Diffusion in soils

Abstract

Diffusion processes in soil comprise transport of water under unsaturated conditions, transport of gases in liquid and gaseous phase, transport of heat in all the three phases, as well as transport of solutes within liquid phase. The flux per unit cross-sectional area and per unit time is proportional to driving forces of these processes which are the gradients of water content, of gas concentration, of temperature, and of solute concentration, respectively, as well as to the parameter called diffusion coefficient (dimension m2 s1 ), characterizing properties of the medium in which the diffusion process takes place. Combination of these equations with the conservation law gives the formulas of the second Fick’s equation type for all the fluxes with the source/sink term. It should be underlined that under field conditions diffusion processes are usually coupled one with other or with mass flow processes caused by infiltration and lateral flow of water as well as by gas movement due to pressure gradient.