Geoscience Reference
In-Depth Information
On the one hand, it is proved that the hydraulic conductivity
(permeability) increases with salt content of the solution (increasing
slope of the curves). On the other hand, it is also proved that hydraulic
conductivity is lower at the same salinity of the water when the
proportion of sodium adsorbed on the clay is greater (compare two
points on the same vertical line).
13.3.3 Consequences
So, an attempt at desalting a soil with pure water is liable to disperse
2/1 clay minerals, destroy the aggregates, close the pore space, make
the soil impermeable and, finally, make later elimination of salts and
agricultural use impossible. In such a case, there is nothing else to do
but to abandon to nature a kind of compacted modelling clay totally
impermeable to water. It is also impermeable to air, often reducing, black
and foul smelling (cf. Chap. 12). The hydraulic conductivity drops to a
few mm per month. Ploughing and subsoiling are of no use. If the soil
is dried, a sort of concrete is obtained. Only massive additions of organic
wastes can then improve the situation (Tejada
et al
. 2006).
It is seen that these plastic and swelling properties of swelling
clays saturated with sodium are used in civil engineering.
Bentonite
corresponds to this kind of material; it is used in thin layers to prevent
various types of infiltration.
On the other hand, there is no difficulty in sandy soils: fresh water
percolates and removes the salts without harming the soil.
13.4
DYNAMICS OF SALTS
13.4.1
Spatial Evolution of Salinity
Field studies were conducted in the Camargue (France), that is, the
Mediterranean environment, on natural soils with sparse plant cover.
These studies brought out a change in salinity during the year
(Servant 1975). In this case, the origin of the salts is marine groundwater
(Fig. 13.8).
This is only an example, but it well shows the difficulty in
characterizing salinity that, in a given soil, sometimes changes rapidly
over time. Study of salinity is not worth much if the processes are not
followed over a sufficiently long time!
Role of the seasons
