Aquifers, Aquiclude, Aquifuge and Aquitard, Unconfined Aquifer, Confined Aquifer and Perched Aquifer

Aquifers, Aquiclude, Aquifuge and Aquitard:

A permeable stratum or a geological formation of permeable material, which is capable of yielding appreciable quantities of ground-water under gravity, is known as an Aquifer. In the regions, where ground-water available with great difficulty, even fine-grained materials containing very less quantities of water may be classified as principal aquifers.

           When an aquifer is overlain by a confined bed of impervious material, then this confined bed of overburden is called an Aquiclude. A clay layer is an example of Aquiclude.

            The geological formation, which is, neither porous nor permeable; and hence it neither contains nor yields ground water is called Aquifuge. Granite rock is an example of aquifuge.

            And the geological formation, which does not yield water freely to wells due to its lesser permeability, although seepage is possible through it is called Aquitard. The yield from such a formation is, thus, insignificant. Sandy clay is an example of aquitard.

Generally aquifers fall into three categories, i.e.

1. Unconfined or Non-artesian aquifer
2. Confined or Artesian aquifers; and
3. Perched aquifer.

Unconfined or Non-artesian aquifers:  Unconfined aquifer (also known as water table aquifer) is one in which a free water surface i.e. a water table exists. Only the saturated zone of this aquifer is of importance in ground water studies. Recharge of this aquifer takes place through infiltration of precipitation from the ground surface. A well driven into an unconfined aquifer will indicate a static water level corresponding to the water table level at that location.

Confined or Artesian aquifers:  A confined aquifer is an aquifer which is confined between two impervious beds such as aquicludes. Recharge of this aquifer take place only in the area where it is exposed at the ground surface. The water in the confined aquifer will be under pressure and hence the piezometric level will be much higher than the top level of the aquifer. At some location the piezometric level can attain a level higher than the land surface and a well driven into the aquifer at such a location will flow freely without the aid of any pump. Instances of free flowing wells having as much as a 50m head at the ground surface are reported. A confined aquifer is called a leaky aquifer if either or both of its confining beds are aquitards.

Perched Aquifer: Sometimes a lens or localized patch of impervious stratum can occur inside an unconfined aquifer in a way that it retains a water table above the general water table. Such a water table retained around the impervious material is known as perched water table. This type of aquifer is known as perched aquifer. Usually the perched water table is of limited extent and the yield from such a situation is very small. In ground water exploration a perched water table is quite often confused with a general water table.