Environmental Engineering Reference
In-Depth Information
to relect the observation that biotic and abiotic chemical reactions in the aqueous phase
have not been factored into the structuring of the relationship. The chemical reactions
discussed in Section 9.4 cannot be ignored since they not only compete with the soil solids
for partitioning of the contaminants, but they may also transform the contaminants—
especially the organic chemicals. The reactions and transformations will not only change
the character of the contaminants, but they will also change the distribution of contaminants
in the zone of interest. The problem is magniied in ield situations because of multispecies
contaminants and a mixture of both inorganic and organic chemicals. Unlike laboratory
leaching column and batch equilibrium tests, real ield situations provide one with a com-
plex mix of contaminants transporting through an equally complex subsoil system.
9.7.2.1 Chemical Reactions and Transport Predictions
To meet the objectives of sustainability of the land environment, proper prediction of
transport and fate of contaminants requires knowledge of how the abiotic and biotic reac-
tions affect the long-term health of the terrain system—especially the subsoil system.
From the myriad of possibilities in handling the complex problem of chemical reactions
and reaction rates, and transformations, there exist at least four simple procedures that
provide some accounting, to a greater or lesser degree, of the various processes control-
ling transport. These include (a) the addition of a reaction term
r
c
in the commonly used
advection-diffusion equation given as Equation 2.2 or Equation 9.8, (b) accounting for the
contaminant adsorption-desorption process, (c) use of irst-, second-, or higher-order reac-
tion rates, and (d) combining transport models with geochemical speciation models. None
of these appear to handle biotransformations and their resultant effect on the transport
and fate processes.
Addition of a reaction term
r
c
to Equation 9.8 is perhaps the most common method used
to accommodate a kinetic approach to fate and transport modeling. The resultant formula-
tion is a linearly additive term to Equation 9.8 as follows:
2
∂
∂
c
t
∂
∂
c
∂
∂
c
x
R
=
D
−
v
+
r
.
(9.9)
L
c
2
x
The last term in Equation 9.9 can be expressed in the form of a general rate law as follows:
ab
r
=−
k
AB
,
(9.10)
c
where
r
c
in this case is the rate of increase in concentration of a contaminant of species
A
,
k
is the rate coeficient, ϑ represents the volume of luid under consideration,
A
and
B
are
the reactant species, and
a
and
b
are the reaction orders.
The use of an adsorption-desorption approach to fate and transport modeling recog-
nizes that in ield situations, desorption (or displacement) occurs as part of the ion exchange
process. Determination of the transport and fate of contaminants using the partitioning
approach typiied by the advection-diffusion relationship with adsorption-desorption
consideration requires that one can write a relationship for
c
* in Equation 9.6. Curve itting
procedures are commonly used to deduce information obtained from batch equilibrium
and/or low-through (leaching column) tests. The Freundlich and Langmuir curves, for
example (see Figure 2.15), are speciic cases of such procedures.
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