Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 117

Several factors determine the ease of infi ltration through a permeable
surface: the topography, the degree of fracturation in rock masses, the
nature and thickness of the weathering mantle, and the development of
an unconsolidated superfi cial cover.
Planar surfaces (plateaus and valley floors) facilitate infiltration,
whereas inclined surfaces (hillslopes) favor runoff.
The fracture density of a rock formation, as well as its degree of
openness, play a primary role in the penetration of surface water into
otherwise intrinsically impermeable rock, in the absence of fractures. The
same can be said for detrital rock lacking a clay matrix or highly degraded
through weathering.
Inversely, superfi cial deposits covering an aquifer can sometime trap
surface water until they are saturated, and only then allow its slow and
delayed infi ltration into the underlying reservoir. This is particularly
common in the most decompressed zones of fractured and karst masses (at
the surface of plateaus and on the edges of reliefs), where the opening of
fractures and their more or less pronounced infi lling by terrigenous material
favor the temporary storage of a suspended superfi cial reservoir, allowing
a delayed infl ow to the deeper reservoir. This suspended aquifer, termed
epikarst aquifer in karstifi ed regions, explains the frequent coexistence in
such regions of large springs at the base of the reservoir and superfi cial
wells dug by humans in the depressions at the top of the plateau.
Over permeable surfaces, infiltration is generally diffuse, and of
variable amount depending on the nature of the environment. It can also
be concentrated and can bring to the reservoir a volume of water collected
over a large catchment area. The amount of water infi ltrated at preferential
points can reach large values and plays an important role in the recharge
of the aquifer. This is the case for swalow-holes in valleys, and, in a more
general sense, for the surface of karst masses, for allocthonous perched
valleys, dry valleys, and closed depressions.
When one has the benefi t of access to results calibrated on experimental
or well-equipped hydrologic systems, or of well-tested experience in
comparable aquifers and in similar situations, one can consider using the
specifi c infi ltration modulus (given in L·s -1 ⋅km -2 or in mm⋅km 2 ), based on
the global relationship between rainfall, the recharge area of the aquifer, and
the pumped discharge. This modulus evaluates the value of subterranean
fl ow as a yearly average over sites particularly infl uenced by infi ltration
(rain and snowmelt). It allows comparisons between different aquifers of
a similar nature, and provisional estimates of theoretical catchment areas
or pumping discharges (Table 10).