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dissolved CO 2 increases (increase of HCO 3 - and H + ). This limits
the rise in pH. Also, sulphur is reduced and sulphates do not
form. It is the reverse in oxidizing medium.
The approach that we have seen above is, therefore, too simplistic
although it has instructive value.
13.4.4 Numerical Simulation
For predicting the fate of salts in the soil, therefore, it is indispensable
to resort to numerical simulation. A large number of models exist that
are differentiated by the way they treat (or ignore) transport, cation
exchange, precipitation and dissolution of salts and root absorption.
Reviews have been written (Cheverry 1984; Condom 2000). Some other
models, not included in these bibliographical analyses, deserve to be
pointed out. For example, EVAPOR and associated models (Al Droubi et
al . 1976) or BILION (Job and Cochonneau 1985), PORSAL (Vallès 1988),
EXPRESSO (Rieu et al . 1998; Wang et al . 2007), etc. One will be hard to
put to select the best among them, if there is one!
In regard to precipitation phenomena, the models work step by
step by taking into account the composition progressively acquired
by the solution, activities of ions and solubility products. The species
undissociated in solution CaCO 3 0 , MgCO 0 , CaSO 4 0 , MgSO 4 0 as well as
complex ions ( HSO 4 - , MgHCO 3 - , etc. ) are generally considered. The
corresponding concentrations are far from being negligible.
Figure 13.12 gives an example of a model for following the evolution
in the concentration of a water (Al Droubi et al . 1976).
pH scale
Na +
pH
0
Cl -
-2
HCO -
SO 2-
4
-4
Ca 2+
-6
Mg 2+
-8
Concentration of the water during evaporation
1 time
Fig. 13.12 Modelling of the change during evaporation of the chemical composition of a
water loaded with salts (Al Droubi et al . 1976). For easy reading, many curves corresponding
to several other ions have not been given here.
 
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