Agriculture Reference
In-Depth Information
use of such models yields a set of parameters that are only applicable for a
specific reaction time. On the other hand, the multireaction model presented
here provides a comprehensive accounting of the sorption-desorption pro-
cesses, where a single set of parameters is applicable for an entire data set
and for a wide range of initial (or input) concentrations.
In order to describe transport of reactive chemicals in soils and geologi-
cal media, it is necessary to incorporate the multireaction model equations
described above with the CDE described in Chapter 3. For steady water flow
conditions, the resulting CDE equation becomes
2
∂
∂
C
t
+
ρ
θ
∂
∂
S
x
∂
∂
C
x
∂
∂
C
x
=
D
−
v
(5.12)
2
where
x
is distance (cm),
D
is dispersion coefficient (cm
2
h
-1
),
v
(=
q
/θ) is aver-
age pore water velocity (cm h
-1
), and
q
is Darcy's water flux density (cm h
-1
).
The appropriate initial and boundary conditions for a finite soil column are
Cx
()
=
Ct
init
=
0
(5.13)
Sx
()
=
St
init
=
0
(5.14)
vC
tT
∈
∂
∂
C
x
o
p
(5.15)
−
D
+
vC
=
0
tT
x
=
0
p
∂
∂
C
x
=
0
t
>
0
(5.16)
xL
=
where
C
init
is the initial solution concentration (mg L
-1
),
S
init
is the initial
amount of sorption (mg kg
-1
),
C
o
is the input solute concentration (mg L
-1
),
T
p
is the duration of applied solute pulses, and
L
is the length of the column
(cm). The above Equations (5.12 to 5.16) were solved numerically using the
finite-difference, Crank-Nicholson explicit-implicit approximation, which
was discussed in Chapter 3.
5.3 Applications
Multireaction models have been applied successfully to describe the transport
and retention of numerous reactive chemicals in soils and geological media,
including trace elements and heavy metals, radionuclides, military explosives,
herbicides, and pesticides. Selected examples are presented in this section.
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