Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 123

on the contrary, highlight both fl agrant and possible anomalies, directing
subsequent research towards the understanding of leakages or towards
additional inputs.
Such exchanges often provide the majority of inputs to superfi cial
aquifers:
￿ in the case of hillside aquifers, where an aquifer at a higher elevation
is partially drained by accumulations of scree and fallen rock or of
fl uvio-glacial sediment covering the basal contact with underlying
impermeable rock (Figure 65a);
￿ in the case of alluvial aquifers, where stream losses and hillside
contributions jointly provide the inputs to the aquifer (Figure 66).
Figure 66 Representative diagram of an alluvial aquifer fed by its banks.
Vertical exchanges can occur as well, when several aquifers are
superimposed, due to rock types variations or tectonic offsets locally
disturbing the continuity of intermediate impermeable horizons
(drainage).
Such arrangements are commonly observed in alluvial aquifers, where
the superfi cial unconfi ned aquifer can receive upwelling inputs from semi-
confi ned or confi ned deeper aquifers (Figure 61).
The same is true in large rock masses, where a large reservoir at a higher
elevation can contribute to the supplying of underlying aquifers thanks to
tectonic discontinuities. For example, in the Dévoluy (north-south subalpine
chains), the great karstifi ed Senonian and Eocene series, the framework of