Reactive solute transport in heterogeneous porous media: Cadmium leaching in acid sandy soils

This thesis describes the transport mechanisms of cadmium in heterogeneous sandy soils, at different spatial scales. Sorption of cadmium in selected soil horizons of an acid sandy soil (i.e., Spodosol) was described best using an extended Freundlich isotherm of which the sorption constant is related to the organic matter content, pH and the calcium activity of the soil solution. Displacement experiments with a 111Cd-isotope in 10 cm columns filled with undisturbed soil horizons showed asymmetric breakthrough, characterized by a sharp adsorption front and a wide desorption front.; The influence of soil morphology on solute transport was investigated for two distinctly different Spodosols, i.e., one developed under dry conditions (relatively deep groundwater table), and another developed under wet conditions (shallow groundwater table). Measurements of resident concentrations in 1-m-long soil profiles with a network of 36 Time-Domain-Reflectometry probes showed that the Bh horizon in the dry Spodosol act as a redistribution layer between heterogeneous transport in the topsoil layers and homogeneous transport in the subsoil. The transport in the dry Spodosol could be adequately described using the classical convection-dispersion equation, whereas a stochastic-convective transport model described the transport behavior in the wet Spodosol best.; The results of the migration experiments with cadmium in the lysimeters confirmed the relationship between the mass transfer coefficient and pore water velocity, which was already found in the small columns. This demonstrates that transfer of cadmium from the solid soil phase to the soil water is rather limited by diffusion than by pure reaction kinetics.; A Monte-Carlo sensitivity analysis showed that variations in cadmium flux concentrations were not affected by variations in retention curve parameters. Variations in deposition rate and in sorption parameters considerable affected variations in cadmium flux to groundwater. Variations in longitudinal dispersivity affected cadmium flux at the initial stages and at the end of the contamination. The results showed that the cadmium profile was reconstructed better using a nonlinear Freundlich sorption model as compared to the linear model. Cadmium flux model predictions showed that a considerable fraction of cadmium deposited was still present in the soil and is leaching to groundwater. The leaching occurs over hundreds of years which means that it remains a possible threat for the ages to come. (Abstract shortened by UMI.)...