Diffusion effects in miscible displacement occurring in saturated and unsaturated porous materials

Abstract

Miscible displacement has been studied in several porous materials, saturated and unsaturated, at different average flow velocities. The tracer appeared at the end of the soil column well in advance of what would have been expected had no mixing occurred at the boundary of the tracer and tracer-free water. Physical differences between porous materials were manifested by changes in shape and position of breakthrough curves due to ionic diffusion. One of the more important physical features was the magnitude of the volume of water not readily displaced at saturation and its increase when the soil was desaturated. Because the total flux of water moving through field soils is generally small, the role of hydrodynamic dispersion and diffusion in transporting dissolved solutes must be included in the theory of most soil-water processes.