Karst Aquifers - Hydrogeology: Objectives, Methods, Applications - page 51

Karstifi cation is therefore an evolving process, which increases the
permeability of the rock mass as long as the rock remains capable of
evacuating its insolubles.
The speed of this evolution is a function of the state of undersaturation
of the solution relative to the minerals composing the surrounding rock,
but, above all else, the solubility of these minerals and the fl ux of water
carrying the ions going into solution (effective rainfall).
As a result, the evolution of karstifi cation in gypsum is visible on the
yearly scale, while the karstifi cation of carbonates is visible on the century
scale.
The spectacular aspect of karst is its surface and subterranean
morphology (Figure 25), with the accompanying hydrogeologic
consequences:
• differentiation between zones of diffuse (bare or covered karren),
locally favoured (sinkholes), or concentrated (ponors or swallow
holes) infi ltration;
• hydraulic equilibrium with a high piezometric variation between
surface water and karst water table (poljes, estavelles, overfl ows) due
to the saturation of conduits;
• small number of springs with a high discharge, fl owing out from
conduits draining a vast watershed.
karrenfi eld
swallow hole
Figure 25 Karst model and geologic functioning.
1.2 Polyphase system
Karstifi cation begins as soon as carbonate rocks are subjected to the erosive
action of fresh water. For example, it affects coral reef fl ats as soon as they