Geoscience Reference
In-Depth Information
For example, it is necessary to wait till the volume of chlorinated
water percolating through reaches a volume equal to twice or even
four times the pore volume of the soil sample for the concentration of
chloride at the outlet of the apparatus to become almost equal to that
introduced at the inlet (Condom 2000). Many more volumes of solution
would be necessary if the ion introduced were liable to be fixed by the
solid phase (K
+
, for example).
Another simple model consists of assuming there is perfect and
permanent mixing between the chlorinated water introduced and the
soil solution. In this hypothesis, theoretically, infinite time will be
required to obtain, at the outlet, the concentration of chlorine added at
the inlet!
The reality is, therefore, situated between the extreme hypotheses
mentioned (piston effect and perfect dilution). A model seeks to
formalize the middle path:
C
d
= a
C
soil
+
(
1 - a
)C
i
where
C
d
is the concentration of drainage water,
C
soil
that of the soil
solution and
C
i
that of the irrigation water. This returns to stating that
drainage water is a mixture of irrigation water and soil solution in the
ratio of a:(1 - a). In actual fact, a varies with time and, moreover, the
soil solution is in finite quantity.
All these attempts point out the need for a true modelling procedure
based on appropriate consideration of the essential mechanisms:
Water balance for simulation of all movements of water: inputs,
evapotranspiration, and drainage.
Precipitation or redissolution of salts in the soil when it is
irrigated.
Transport in relation to texture and the size of pores.
13.4.3 Simplified Modelling of Precipitation of Salts
The salts accumulated in the regions affected by confirmed salinization
are very varied.
Table 13.1 lists the salts that are commonly found in soils, waters and
certain mineral deposits.
Ordinary salts
