Karst Aquifers - Hydrogeology: Objectives, Methods, Applications - page 53

2 THE KARST HYDROSYSTEM: SPATIAL
ORGANISATION
2.1 General principles
Unlike a porous aquifer, karst displays a heterogeneous and anisotropic
structure on all scales. The concept of REV does not apply, this
volume corresponding in some cases to the total volume of the karst
hydrosystem.
Spatially, the karst hydrosystem is in opposition between three
permeability structures (Figure 25):
• high-capacity, low-transmissivity micro-fissured blocks, with
a hydraulic behavior similar to that of a coarse-grained porous
medium;
• lower-capacity, high-transmissivity drainages (velocities of tens
to hundred of meters per hour). The geometry of these drainage
networks varies signifi cantly from upper (numerous submillimeter-
scale fractures) to lower (single meter-scale passageway, accessible to
cavers) parts. It is therefore very hierarchical;
• high-capacity annex-to-drain systems, poorly connected to the main
drain. Some of these cavities, located near the principal drainage axis,
may have been connected to it in the past (paleokarsts).
From the upper portions of the basin to the drainage exit, karst appears
as a surface hydrographic network where the small drainages from the
upper regions fl ow together towards the lower, giving greater and greater
drainages in smaller and smaller numbers.
Vertically, karst differentiates itself from other hydrosystems by its
unsaturated zone, with a differentiated structure (Mangin, 1975). From top
to bottom, it can be divided as follows:
• the epikarst, decompressed zone within which dissolution has acted in all
directions to enlarge discontinuities. The epikarst is less anisotropic than
the rest of the unsaturated zone. It is a highly permeable environment,
capable of storing a non-negligible percentage of precipitation. In
Mediterranean karsts, the epikarst corresponds to an AWC of 15 to 30
mm. The superfi cial character of the aquifer that it can contain (epikarst
aquifer) allows the reconcentration of heavy isotopes (oxygen-18,
deuterium) and of rain-derived solutes (halides, alkalis);
• the unsaturated zone in the strictest sense, not very open, cut through
by tectonic accidents. Unsaturated indicates that the entirety of this
zone is not fi lled with water, nevertheless, perennial outfl ows are
observed, as in the European Inter-Disciplinary Underground Science
and Technology Laboratory (i-DUST) in Rustrel (in the underground