Geology Reference
In-Depth Information
Table 2:
Main scientific results on hard rock aquifers and corresponding
main hydrogeological applications
Scientific results
Applications
1. Knowledge of the structure of hard rock aquifers and particularly
demonstration of the relationship existing between their permeability
and the weathering processes
they were submitted to in the past. The
hydrodynamic properties of the main permeable layer (the
fissured
horizon
), 50 metres thick approximately, located below the
saprolite
,
are due to the development of weathering-induced fissures (sub-
horizontal in granites) caused by the strain induced in the rock by the
swelling of some minerals (especially biotite).
Hard rock thus comprises two kinds of permeability: a fissure
permeability related to weathering and a fracture permeability related
to tectonic fracturing.
Most of the water wells, particularly those used
for village's hydraulics or irrigation, are tapping the weathering induced
fissures.
These results have been demonstrated in numerous regions of
the world.
1.1.
Mapping of hard rock aquifers, potentialities evaluation at large
areas scale
(several hundreds, thousands, or tens of thousands km
2
),
on the basis of (i) the susceptibility of the various kinds of hard
rocks to the weathering processes (development of an efficient
fissured layer, mainly as a consequence of the rocks' mineralogical
composition), (ii) the state of the weathering profiles erosion
(iii) the depth to the water table, etc. (Fig. 13) (Lachassagne et al.,
2001).
1.2.
Mapping and location of the areas
(a few km
2
to tens of km
2
)
favourable for field hydrogeological prospecting
(before the
implementation of geological or geophysical investigations).
(Lachassagne et al., 2001).
1.3.
Hard rock aquifers vulnerability maps
2. Characterisation of the
hydrodynamic properties of the fissured horizon
(thickness, density and orientation of the various kinds of fractures,
anisotropy, permeability, storage properties, etc.).
Specially designed for hard rock aquifers
pumping tests
(“tool box”)
and interpretation.
(Maréchal et al., 2004; Dewandel et al., 2006)
3. Development of
geophysical methods
(especially PMR
1
) for the
characterisation of the thickness and the hydrodynamic properties
-
especially specific yield - of the main constitutive layers (alterites,
weathered horizon) of hard rock aquifers. (Wyns et al., 2004)
2.1.
Long term forecast of high yield wells
2.2.
Parameters for the deterministic modelling
of hard rock aquifers.
3.1. Evaluation of the
groundwater storage
within the aquifers.
(Fig. 14).
3.2. Evaluation of the
durability of diffuse source pollutions
within the
aquifer.
(Contd.)
