Geoscience Reference
In-Depth Information
1) Seaspray
(up to 750 kg salt carried per ha
per year)
2) Submergence by tides and storms
on
shores of lagoons
Freshwater level linked with rain
Supersaline
water
(e.g. 100 g
per litre)
Sea
salts
Saline groundwater
3) Subterranean penetration of seawater
(sandy coasts)
Fig.
13.4
The three types of transport of marine salts on to the continent.
13.2.2 Weathering in Dry Climate
Such weathering is the principal origin of salinization of soils. Sodium
is present in many minerals liable to be dissolved or hydrolysed when
water is present. To give an exhaustive list would be impossible. Some
typical examples (Servant 1978) are:
Sodic plagioclases, for example albite
6SiO
2
,Al
2
O
3
,Na
2
O
Feldspathoids: nepheline (
Na,K
)
AlSiO
4
Amphiboles: riebeckite [
Na
2
]
Fe
2+
Fe
3+
[
Si
8
O
22
](
OH
)
2
Zeolites: analcime
NaAlSi
2
O
6
,H
2
O
and sodalite
Na
4
Al
3
Si
3
O
12
Cl.
These minerals are found in magmatic rocks (3 or 4%) and/or
volcanic rocks. The earth's crust contains 2.8 per cent Na
2
O.
When rainwater washes rocks containing sodium and other bases, the
dissolved cations are equilibrated by anions coming from atmospheric
carbon dioxide (
CO
3
2-
and
HCO
3
-
). Potentially, this gives the carbonates
of K, Na, Mg and Ca in solution. These are salts of a weak acid (H
2
CO
3
)
and strong bases [KOH, Ca(OH)
2
, etc.]. They lead to an alkaline reaction
in solution, that is, high pH.
Salts such as trona (
Na
2
CO
3
,
NaHCO
3
,2
H
2
O
), nahcolite (
NaHCO
3
),
burkeite (2
Na
2
SO
4
,
Na
2
CO
3
), etc., are formed by precipitation.
The concentration of salts takes place mostly in climates with a wet
season (dissolution and transport) as well as a dry season (concentration
of brines). Of course, accumulation occurs mostly in the depressions
on the footslopes (cf. chotts of the Maghreb). But surprisingly, in
some areas the quality of river water is better in summer: thawing
of glaciers upstream and no leaching of saline catchments by rain
(Crosa
et al
. 2006).
