Geoscience Reference
In-Depth Information
Steady state water flow through soil column with:
water flux q = -1 cm d
-1
water content
θ
= 0.5 cm
3
dry bulk density
ρ
d
= 1 g cm
-3
dispersion length
L
dis
= 1 cm
linear adsorption isotherm
no decay
Pesticide applied at concentration of 1 mg L
-1
during 10 days
Figure 5.2
Solute transport through a soil column with some general data.
laminar low conditions, which is almost always the case in natural conditions,
D
dis
is
proportional to the pore water velocity
v
(=
q
/
θ
) (Bolt,
1979
):
DLv
dis
=
(5.4)
dis
with
L
dis
the dispersion length (m), which depends on the scale over which the water
lux and solute convection are averaged. Typical values of
L
dis
are 0.5-2.0 cm in
packed laboratory columns and 5-20 cm in the ield, although they can be consid-
erably larger in regional groundwater transport (Jury et al.,
1991
). Unless water is
lowing very slowly through repacked soil, the dispersion lux is much larger than the
diffusion lux.
Question 5.3:
Assume a soil with the following conditions:
q
= 2 mm d
-1
,
θ
= 0.25 cm
3
cm
-3
,
L
dis
= 5 cm, and
D
dif
= 0.156 cm
2
d
-1
. What is the percent of the diffusion lux with
respect to the dispersion lux?
The total solute lux
J
(kg m
-2
d
-1
) is the sum of the diffusion, convection and disper-
sion lux:
DD
C
z
∂
∂
JJ
=++=−
J
J
qC
θ
(
+
)
l
(5.5)
dif
con
dis
l
dif
dis
We here illustrate the transport processes for a soil column. Assume a steady-state
water lux with soil physical properties as listed in
Figure 5.2
. A pesticide with a
concentration of 1 mg L
-1
is applied during a period of 10 days.
Figure 5.3
shows the
resulting solute concentrations in the case of only convection and in the case of con-
vection plus dispersion after periods of 10 and 50 days. When dispersion is included,
the solute proile has a shape similar to the normal Gauss distribution. Some solutes
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