Identifier: TDX:319
Authors: Silva Rojas, Orlando
Abstract:
The presence of some chemicals in the vadose zone can affect the physical properties of the fluid phases leading to transport processes known as non-passive. Common numerical models do not account for the simultaneous interaction of various non-passive effects on unsaturated flow and transport. The main objective of this thesis was to develop a detailed model that describe the isothermal non-passive transport of water-soluble organic compounds in the unsaturated zone. The model includes the dependence of density, viscosity, surface tension, molecular diffusion coefficient in the liquid phase, and gas-liquid partition coefficient on the aqueous mixture composition. It also takes into account the decrease in the gas-liquid partition coefficient at high capillary pressures, in accordance with Kelvin's equation for multi-component mixtures.The model was implemented in two numerical codes: 1D and 2D with cylindrical symmetry. It was used to illustrate the one-dimensional infiltration and redistribution of alcohol-water mixtures. Simulation of butanol-water mixtures infiltration in sand was in agreement with the experimental data and simulations reported in the literature. Simulation of infiltration, redistribution and volatilization of aqueous mixtures of methanol in two different soils showed that composition significantly affects volumetric liquid content and concentration profiles, as well as the normalized volatilization and evaporation fluxes. Dispersion in the liquid-phase was the predominant mechanism in the transport of methanol when dispersivity at saturation was set to 7.8 cm. Liquid flow was mainly due to capillary pressure gradients induced by changes in volumetric liquid content. However, for dispersivity at saturation set to 0.2 cm, changes in surface tension due to variation in composition induced important liquid flow and convection in the liquid-phase was the most active transport mechanism. When the Kelvin effect was ignored within the soil, the gas-phase diffusion was significantly lower, leading to lower evaporation flux of water and higher volumetric liquid contents near the soil surface.The model was also applied to the simulation of two-dimensional infiltration of aqueous mixtures of methanol from a disk source, its redistribution and volatilization in both homogeneous and heterogeneous soils. Simulations showed that, for homogeneous 2D soils, results and conclusions are analogous to those obtained in 1D. Concentration-dependent viscosity had the major impact on the liquid flow, and acted in the same or opposite direction of surface tension effects, depending on the composition and the magnitude of the concentration gradients developed in the soil. Heterogeneity favored the non-passivity.Finally, in this work a full-range water retention function that takes advantage of the physical consistence of BET adsorption to describe the very dry end, and preserves the capillary behavior of the classical Brooks and Corey function in the wet range is proposed. The transition from capillary to adsorption mechanisms is accounted for by a generalization of the Bradley's isotherm. Tests on seven widely studied soil data sets show that the experimental water retention curves are well fitted by the proposed retention model. The proposed soil-water retention function was evaluated in a water transport model. Simulationswere compared to experimental data for water transport under very dry conditions, found in the literature. The comparison shows that the proposed retention model leads to predictions as good as those resulting from previous full-range soil-water retention functions, while using a physical-based description of the process in the dry region. It is well known that chemical sorption on soils is greatly influenced by relative humidity. Therefore, it is expected that a physically accurate description of the moisture behavior in a very dry soil could also help improve chemical transport and volatilization simulations.
Repositori URV