The Infiltration of Water Below Ground and the Concept of an Aquifer - Hydrogeology: Objectives, Methods, Applications - page 31

C HAPTER A5
The Infiltration of Water Below
Ground and the Concept of
an Aquifer
1 THE MECHANISMS OF INFILTRATION
The term infi ltration is imprecise, as it indicates both the process of water
seeping into the upper soil layer (zone of infi ltration or zone of aeration) and
that of water reaching the water table, the surface of the zone of saturated
or vadose zone. This permanently saturated area is called groundwater.
Porosity is the volume of open space in a material divided by its total
volume.
Role of the soil: interface between the atmosphere and the aquifer, the
soil dampens incoming signals and spreads them out over time. The soil's
infl uence depends on its clay content, the slope, and the nature of the surface.
Within the soil, the infi ltration of water depends on the relationship between
microporosity (slow fl ow) and macroporosity (rapid fl ow).
Upon encountering clayey bedrock, water seeps into a thin layer
of weathered material and circulates at the top of the unaltered clay
(hypodermic fl ow).
In porous terrain, rainwater seeps downward with a more or less even
infi ltration front. For example, in the pozzolanas of the Puys range (center
of France), a 60 mm rainfall will penetrate to a depth of 1 to 2 m, depending
on the local porosity of the material and its permeability.
In very open, fractured regions (tectonic accidents, karstifi ed areas),
downward drainage is rapid.
Water coats the surfaces of solid materials, and then replaces the
entire atmosphere present within the soil (improperly called “air”).
Infi ltrated water can therefore temporarily saturate the porosity of the soil
(Figure 12).