonreactive tracer transport, and Triton X-100 transport. We observed and quantified removal of residual PCE by a number of processes (dissolution, micellar emulsions/microemulsion transport, macroemulsion transport, and DNAPL mobilization) as a function of the surfactant concentration used to elute the PCE from the porous medium. We concluded that macroemulsion transport was an important process, accounting for up to 30% of total PCE removal-a process which, to date, has not been accounted for in mathematical models of surfactant-enhanced remediation. We also observed that viscous fingering developed during elution of surfactant in the idealized one-dimensional column system, suggesting that this phenomenon will also affect the efficiency of surfactant recovery in field-scale applications.

The removal of denser than water nonaqueous phase liquids (DNAPLs) trapped at residual saturation is an important problem at many contaminated ground-water 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 an 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, nonequilibrium PCE dissolution, and Row bypassing were allobserved. 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.

Pump and treat is the most commonly used method for removing contaminants from groundwater but has proven ineffective in the case of nonaqueous phase liquids (NAPLs). Two methods that have been used to enhance recovery of crude oil, hot water and alcohol flooding, were investigated in the laboratory as a means of enhancing the dissolution and recovery of NAPLs in groundwater. A ubiquitous NAPL, tetrachloroethylene (PCE), was chosen for recovery in this investigation. The PCE/methanol/water system was studied to evaluate the effect of methanol concentration on interfacial tension, equilibrium phase composition, and phase density.