| Abstract
| - The purpose of this work is to examine the effect ofnonuniform distributions of immiscible organic liquid ondissolution behavior, with a specific focus on the conditiondependency of dissolution (i.e., mass transfer) ratecoefficients associated with applying mathematical modelsof differing complexities to measured data. Dissolutionexperiments were conducted using intermediate-scale flowcells packed with sand in which well-characterizedzones of residual trichloroethene (TCE) and 1,2-dichloroethane(DCA) saturation were emplaced. A dual-energy gammaradiation system was used for in-situ measurement of NAPLsaturation. Aqueous concentrations of TCE and DCAmeasured in the flow-cell effluent were significantly lessthan solubility, due primarily to dilution associated with thenonuniform immiscible-liquid distribution and bypassflow effects associated with physical heterogeneity. Aquantitative analysis of flow and transport was conductedusing a three-dimensional mathematical model whereinimmiscible-liquid distribution, permeability variability, andsampling effects were explicitly considered. Independentvalues for the initial dissolution rate coefficients were obtainedfrom dissolution experiments conducted using homogeneously packed columns. The independent predictionsobtained from the model provided good representations ofNAPL dissolution behavior and of total TCE/DCA massremoved, signifying model robustness. This indicates thatfor the complex three-dimensional model, explicitconsideration of the larger scale factors that influencedimmiscible-liquid dissolution in the flow cells allowed the useof a dissolution rate coefficient that represents only local-scale mass transfer processes. Conversely, the use ofsimpler models that did not explicitly consider the nonuniformimmiscible-liquid distribution required the use of dissolutionrate coefficients that are ∼3 orders of magnitude smallerthan the values obtained from the column experiments.The rate coefficients associated with the simpler modelsrepresent composite or lumped coefficients that incorporatethe effects of the larger scale dissolution processesassociated with the nonuniform immiscible-liquid distribution,which are not explicitly represented in the simplermodels, as well as local-scale mass transfer. Theseresults demonstrate that local-scale dissolution ratecoefficients, such as those obtained from column experiments,can be used in models to successfully predict dissolutionand transport of immiscible-liquid constituents at largerscales when the larger scale factors influencing dissolutionbehavior are explicitly accounted for in the model.
|
