Geology Reference
In-Depth Information
erosion, can now be mapped, even at the catchment scale. Thus, this newly
developed approach enables to regionalize hard rock aquifers properties, and
to find numerous practical applications: from the mapping of groundwater
potential on a regional scale, through well siting techniques and methods, to
water resources management at the watershed scale, crucial in areas where
ground water is heavily exploited.
These recent research results and their practical applications are summarized
within the present paper.
STRUCTURE AND HYDRODYNAMIC PROPERTIES
OF HARD ROCK AQUIFERS
The Classical Concept of Discontinuous Aquifer
The "hard rock" aquifers, or "fissured aquifers", that are present near the
surface (within the first 100 m below ground surface) are considered as
"discontinuous aquifers", as a consequence of their discrete hydraulic
conductivity. In fact, during a drilling, the first significant water bearing
zones appear within the fresh (hard) rock. The well intersects an impermeable
rock that is only very locally (along a few centimetres or decimetres) showing
significantly permeable zones. Most of the wells exhibit a few of these water
strikes (from 0 to 4 or 5 water bearing zones).
The classical concept of discontinuous aquifer has been developed during
the seventies, mainly on the basis of the results of the large drilling campaigns
performed in Africa (Detay et al., 1989). It considers that these water bearing
zones are tectonic open fractures (Fig. 1).
Figure 1.
The classical concept of discontinuous aquifer [translation - upto bottom:
superficial and consolidated weathering cover (a few metres), piezometric level,
aquifers in isolated fractures].
