Geology Reference
In-Depth Information
Diagenesis, gravity, and tectonic constraints have created diaclases
within them, transforming poorly porous environments into fractured
environments. In addition to the fractures on a sub-meter scale, one must
also consider all the fractures up to a multi-kilometer scale.
Fractured aquifers develop in igneous rock (granitic and gabbroic
masses, extrusive fl ows), but also in metamorphic (gneiss, mica schist,
pelite) or sedimentary (sandstone, carbonate) rock.
Generally, fractured aquifers can be found in series with poorly soluble
minerals resistant to weathering (silicates). Of particular note are series of
fractured rocks composed of soluble minerals (evaporites, carbonates), as
they will have undergone the process of karstifi cation. Pure water is, indeed,
capable of dissolving a large quantity of halite, and a lesser quantity of
gypsum, and water containing carbon dioxide (CO
2
) is capable of dissolving
calcite, aragonite, and dolomite. In such a situation, the porosity and
permeability of fractures is greatly increased from the original fractured
environment. This increase in permeability can occur only if the rock unit
can eliminate ions in solution, as well as insoluble residue (clay, quartz,
silicates), and if the carbonate minerals do not precipitate in the fractures
that were previously enlarged through dissolution. If this is not the case
(insoluble particles accumulate, carbonate precipitates), evolution of the
karst system will be blocked.
Karstifi cation remains functional when hydraulic gradients are high
enough to maintain a steady fl ow.
These three types of aquifers therefore present different characteristics.
The size of the representative elementary volume (REV), a concept describing
the properties of a medium, varies with the medium's homogeneity.
The size of the REV of the three aquifer types changes in orders of
magnitude: indeed, to statistically represent the average properties of a
water-bearing medium, one must consider a few m
3
of a porous marine
rock, a few tens of m
3
of a porous continental rock, a few hundreds of m
3
of a fractured rock, and a few km
3
of a karstifi ed unit. The size of this last
REV may include the entire hydrosystem.
4 RELATIVE IMPORTANCE BASED ON STRUCTURAL
ASSEMBLAGES
Depending on the nature of the outcropping bedrock, on the aquifers'
lithology, but also on the geodynamic conditions under which they formed,
aquifers of each of the three major types can be found in different geologic
contexts:
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