Environmental Engineering Reference
In-Depth Information
“Exploded” view of unit
of soil mass
Biologically
mediated transfer
Redox and
biological
transformations
Chemical mass transfer
Sorption—contaminant
partitioning
Dissolution/precipitation—new
solids
Acid/base reactions—proton
transfer
Redox reactions—electron
transfer
Hydrolysis/substitution
Complexation/speciation—
ligand-cation complexes
Electron acceptors
and donors, sorption
sites, ligands, pH,
Eh,......
Attenuated leachate leaving
unit of soil mass
FIGURE 9.19
Elements of basic mass transport and mass transfer in attenuation of contaminants in leachate transport
through a soil element.
is of particular concern, radioactive decay is one of the important chemical transfer pro-
cesses that need to be considered. Finally, biological mediated transfer of contaminants
completes the three main categories of contaminant transfer from porewater to soil solids.
Transformation and degradation of the organic chemical contaminants require as much
attention as sorption and transport of the organic chemicals.
Many of the chemical mass transfer processes are kinetic processes, and reactions are not
instantaneous. However, in a majority of the analytical considerations, most of the reac-
tions are considered as instantaneous and chemical equilibrium is immediately obtained.
This leads to modeling of transport of contaminants as a nonreactive process, an outcome
that is not always appreciated by users of developed models. Consideration of transport
and fate as a reactive process requires incorporation of the many transfer mechanisms that
are time dependent, and also the transformations and degradations of the chemical. This is
not easily accomplished. In the recent attempts to incorporate the reactive processes, incor-
poration of geochemical models into the “standard” transport models has been attempted,
with differing degrees of complexity and success.
9.7.1 Mass Transport
In the conventional treatment of mass transport of contaminants, the three mechanisms
for mass transport include advection, diffusion, and dispersion. Advection refers to the
lux generated by the hydraulic gradient and is given in terms of the advective velocity  v .
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