Geoscience Reference
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Concentration in liquid phase in soil (mg L
-1
)
0.5
0
1
0
-50
-100
Convection
Convection + dispersion
Solute profiles after 50 days
-150
Figure 5.9
The effect of convection, dispersion and adsorption on the solute concen-
tration proiles after 50 days. The experimental data of
Figure 5.2
apply.
600
R
= 1
400
R
= 2
200
R
= 4
0
0
50
100
150
Time (days)
200
250
300
Figure 5.10
Breakthrough curves at different retardation factors. No decomposition
occurs. The experimental data of
Figure 5.2
apply.
5.6 Reactions of Chemicals in Soil
Many organic chemicals in soil decompose by microbial or chemical reactions.
Although this reaction may depend in a complex manner on temperature, pH, micro-
bial population density, carbon content and chemical history, for optimum conditions
irst-order kinetics in general provide a useful approximation (Hamaker,
1972
). A
chemical amount
M(t)
(g) subject to irst-order decay loses material at a rate propor-
tional to its mass. This loss rate can be expressed mathematically as:
d
d
M
t
=−
µ
M
(5.19)
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