Transport in Nonuniform Media - Transport and Fate of Chemicals in Soils: Principles and Applications - page 301

9
Transport in Nonuniform Media
Soils are defined as heterogeneous systems that are made up of various con-
stituents having distinct physical and chemical properties. A major source of
soil heterogeneity is soil stratifications or layering. The phenomenon of strat-
ifications or layering of the soil profile is an intrinsic part of soil formation
processes and has been documented for several decades by soil survey work.
Another source of soil heterogeneity is due to mixing of different soils or
geological media. Mixing of soils and/or geological media may be the result
of various industrial and agricultural activities. Such activities include site
remediation, construction and land leveling, incorporation of industrial and
municipal waste, manure application and mixing, and crop residue manage-
ment. Therefore, approaches that account for the reactivities in mixed media
are necessary to predict the fate and transport of reactive dissolved chemi-
cals in such media.
9.1 Layered Media
The transport processes of dissolved chemicals in stratified or layered soils
have been studied for several decades by Shamir and Harleman (1967),
Selim, Davidson, and Rao (1977), Bosma and van der Zee (1992), and Wu,
Kool, and Huyakorn (1997), among others. Solute transport in layered soils
can be investigated through numerical methods as well as approximate ana-
lytical solutions. An early analytical method was proposed by Shamir and
Harleman (1967), who used a system's analysis approach. They assumed that
different layers were independent with regard to solute travel time. Each
layer's response served as the boundary condition for the downstream layer
and so on. Later, Selim, Davidson, and Rao (1977) discussed the movement
of reactive solutes through layered soils using finite-difference numerical
methods. They considered both equilibrium and kinetic sorption models of
the linear and nonlinear types. In the late 1980s, Leij and Dane (1989) devel-
oped analytical solutions for the linear sorption-type models using Laplace
transforms. Their solutions were based on the assumption that each layer
was semi-infinite. Bosma and van der Zee (1992) also proposed an approxi-
mate analytical solution for reactive solute transport in layered soils using
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