Chapter 6 - Soil-Water Interaction

Abstract

This chapter begins by discussing the main functional groups, the atoms and bonds, that determine the characteristic chemical reactions between molecules and hence, the accumulation, persistence, and fate of compounds in the soil-water system. It continues by providing a detailed exposition of the forces between molecules, which include intermolecular forces, inner-sphere and outer-sphere surface complexes, and the diffuse double layer of clay platelets. The theory of interaction energy based on the diffuse double layer and the Derjaguin, Landau, Verwey, and Overbeek models is presented for the three principal modes of clay particle interaction (face-to-face, edge-to-edge, and face-to-edge). Subsequently, the potential theory as it relates to subsurface water is presented, which includes a mathematical exposition of the gravitational potential, the pressure potential, and the osmotic potential. The discussion on capillary pressure, soil-moisture characteristic curve, and hysteresis provides a solid introduction in the study of flow and contaminant transport in unsaturated soils. The chapter concludes by discussing the basic unsaturated flow equations and the movement of moisture in unsaturated soils that results from thermal gradients.