[1] This study investigates connectivity in a small portion of the extremely heterogeneous aquifer at the Macrodispersion Experiment (MADE) site in Columbus, Mississippi. A total of 19 fully penetrating soil cores were collected from a rectangular grid of 4 m by 4 m. Detailed grain size analysis was performed on 5 cm segments of each core, yielding 1740 hydraulic conductivity (K) estimates. Three different geostatistical simulation methods were used to generate 3-D conditional realizations of the K field for the sampled block. Particle tracking calculations showed that the fastest particles, as represented by the first 5% to arrive, converge along preferential flow paths and exit the model domain within preferred areas. These 5% fastest flow paths accounted for about 40% of the flow. The distribution of preferential flow paths and particle exit locations is clearly influenced by the occurrence of clusters formed by interconnected cells with K equal to or greater than the 0.9 decile of the data distribution (10% of the volume). The fraction of particle paths within the high-K clusters ranges from 43% to 69%. In variogram-based K fields, some of the fastest paths are through media with lower K values, suggesting that transport connectivity may not require fully connected zones of relatively homogenous K. The high degree of flow and transport connectivity was confirmed by the values of two groups of connectivity indicators. In particular, the ratio between effective and geometric mean K (on average, about 2) and the ratio between the average arrival time and the arrival time of the fastest particles (on average, about 9) are consistent with flow and advective transport behavior characterized by channeling along preferential flow paths.

The limitations associated with conventional pump and treat technology have generated interest in using enhanced in-situ flushing as an alternative for remediating source zones contaminated with immiscible liquid. This research investigates the effectiveness of cyclodextrin as a solubility-enhancement agent to enhance the removal of tetrachloroethene (PCE) from a physically isolated section of an aquifer. An important component of this project was the implementation of reagent recovery and reuse. This field experiment presented the rare opportunity, under strict regulatory guidance, to inject PCE into the surficial aquifer cell created with two sets of sheet piles driven into an underlying clay unit. The well-controlled conditions specific to this experiment allowed quantification of mass balances, which is problematic for many contaminated field sites. The fact that mass balances can be obtained provides the ability to determine remediation effectiveness with unusual accuracy for a field project. The saturated zone within the test cell was flushed with a 15 wt % cyclodextrin solution. The cyclodextrin solution increased the aqueous concentration of PCE in the extraction-well effluent to as much as 22 times the concentrations obtained during the water flush conducted prior to the complexing sugar flush (CSF). The seven pore-volume CSF removed the equivalent of approximately 33 L of PCE from the subsurface. This equates to 48% of the total initial mass, based on the volume of PCE present prior to the CSF (68.6 L). Conversely, the seven pore-volume water flush conducted prior to the CSF removed the equivalent of 2.7 L of PCE. The use of cyclodextrin as a flushing agent, especially in a recycling configuration, appears to hold promise for successful remediation of chlorinated-solvent-contaminated source zones.

The purpose of this work is to examine the effect of nonuniform distributions of immiscible organic liquid on dissolution behavior, with a specific focus on the condition dependency of dissolution (i.e., mass transfer) rate coefficients associated with applying mathematical models of differing complexities to measured data. Dissolution experiments were conducted using intermediate-scale flow cells packed with sand in which well-characterized zones of residual trichloroethene (TCE) and 1,2-dichloroethane (DCA) saturation were emplaced. A dual-energy gamma radiation system was used for in-situ measurement of NAPL saturation. Aqueous concentrations of TCE and DCA measured in the flow-cell effluent were significantly less than solubility, due primarily to dilution associated with the nonuniform immiscible-liquid distribution and bypass flow effects associated with physical heterogeneity. A quantitative analysis of flow and transport was conducted using a three-dimensional mathematical model wherein immiscible-liquid distribution, permeability variability, and sampling effects were explicitly considered. Independent values for the initial dissolution rate coefficients were obtained from dissolution experiments conducted using homogeneously packed columns. The independent predictions obtained from the model provided good representations of NAPL dissolution behavior and of total TCE/DCA mass removed, signifying model robustness. This indicates that for the complex three-dimensional model, explicit consideration of the larger scale factors that influenced immiscible-liquid dissolution in the flow cells allowed the use of a dissolution rate coefficient that represents only local-scale mass transfer processes. Conversely, the use of simpler models that did not explicitly consider the nonuniform immiscible-liquid distribution required the use of dissolution rate coefficients that are approximately 3 orders of magnitude smaller than the values obtained from the column experiments. The rate coefficients associated with the simpler models represent composite or lumped coefficients that incorporate the effects of the larger scale dissolution processes associated with the nonuniform immiscible-liquid distribution, which are not explicitly represented in the simpler models, as well as local-scale mass transfer. These results demonstrate that local-scale dissolution rate coefficients, such as those obtained from column experiments, can be used in models to successfully predict dissolution and transport of immiscible-liquid constituents at larger scales when the larger scale factors influencing dissolution behavior are explicitly accounted for in the model.