The ground water is the underground Aquifère that one meets at a shallow depth and who traditionally feeds the well in Drinking water. It is obviously the tablecloth most exposed to the Pollution coming from surface.
The imaginary collective generally represents the ground water like kinds of underground lakes or caves, which is of course completely false. By tablecloth , one understands the saturated part of the ground, i.e. that where the interstices between the solid matter constituents are entirely filled with water, which makes it possible this one to run out. Above, one can find grounds unsaturated, in which the interstices contain also air. This layer is called the unsaturated zone or Zone vadose. It can be enough to a small additional water contribution coming from surface to make rock the layer unsaturated with the state saturated . If the thickness of this section of ground is important, and if the topography of the places lends itself to it, this mechanism can start a Inondation by increase of the ground water. This phenomenon worsened the risings of the Somme in 2001.
The tablecloth is known as free when its level can vary without being blocked by an impermeable layer. If one creates a well in such a tablecloth, the level of water remains unchanged. In the contrary case, one speaks about captive tablecloth '' ''. It is “under pressure” and when that one opens a well there, water rises up to a level of higher balance. It even arrives that the tablecloth spouts out ground; it is the phenomenon of artesianism.
There exist also the tablecloths semi-captive or with drainance. The roof or the substratum (sometimes both) of the aquifer is frequently consisted semipermeable formations. When the hydrodynamic conditions are favorable, there can be exchange of water with the aquifer superimposed or subjacent, it is the phenomenon of drainance.
In France, the largest tablecloth is that of Beauce whose surface is of almost 9000 km ² out of six departments. Its reserves are estimated at nearly 20 billion cubic meters.
A drilling makes it possible to locate the higher level of the tablecloth. It is the level above with which the interstices of the rock are not saturated with water. The variations of this level inform about the degree of filling of the container
- Cycle of water
- alluvial Nappe
- Nappe captivates
- Nappe of subterranean water
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