Water table
Introduction
A water table or subterranean layer is an accumulation of groundwater that is located at a relatively small depth below ground level. More precisely, it is a relatively shallow aquifer, since aquifers can also be at greater depths.
Wells and water sources, whether potable or not, are fed by them. They are the aquifers most exposed to contamination from the surface.
A water table is usually limited by two surfaces. The lower one is usually a stratum of impermeable terrain at a greater or lesser depth. Above there is a saturated zone, the water table itself, whose upper limit may or may not be an impermeable stratum. This limit is called the phreatic level.[1] If the land above that level is permeable, it will normally be an unsaturated zone.
Phreatic layer refers to the part of the soil saturated with water, that is, the part in which the spaces between the grains of soil are completely filled with water. If the stratum above is not impermeable, there will be unsaturated soils, whose interstices contain, in addition to water, also air.
A supplementary and reduced contribution of water may be sufficient to convert the unsaturated layer to the saturated layer, that is, to raise the water table. And, if the thickness of this unsaturated layer was originally small (shallow water table) and the topography of the place lends itself to it, the water would rise to the surface, in the form of a pond, lagoon or lake, and may even trigger a flood.
Types of water tables
• - A layer is said to be free when its upper level can vary without being constrained by an upper substrate of impermeable terrain. Drilling a well on this type of layer will not influence the water table, which will only vary for other reasons.
• - Otherwise it would be called a confined layer. The water will be under pressure and if a well is drilled, the water rises to its equilibrium level, which will be the one at which the pressure of the aquifer equals the atmospheric pressure. If this balance occurs above ground level, it is called an artesian well.
• - There is also another type of layer, confined, but without any contact with the outside and which has not been recharged since very distant times, and it is called .[2].