Diffusive And Advective-diffusive Transport Through Saturated And Unsaturated Soils

This thesis describes a series of laboratory experiments together with the theoretical analysis of NaCl migration through single and double layer soil systems for a range of physical and chemical characteristics. The soil materials were tested under saturated and unsaturated (gravity drainage) conditions. The small and large scale apparatus were designed so that soil samples could be tested for diffusive and advective-diffusive migration.;The experiments examining NaCl diffusion through the saturated fine sand and clayey silt were used to obtain the reference diffusion coefficients for the individual types of soil. The fine sand and clayey silt were then used in a combined system to simulate a compacted clayey liner over a natural deposit of nearly saturated fine sand. The migration process involved downward advective-diffusive transport of NaCl from a source solution placed on top of the clay plug through the clay and fine sand and into a receptor solution. Using the contaminant transport model POLLUTE and Cl{dollar}\sp-{dollar} diffusion coefficient in the fine sand and clay determined from diffusion experiments conducted on isolated soils, the transport of Cl{dollar}\sp-{dollar} in the combined soil system was predicted and found to agree well with the experiments indicating the reproducibility of the transport parameters in the combined soil systems.;The diffusion characteristics of the coarse sand and fine gravel were studied by first testing the materials under saturated conditions and determining the reference diffusion coefficients of Cl{dollar}\sp-{dollar} and Na{dollar}\sp+{dollar}. These materials were then tested for unsaturated conditions under gravity drainage with atmospheric pressure being maintained at the bottom of the samples. Relatively slow migration of the ions was observed in the unsaturated portion of the samples. The unsaturated diffusion coefficients were predicted using the reference diffusion coefficients and assuming a linear relationship to exist between the volumetric moisture content and diffusion coefficient of the unsaturated sample.;The diffusion coefficients determined from diffusion tests on the isolated clayey soil and unsaturated fine gravel were used to predict the advective-diffusive migration of the Cl{dollar}\sp-{dollar} and Na{dollar}\sp+{dollar} ions in a combined soil systems.;Using the large scale advection-diffusion apparatus, laboratory experiments were conducted to simulate a compacted silt or clayey silt liner over an unsaturated secondary leachate collection system. Experiments were conducted under high , moderate and low flow rates. Relatively steep and uniform Cl{dollar}\sp-{dollar} concentration profiles were observed in the unsaturated stone layer. This behaviour was accurately modelled by the advection-diffusion theory using POLLUTE and transport parameters obtained from the previous experiments. A small amount of dispersion was inferred from the results of the theoretical modelling in the unsaturated stone layer and the effect of groundwater velocity was found to be significant in migration through the stone, even in the low flow rate system. (Abstract shortened by UMI.