Karst Aquifers - Hydrogeology: Objectives, Methods, Applications - page 50

C HAPTER A8
Karst Aquifers
At once relief, hydrosystem, and water resource, karst is a particular
environment, defi ned as the group of surface and subterranean features
created by the dissolution of certain rocks, particularly carbonate rocks , a
process called karstifi cation.
Karst is therefore presented as a typical morphology, fi rst defi ned in
the Kras in Slovenia, the formation conditions of which include soluble
lithology and underground drainage. The rock is resistant enough to allow
the preservation of forms created through dissolution.
The hydrogeologic consequence is the existence of a very heterogeneous,
atypical aquifer, which is anisotropic on all scales.
1 KARSTIFICATION
1.1 The phenomenon
The phenomenon of karstifi cation can develop only in rocks composed of
minerals which are soluble in pure water (gypsum) or in water made more
aggressive by the addition of a strong acid (oxidation of metallic sulfurs,
sulfured hydrogen in oilfi elds), or in the great majority of cases, by the
dissociation of a weak acid, carbon dioxide (CO 2 ) dissolved in the water.
The principal sources of CO 2 (Bakalowicz, 1979; Plagnes, 1997) are the
soil (meteoric karsts) or deep infl ows of water (hydrothermal karsts). They
also include the mineralisation of natural organic material (humic and fulvic
acids), which can provide up to 30% of the bicarbonates in water, as well
as polluting anthropogenic organic material, such as the organic carbon in
sewage, in livestock farming effl uents, or in fertilizers.
In order for acidic water to seep into the ground, dissolution must occur
in preexisting discontinuities (stratifi cation joints, diaclases, faults), which
will then allow a subterranean drainage path for water and the ions and
insoluble residues produced by dissolution (clay, silicates) that it carries.