The attenuation of gamma radiation was utilized to measure changing residual trichloroethylene (TCE) saturation in an otherwise water‐saturated porous medium as clean water was flushed through the medium. A front over which dissolution actively occurred was observed. Once developed, this front varied in length from ≈11 mm to ≈21 mm, lengthening as it moved through the porous medium. Gamma attenuation measurements and analyses of effluent water samples indicate that there was minimal if any transport of TCE as colloidal droplets. Even as trapped TCE ganglia decreased in size due to dissolution, there is no evidence that they became mobile and advected downgradient. An extraction of the porous medium at the completion of one experiment indicated that less than 0.002% of the original TCE mass remained, suggesting that minimal amounts of separate phase TCE remained trapped within the medium after flushing with 290 pore volumes. Mass transfer rate coefficients were computed and are shown to be a function of Darcy flux, TCE volumetric content, and distance into the region of residual TCE.

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The dissolution of nonaqueous phase liquids (NAPLs) trapped at residual saturation is an important problem at many contaminated groundwater sites. It is well known that NAPL ganglia trapped within the pore space reduce the permeability of the medium to aqueous phase flow. When fluid flow is imposed on such a system, the aqueous phase may interact with the dissolution‐induced permeability changes, leading to fingered patterns. This mechanism is very similar to that of mineral dissolution instabilities, which are a particular example of reactive infiltration instabilities. Extending that literature, we present a nonlinear model describing the dissolution of NAPL ganglia and perform a linear stability analysis of the resultant moving free boundary problem, demonstrating that instabilities may develop from a planar dissolution front. Predicted finger wavelengths are a function of both residual NAPL saturation and the imposed aqueous phase flow rate; they range from centimeters to meters. Experimental observations of dissolution fingering are presented in a companion paper [Imhoff et al., this issue] and are compared with predictions from this model. Dissolution fingering may affect the solubilization of NAPL ganglia in natural environments and in experimental studies of NAPL dissolution intended to quantify mass transfer rates.

Nonaqueous phase liquids (NAPLs) are a common source of contamination at polluted groundwater sites, where they frequently remain trapped within the pore space at residual saturation and reduce the permeability of the medium to aqueous phase flow. The model presented in a companion paper [Imhoff and Miller, this issue] suggested that when fluid flow is imposed on such a system, the aqueous phase may interact with dissolution‐induced permeability changes, and lead to fingered patterns. In this investigation, a two‐dimensional flow cell was used to study the effects of porous medium structure, Darcy flux, initial residual NAPL saturation, median particle diameter, gravity, and NAPL composition on dissolution fingering. Fingering occurred when two conditions were met: (1) 11 to 80 e‐fold times had elapsed, where e‐fold time is the time required for the instability to grow by a factor e and was predicted from the linear stability analysis in the companion paper; and (2) the length of the dissolution front before finger development was smaller than the zone of NAPL residual. Where fingers formed, finger structure was similar and showed no systematic variation within the parameters investigated. Observed finger wavelengths compared well with model predictions. A single experiment in a three‐dimensional cell, 1 m long, demonstrated that fingers can grow to at least 30 cm in length. When experimental observations in this cell were compared with predictions of NAPL dissolution based on models that did not include fingering, the measurements of changing NAPL saturation differed significantly from model predictions.

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, nonequilibrium 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. BackgroundRecently, surfactants, alcohols, and mixtures of these chemicals have been suggested for in situ treatment of aquifers contaminated with hazardous organic compounds. These technologies originally were developed in the petroleum industry, and several reviews have been written f Present address: 121 S, Lippincott Ave., Maple Shade, NJ 08052.

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