This study examines solute transport phenomena in subsurface systems, with emphasis on molecular diffusion. The exchange of solutes between high and low-permeability zones is of considerable interest for solute and contaminant fate and transport. For example, solutes in low-permeability zones can serves as long-term sources of aquifer contamination. This study develops experimental and analytical approaches to investigate diffusive solute transport between aquifers and aquitards with a variety of initial and boundary conditions. In Chapter 2, a laboratory technique for examining dye diffusion in clays using light reflection visualization (LRV) experiments is demonstrated. One- and two-dimensional experimentally-measured LRV concentration profiles in the clay were found to be in very good agreement with those predicted from a one-dimensional (1D) analytical solution. The results from Chapter 2 demonstrate that the LRV approach is an attractive non-invasive tool to investigate the concentration distribution of dye tracers in clays in laboratory experiments. In Chapter 3, a series of well-controlled laboratory experiments were conducted to quantify solute diffusion from a high-permeability sand into and subsequently out of thin kaolinite clay layers. One-dimensional analytical solutions were developed for forward and back diffusion in a finite aquitard using the method of images and showed very good agreement with measured breakthrough curves and concentration profiles measured in situ by LRV. A dimensionless diffusion length scale was introduced and was used to illustrate that the solutions using image sources should be applied in cases with rapid solute diffusion and/or thin clay layers. In Chapter 4, novel 1D analytical solutions are developed for solute diffusion between aquifers and single aquitard systems, validated in well-controlled experiments, and applied to data from laboratory and field-scale problems with diffusion time and length scales ranging from 10-2-10 8 years and 10-2-102 m. The diffusion models were developed using the method of images to consider a finite aquitard bounded by two aquifers at the top and bottom, or a semi-infinite aquitard bounded by an aquifer. Measured resident solute concentration profiles in aquitards and flux-averaged solute concentrations in surrounding aquifers were accurately modeled by appropriately accounting for generalized dynamic aquifer-aquitard boundary conditions, including concentration gradient reversals. In Chapter 5, the impact of source strength functions on forward and back diffusion was evaluated using three laboratory experiments that were compared to and with analytical solutions. Aquitard solute concentration profiles and aquifer breakthrough curves were compared among three source strength functions (Γ = 0, 0.5, and 1). The results suggested that the magnitude of back diffusion flux from a clay layer decreases with an increase in the value of Γ. Overall, the modeling approach used in this study can be used as a decision tool for field sites to evaluate the relative significance of forward and back diffusion.
|Advisor:||Jawitz, James W., Annable, Michael D.|
|School:||University of Florida|
|School Location:||United States -- Florida|
|Source:||DAI-B 78/04(E), Dissertation Abstracts International|
|Subjects:||Hydrologic sciences, Water Resource Management, Environmental engineering|
|Keywords:||Analytical solution, Aquitard, Back diffusion, DNAPL, Diffusion, Forward diffusion|
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