Mobile, Immobile, and Multiflow Domain Approaches - Transport and Fate of Chemicals in Soils: Principles and Applications - page 257

8
Mobile, Immobile, and Multiflow
Domain Ap proaches
Nonequilibrium transport of solute in soils and geological media is com-
monly attributed to two main processes: (1) the rate-limited mass transfer of
aqueous species to or from the regions with limited or no advective flow; and
(2) the time-dependent chemical reaction at the surfaces of solid materials.
The first process is often referred to as transport-related nonequilibrium or
physical nonequilibrium and the second process is referred to as reaction-
related nonequilibrium or chemical nonequilibrium. As a result of pedogenic
processes, soils and geological media are often characterized by extensive
chemical and physical heterogeneities. Soil heterogeneity has a profound
effect on the transport of chemicals in the soil profile. The physical heteroge-
neity is mainly attributed to the highly nonuniform pore space as a result of
the soil aggregates, soil layers, as well as cavities developed from natural and
anthropogenic activities. The highly irregular flow field resulting from the
spatial variance of pore sizes and connectivity is a major driving force of the
nonequilibrium transport of solutes in soils (Biggar and Nielsen, 1976; Dagan,
1984). Conceptually, the nonequilibrium transport of nonreactive solutes in
structured soils can be described as a rapid movement of water flow and sol-
utes within preferential flow paths and a diffusive mass transfer of solutes to
a stagnant region. This mobile-immobile dual-region concept is the founda-
tion of the mathematical models used for simulating the physical nonequilib-
rium transport (van Genuchten, 1981; Reedy et al., 1996; Simunek et al., 2003).
8.1 Matrix Diffusion
In general, the nonequilibrium mass transfer of solutes in soils proceeds in
two stages: (1) external or film diffusion from flow water to the solid inter-
face; and (2) internal diffusion through the porous network of soil aggregates
(Brusseau, 1993). Film diffusion is of importance only at low low velocities.
Under environmental conditions, diffusive mass transfer in the micropores
inside soil aggregates is an important process influencing the mobility of
reactive metals in soils. The intra-aggregate diffusion serves as a reservoir in
the transport of contaminants by spreading them from flowing water to the
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