Geology Reference
In-Depth Information
Several authors noticed a decrease of the occurrence of such water bearing
zones with depth (within the first 100 m below the ground surface) and
attributed it to the "closure" of these tectonic fractures, as a consequence of
the increase of the lithostatic strain. These concepts influenced in the past,
and still presently influence the methodologies used in such areas, for instance
for water well sitting.
The New Concept of Continuous Stratiform Aquifer due to the
Weathering Processes
Most of the areas where metamorphic or plutonic rocks do outcrop are stable
areas, emerged since a long time, that were thus exposed during very long
periods (several tens of millions years) to the weathering processes, under
rather humid climates. The outcropping rocks thus generally comprise a
several tens of metres thick superficial weathered layer, where it has not
been eroded. This superficial layer corresponds to a laterite type weathering
profile (Dewandel et al., 2006).
Figure 2.
Stratiform conceptual model of the structure and the hydrogeological
properties of hard rock aquifers (after Wyns et al., 2004).
From recent results (see for instance Dewandel et al., 2006; Wyns et al.,
2004), a typical weathering profile (Fig. 2) comprises the following layers
that have specific hydrodynamic properties. All together (where and when
saturated with ground water), these various layers constitute a composite
aquifer. From the top to bottom, the layers are the following (Fig. 2):
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The laterite (or iron or bauxitic crust) that can be absent, due to erosion
or rehydration of hematite in a latosol (for iron crusts), or resilicification
of gibbsite/boehmite into kaolinite (for bauxitic crusts).
