Influence Of Porous Media Heterogeneity On Nonaqueous Phase Liquid Dissolution Fingering And Upscaled Mass Transfer

Laboratory experiments and numerical simulations in homogeneous porous media were used to investigate the influence of porous medium wettability on the formation and growth of preferential dissolution pathways, dissolution fingers, during nonaqueous-phase liquid (NAPL) dissolution. As the porous medium became increasingly NAPL-wet, dissolution fingers grew wider and slower. This result was observed in physical experiments with 0 and 100% NAPL-wet conditions and confirmed with numerical simulations at these and intermediate wettabilities. A previously derived expression for an upscaled mass transfer rate coefficient that accounts for the growth of dissolution fingers was used to quantify the effect of fingering on overall NAPL removal rates. For the test cases evaluated, NAPL dissolution fingering controlled the overall rate of NAPL dissolution after the dissolution front moved 9 cm in 0% NAPL-wet conditions and 34 cm in 100% NAPL-wet conditions. Thus, even in completely NAPL-wet media dissolution fingering may control the overall rate of NAPL dissolution after relatively short travel distances. The influence of aqueous-phase boundary conditions on dissolution fingering was also investigated. For the range of conditions studied here, constant head and constant flux boundary conditions resulted in similar finger growth and morphology of the interface between clean and NAPL-contaminated regions: the interface roughness and the growth of this roughness with pore volumes flushed were independent of boundary condition type. Laboratory studies were conducted to evaluate the influence of heterogeneity on NAPL dissolution fingering. During a spill event heterogeneous porous media favor the formation of nonuniform NAPL saturation fields, including NAPL residual and NAPL pools, that may affect the fingering mechanism. A light transmission technique was used to measure trichloroethylene saturation fields at a 0.05-cm resolution that resulted from spills in two heterogeneous packings of a laboratory test cell. The correlation length of the permeability field transverse to the mean water flow direction was selected to be similar to (1.0 cm) or significantly greater than (6.0 cm) the expected wavelength of dissolution fingers. As the entrapped NAPL dissolved into water, preferential NAPL dissolution patterns occurred in both experiments, with patterns strongly affected by the heterogeneities. Experimental results were used to validate the utility of a numerical simulator for capturing the growth of centimeter-scale preferential NAPL dissolution patterns. Using data from these experiments, four different methods for upscaling the mass transfer rate coefficient for NAPL dissolution were examined [Basu et al., 2008; Christ et al., 2006; Imhoff et al., 2003a; Saenton and Illangasekare, 2007]. These models were developed to account for the influence of dissolution fingering or NAPL architecture on the long-term flux of contaminants from NAPL source zones. In the packing where the correlation scale of permeability perpendicular to the mean water flow direction was 6.0 cm, greater than the scale of the dissolution fingers, all upscaling approaches predicted effluent concentrations reasonably well. When the correlation scale of the heterogeneities was smaller (1.0 cm), the models performed much more poorly. These results and their implications on the applicability of the upscaling models will be discussed. Among the four different upscaling approaches tested in Chapter 4 the stream-tube approach developed by Basu et al. [2008] seemed very promising in predicting the upscale mass transfer and effluent concentrations in the two physical experiments we conducted with different degrees of heterogeneities in permeability and NAPL saturation distribution. A simple stream-tube approach was developed to predict the dissolution front dynamics and effluent concentration profiles. This simple model worked poorly when used for the large correlation length system where the permeability heterogeneity was close to homogeneous, where the dissolution rate is controlled by dissolution fingering. However, when the correlation scale of heterogeneity decreased for the small correlation length system, the dissolution rate was controlled solely by the heterogeneities in permeability and NAPL saturation fields and not by the dissolution fingering mechanism. In this case the stream-tube model was able to predict the effluent concentration profile and capture the tailing in that very accurately. This model needs to be combined with the dissolution fingering model and be tested for wider range of heterogeneous systems. This is outlined as a recommendation for the future work.

This monograph provides state-of-the-art theoretical and computational findings from investigations on physical and chemical dissolution front instability problems in porous media, based on the author’s own work. Although numerical results are provided to complement theoretical ones, the focus of this monograph is on the theoretical aspects of the topic and those presented in this book are applicable to a wide range of scientific and engineering problems involving the instability of nonlinear dynamic systems. To appeal to a wider readership, common mathematical notations are used to derive the theoretical solutions. The book can be used either as a useful textbook for postgraduate students or as a valuable reference book for computational scientists, mathematicians, engineers and geoscientists.

A comprehensive account of the state of the science of environmental mass transport Edited by Louis J. Thibodeaux and Donald Mackay, renowned experts in this field, the Handbook of Chemical Mass Transport in the Environment covers those processes which are critically important for assessing chemical fate, exposure, and risk. In a comprehensive and authoritative format, this unique handbook provides environmental chemists, geoscientists, engineers, and modelers with the essential capabilities to understand and quantify transport. In addition, it offers a one-stop resource on environmental mass transfer and mass transport coefficient estimation methods for all genres. The book begins by discussing mass transport fundamentals from an environmental perspective. It introduces the concept of mobility — key to environmental fate, since transport must occur prior to any reaction or partitioning within the natural multimedia compartments. The fugacity approach to environmental mass transfer and the conventional approach are examined. This is followed by a description of the individual mass transport processes and the appropriate flux equations required for a quantitative expression. The editors have identified 41 individual processes believed to be the most environmentally significant, which form the basis for the remainder of the book Using a consistent format for easy reference, each chapter: Introduces the specific processes Provides a detailed qualitative description Presents key theoretical mathematical formulations Describes field or laboratory measurements of transport parameters Gives data tables and algorithms for numerical estimates Offers a guide for users familiar with the process who are seeking a direct pathway to obtain the numerical coefficients Presents computed example problems, case studies and/or exercises with worked-through solutions and answers The final chapter presents the editors’ insight into future needs and emerging priorities. Accessible and relevant to a broad range of science and engineering users, this volume captures the state of the transport science and practice in this critical area.