The Infiltration of Water Below Ground and the Concept of an Aquifer - Hydrogeology: Objectives, Methods, Applications - page 33

(“true” infi ltration). The zero-fl ux plane is defi ned by its maximum water
content; above this plane, the salinity is increased and the heavy isotope
content is enriched. The general characteristic of water circulation during
infi ltration is, with the exception of lithologic heterogeneities, a strong
vertical component. However, the presence of clay lenses can force water
to fl ow laterally before reaching the water table. The downwards transfer
of water translates into a progressive de-saturation of the soil (Figure 14).
While the liquid phase is dominant, water fl ows downward due to gravity
while the gas in the soil is immobilized. The stage at which the gas phase
becomes continuous and is in open exchange with the atmosphere allows
the evaporation of water, and it is the liquid phase which is no longer
mobile.

liquid

watervapor

Figure 14 Progressive desaturation of a soil.

Once infi ltrated, water reaches the water table, molecules follow a
primarily horizontal path towards the outfl ow point.
The term aquifer includes the group of geologic formations bearing
water; it therefore encompasses the zone of infi ltration as well.

2 THE ROLE OF STRUCTURE AND LITHOLOGY

If the water table is located in porous water-bearing formations in vertical
communication with the atmosphere, the aquifer is termed unconfi ned
(Figure 15). The consequences of such an arrangement are:

• the immediate infl ow of water into boreholes or wells as they are
created as soon as they reach the water table or the fi rst water-bearing
discontinuity (stratifi cation joint or fracture);

• the aquifer's ease of recharge;