Geoscience Reference
In-Depth Information
13.2.3 Fossil Deposits (Evaporites)
Besides the two great types of salinity reviewed above, it is also
necessary to mention salinity of geological origin. This corresponds to
deposits accumulated during ancient times with dry climate. In areas
with supersaline water, of which the Dead Sea is a present-day example,
the continental waters carried salts that precipitated at the bottom and
were accumulated over long periods of time. The principal salts found
in these deposits are:
Gypsum (CaSO
4
,2H
2
O). For example, in Spain (Zaragoza region)
and in Syria (Euphrates plain), considerable areas of gypsum
deposits bear soils that are gypsiferous as well as saline (Machín
and Navas 1998; Florea and Al-Joumaa 1998).
Rock salt (NaCl) also called halite; it is often intercalated in marls
or clays; it forms salt domes (diapirs).
Sylvinite (KCl) is very rare as a deposit because it is much more
soluble than NaCl; it is a well-known fertilizer.
Nitrates (NaNO
3
). Very soluble, greatly utilized by
microorganisms, they are not preserved in deposits after
concentration through evaporation, except when rainwater is
totally absent. This is the case in South America, where the
nitrates of the famous Atacama desert, the most arid in the world,
were mined in the nineteenth century. A war, the
Guerra del Salitre
(1879-1883), allowed Chile to seize the principal deposits.
All these compounds are salts of strong acids and strong bases.
Therefore, the water that dissolves them is more or less neutral.
In addition, in many regions of the world, farmers provoke salinization
of lands by irrigating them with water of mediocre quality. We shall
examine this later on.
13.3 PROPERTIES OF SALINE SOILS
13.3.1 Diminution of Structural Stability
The Emerson test enables easy estimation, in 16 classes, of the tendency
of soil aggregates to disperse. It has been used for the slimy soils
extracted from the marshes of western France (Pons and Gerbaud 2005).
For 118 samples, the following curves (Fig. 13.5) were obtained:
It is seen that instability of structure, as estimated by the Emerson
test, increases with the proportion of sodium on the exchange complex.
Increase is faster and the instability greater when the soil is noncalcareous
(less Ca
2+
on the exchange complex).
