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θ, Volumetric Water Content (cm
3
/cm
3
)
0
0.1
0.2
0.3
0.4
0
A Horizon
Measured
B Horizon
30
60
C Horizon
90
Calculated
120
Windsor soil
150
FIGURE 9.10
Measured and predicted gravimetric soil water content with depth 4 d after wastewater appli-
cation for Windsor soil.
parameters were directly based on experimental measurements and sup-
port earlier findings. Zhou and Selim (2001) simulated the breakthroughs of
Ca and Mg using a two-layered model for reactive solutes. The simulation
results are shown in Figure 9.12 (solid and dashed lines for different layering
arrangements) where the reaction mechanism for the Ca-Mg system was
assumed to be governed by simple ion exchange for a binary system:
KSc
Kc
12
T
S
=
+−
(9.12)
1(
1)
12
where
c
is relative concentration (
C
/
C
o
),
K
12
is the selectivity coefficient
(dimensionless), and
S
T
is the cation exchange capacity (CEC). The simula-
tion curves agree with the experimental data, especially for the adsorption
front and also exhibit tailing of the release curve for both Ca and Mg. Such
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