The Infiltration of Water Below Ground and the Concept of an Aquifer - Hydrogeology: Objectives, Methods, Applications - page 34

• the presence of oxidating conditions favorable to the persistence of
metallic compounds (iron, manganese) in the form of solid minerals
(oxides, hydroxides);
• the mineralization of humic organic compounds or pollutants (positive
aspect);
• the exposure of the resource to inorganic surface pollution (negative
aspect).
Figure 15 Unconfi ned aquifer and confi ned aquifer.
If the surface of the aquifer is constrained by an impermeable roof
preventing water from reaching the piezometric surface, it is said to be
confi ned. Recharge can then occur only through lateral transit from areas
in which the aquifer outcrops at the surface (unconfi ned zone), or through
weak vertical fl ow passing through the poorly-permeable roof (descending
drainage). The result is an environment more protected from surface
pollution, but also more isolated from sources of oxygenation, which leads
to anoxic conditions favoring the persistence of organic molecules and the
dissolution of undesirable metallic cations (iron and manganese).
In an artesian aquifer, any well or borehole will remain dry until it
reaches the geologic roof of the aquifer. During the construction of a well
in a site where the piezometric surface of the confi ned aquifer is at a higher
elevation than its roof, water will rise in the well if the topographic surface
is at a higher elevation than the piezometric surface (ascendant water).
Water will fl ow out at the surface if the topographic surface is at a lower
elevation (artesian fl ow). The term artesian comes from the Artois region,
in Northern France, where this is a well-known phenomenon.
The impermeability of the aquifer depends on the lithologic nature of
the roof formation: in geologic series, clay layers, if they are not fractured
and if they are thick enough to not be disconnected by faults, constitute
effective impermeable caps.
In alluvial plains, water-bearing Quaternary gravel is often covered by
younger silt layers, deposited by a river with a lower carrying capacity. This
less-permeable silt partially constrains alluvial gravel and sand aquifers,
and gives them a semi-confi ned character.