An Aquifer is defined as a rock mass, layer or formation which is saturated with groundwater and which by virtue of its properties is capable of yielding the contained water at economical costs when tapped. The quality of an aquifer will, therefore, depend both on how much quantity of water a rock formation can hold per unit volume and at what rate it can yield water when tapped for supplies. It is a storage reservoir and transmission conduit at the same time.
Gravels, limestone and sandstone generally form good aquifer when occurring in suitable geological conditions and geographic situations.
Types of Aquifer
Aquifer are divided into four types on the basis of physical conditions under which water can exist in therm, they are:
(a) Confined Aquifer
A confined aquifer is that in which aquicludes lie both above and below it. The aquiclude restrict the movement of groundwater and as a result in the confined aquifer the groundwater moves under pressure. The water pressure in such an aquifer depends on the difference in height between it and the recharge area. If the difference is enough, the water will readily flow out of a well drilled into it. Such wells are called “artesian wells”. A region supplying water to a confined aquifer is called “recharge area”. In the recharge area, the rain water infiltrates underground through the soil. Naturally, water held in this type of aquifer is not under atmospheric pressure but under a greater pressure due to the confining medium. The upper surface of water in a confined aquifer is called Piezometric surface.
(b) Unconfined Aquifer
It is also called a water table aquifers , and is most common type encountered in the field. In this type, the upper surface of water-table is under atmospheric pressure which may be acting through the interstices in the overlying rocks. Water occurring in this type of aquifer is called Free Groundwater. When tapped through a test well, the free water will rise to a level in the well equivalent to the water table of the area.
(c) Perched Aquifer
It is the term used for isolated water table in an aquifer held by a small extension of impervious rocks within a large pervious tract. In such cases, the main water table is located much below. Supplied from such isolated reservoirs with perched water-table are often unreliable.
(d) Artesian Aquifer
It is a aquifer of such a geometry developed in suitable geological situation so that the piezometric surface is always above the ground level at many places when projected in elevation. When water is tapped from such a aquifer, it reaches out and may rise to the height equivalent to the projected piezometric surface.