Geology Reference
In-Depth Information
the alteration of silicates in the unsaturated zone and even in the saturated
zones of underlying aquifers.
The unsaturated and saturated zones also represent water-rock
interfaces, in which water becomes enriched in minerals.
2 NATURAL CONTAMINANTS
2.1 Dissolution and alteration
Due to slow kinetic reactions, silicate weathering generally produces a
moderate level of mineralization, with reaction times not exceeding a few
months or years. The weathering of certain primary schists, on the other
hand, in the Anti-Atlas (Morocco), containing 10,000 ppm of Cl
-
in their
unweathered state, can provide 4,300 mg·L
-1
of Cl
-
to water.
Carbonate dissolution, controlled by the calco-carbonic equilibrium
(limiting the solubility of calcite and dolomite), does not generate excessive
mineralization either.
Evaporites, on the other hand, (sulfates, chlorides, nitrates, borates),
with a very rapid dissolution reaction and composed of highly soluble
minerals (gypsum CaSO
4
·2H
2
O, anhydrite CaSO
4
, niter or saltpeter KNO
3
,
borax Na
2
B
2
O
7
,
10
H
2
O), are likely to send the ions they are composed of,
into solution as natural contaminants: they can reach concentrations of
several g·L
-1
of SO
4
2-
and several hundreds of g·L
-1
of Cl
-
. In arid and semi-
arid regions (including the Mediterranean regions), these ions make some
surface and groundwater resources unusable. In France, the Triassic and
Oligocene evaporites in Provence and the Jurassian border are examples of
this phenomenon. Concentrations of 10 g·L
-1
can be found at the Eau Salée
borehole in Malaucène (Vaucluse), located on a Triassic diapir.
The evaporation of surface water (sebkhas, salt pans) and soil moisture
in dry tropical climates causes salt concentrations on the surface and in
the soil. For example, salty effl orescences can be seen at the surface of
outcropping aquifers in Camargue.
2.2 Salt water intrusions
Another source of natural contamination is the presence of a fresh water-salt
water interface below ground. Even though they are in static equilibrium,
lenses of fresh water « fl oat » on salt water from seas or estuaries, and
groundwater fl ow can result in the mixing of the two in a heterogeneous
environment (see chap. A9).
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