Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 67

coordinates, respectively. In the figure, r , ψ, and z and ρ, ψ, and Φ represent
the coordinate systems where r , ρ, and z are distances and the angles ψ and
Φ are known as the azimuth and polar angles, respectively, The continuity
equation in these coordinates is conveniently used in chemical and other
engineering applications. In porous media, spherical coordinate systems are
used in describing solute transport in ideal systems composed of spherical
porous aggregates.

3.2 Transport Mechanisms
It is commonly accepted that there are two types of mechanisms that govern
the transport of dissolved chemicals in soils or geological media. The first
type of mechanism is an active type, which occurs regardless of whether
there is water flow in the soil system. The second mechanism is a passive
one and is applicable only when there is water flow. Diffusion is an active
mechanism and hydrodynamic dispersion and mass transport are passive
mechanisms. As discussed below, dispersion is a unique feature for porous
media.

3.2.1 Mass Flow

This is often called convection and simply refers to the passive transport of a
dissolved chemical with the fluid or water. That is intuitive and states that a
solute moves along with the water through the soil. This mechanism is also
referred to as advection or piston flow. Thus, a solute species in the soil solu-
tion and the water move together at the same flow rate:

U = q C

(3.13)

c

z

where U c is the net transport of solute per unit cross-sectional area and per
unit time (Μg cm -2 h -1 ), q z is the water flux density (cm 3 cm -2 h -1 ) or simply
the water flow velocity in the z direction. This flow velocity q z is referred to
as Darcy's flux (cm h -1 ) and is discussed in subsequent sections.

3.2.2 Molecular Diffusion

Diffusion of ions or molecules takes place due to the random thermal motion
of molecules in solution. Diffusion is an active process regardless of whether
there is net water flow in the soil system. Diffusion results in a net trans-
fer of molecules from regions of higher to lower concentrations. A com-
mon description of the diffusion is Fick's law of diffusion, that solute flux