On the solute transport in an aquifer-aquitard system

Abstract

This dissertation is composed of five chapters and three major contributions are presented in Chapter II, III and IV. Chapter I provided a review of studies on solute transport in aquifer-aquitard system. If the aquitard is considered, two categories of methods address the diffusive flux between the aquifer and aquitard: the old method treats the diffusive flux as a volumetric source in the governing equation of the solute transport in the aquifer; the new method treats the aquifer-aquitard boundary as a strict physical boundary with the requirement of continuity of solute concentration and the vertical flux. The new method is adopted throughout this study. In Chapter II, a review of numerical techniques on Inverse Laplace Transform is provided. By careful comparison between several popular algorithms, the multiple precision Stehfest algorithm is chosen as the method to inverse out solutions on solute transport in Laplace domain throughout this dissertation. In Chapter III, solutions were obtained for two dimensional solute transport in an aquifer-aquitard system with a divergent radial flow field, which can treat different types of solute input function and advection, longitudinal and transverse dispersion in the aquifer, vertical diffusion in the aquitard, retardation and radioactive decay in the aquifer and aquitard are taken into account. Mass exchange via diffusion between the aquifer and aquitard are investigated. The effects of hydrologic properties of the aquitard on solute transport are analyzed. Comparisons were made between the results from this study and those from previous studies. The diffusion along the aquifer-aquitard boundary was treated as a volumetric source term, and proved these solutions yield more accurate solute concentration, while those from previous studies tend to overestimate solute concentration in the aquitard, and underestimate the concentration in the aquifer. In Chapter IV, solutions were derived for the transport of radioactive isotopes in an aquifer-aquitard system with regional flow field. This study focused on the effects of different solute transport processes on the results of groundwater age dating using radiometric techniques. Chapter V summarized the remaining problems in this study and directions for future researches.