Fractured Aquifers - Hydrogeology: Objectives, Methods, Applications - page 47

Due to its low relief, Sub-Saharan Western Africa constitutes a
latitudinal climatic model, with monsoon-type precipitation (winter or
rainy season), increasing from an average of 200 mm in the Sénégal valley
up to 4,000 mm on the coast of the Gulf of Guinea (Liberia). An intense level
of evapotranspiration (over 1,900 mm·year -1 in the Ferlo or around Lake
Chad) modulates this entry signal. The current recharge rate is low (0 to
150 mm·year -1 ), but the inter-annual variability in the hydrologic budget
is very large. In Sub-Saharan Africa, these geotypes are home to a rural
population. The generally low productivity of the wells (1 to 20 m 3 ·day -1 )
is enough to serve the needs of these small communities. The aquifers in
this region seem to have under-used potential.
Drilling is often stopped once the basement rock is reached, but the
major difference with other aquifers is that the principal water stock is
located in the weathered cover rock. One observes a difference between
micaceous metamorphic rocks, which weather into clay and are therefore
a priori not very productive, and granites and gneisses, which weather into
coarse-grained quartz sand that is more capable of storing water.
These shallow aquifers are not costly to exploit, but their storage
capacity is low and their vulnerability high. Their recharge fl ux being
highly variable, fractured aquifers regulate their inter-annual recharge better
than surface waters and their thin alluvium deposits, but less effectively
than porous aquifers, due to their more limited storage capacity and their
discontinuous character.
Due to this discontinuous geometry in fractured aquifers (clay-fi lled
or recrystallized fractures), fl ow laws within them are poorly understood;
prospection in such environments therefore requires probabilistic methods
in choosing drilling locations: remote sensing (Figure 22), emanometry
(radon, CO 2 ), geophysics (geoelectricity, electromagnetism).
These methods detect contrasting parameters, and therefore abnormal
areas, the hydrogeological interpretation of which is possible only after
drilling. Nevertheless, these location guides allow the detection of
hydraulically conductive fractures.
Materials can range from relatively homogeneous, with the presence of
fractures with an equivalent role (Figure 24a), to a double permeability with
small fractures connected by more transmissive accidents (Figure 24b).
The small needs of the concerned human communities, which amount
to a hydrogeology of survival (distance to the one water source for the
village, human labor, parsimonious water use), result in village-scale
projects directly drilling only exploitation wells. On the other hand, urban
development programs drill several reconnaissance wells before equipping
one for exploitation, so that it may then be connected to a mini-distribution
system or to irrigated plots.