Geoscience Reference
In-Depth Information
60.00
50.00
L
dis
= 1 cm
40.00
30.00
L
dis
= 5 cm
20.00
L
dis
= 10 cm
10.00
0.00
0
20
40
60
80
100
Time (d)
Figure 5.6
Breakthrough curves for a solute pulse at soil surface (C(0,
t
) = 1 mg L
-1
,
Δ
t
= 1 d), showing the spreading at different values of dispersion length (
L
dis
= 1, 5
and 10 cm).
Question 5.8:
In the case of the soil column of
Figure 5.2
and the solute pulse
C
0
= C(0,
t
) Δ
t
= 1000 (μg d L
-1
), calculate the concentration at the bottom of the soil column at
t
= 40, 50 and 60 d. Check your answer with
Figure 5.6
.
5.5 Transport of Inert, Adsorbing Chemicals
Certain chemicals, although they do not react chemically or biologically in soil,
adsorb to soil solids like clay platelets and organic matter. For these chemicals, the
transport equation (Eq. (
5.8
)) may be written as (assuming homogeneous soil and
steady-state water lux):
ρ
θ
∂
∂
+
∂
C
t
C
t
∂
∂
2
C
z
−
∂
∂
C
z
b
a
l
l
l
∂
=
D
v
(5.13)
e
2
Equation (
5.13
) differs from (
5.9
) only in the left term, which represents the rate of
change of adsorbed amount of solutes. To solve Eq. (
5.13
), we should know the rela-
tion between the adsorbed concentration
C
a
and the dissolved concentration
C
l
. This
relation at equilibrium is called
an adsorption isotherm
.
Figure 5.7
shows different
kinds of isotherm shapes with their analytical expression as found for various com-
pounds in soil. The linear adsorption isotherm may be expressed as:
CSC
a
d
=
(5.14)
where
S
d
is the slope of the isotherm (m
3
kg
-1
), also known as the distribution coef-
icient. If we assume that the linear isotherm is valid at all times in soil (i.e., the
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