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An aquifer is a water-bearing subsurface zone in which the vertical flow component is so small with respect to the horizontal flow component that it can be neglected.

The groundwater flow in an aquifer is assumed to be predominantly horizontal. The term aquifer is synonymous with water-bearing formation. An aquifer may be porous rock, unconsolidated gravel, fractured rock, or cavernous limestone. Economically important amounts of water may vary from less than a gallon per minute for cattle water in the desert to thousands of gallons per minute for industrial, irrigation, or municipal use.

Aquifers are important reservoirs storing large amounts of water relatively free from evaporation loss or pollution. If the annual withdrawal from an aquifer regularly exceeds the replenishment from rainfall or seepage from streams, the water stored in the aquifer will be depleted. Lowering the pressure in an aquifer by overpumping may cause the aquifer and confining layers of silt or clay to be compressed under the weight of the overburden. The resulting subsidence of the ground surface may cause structural damage to the aquifer and to surface buildings, damage to wells, and other problems.

Depending on the permeability of the layers bordering the aquifer, a distinction is made between: (i) a confined aquifer, (ii) a semi-confined aquifer, (iii) an unconfined aquifer, and (iv) a semi-unconfined aquifer.

A confined aquifer is an aquifer in which groundwater is held under pressures greater than atmospheric pressure by upper and lower confining layers, forcing water to rise in wells to heights above the top of the aquifer (artesian wells; also known as an artesian aquifer). The confined aquifer is typically sandwiched between two impermeable layers of rock or sediments and are recharged only in those areas where the aquifer intersects the land surface. The pressure of the water in confined aquifers is higher than atmospheric pressure, which is why when a well is bored into the aquifer, the water rises up the well tube, to a level higher than the aquifer.

If the water level in an artesian well stands above the land surface, the well is a flowing artesian well. A particular aquifer at one place may be a confined aquifer but in other places may behave as an un-confined aquifer, when the water level falls below the base of the overlying confining layer. In the igneous and metamorphic rocks, groundwater may occur in confined conditions in joints and fractures. In volcanic rocks, the interflow spaces and the vesicular beds form confined aquifers. Examples of flowing artesian wells are found in the hard rock areas of South India.

A semi-confined aquifer is an aquifer that is situated between confining layers with a lower permeability. The upper (and lower) confining layers are semi-pervious, through which vertical leakage takes place due to head difference. These transmit limited quantities of groundwater. The general direction of flow in a semi-confined aquifer is horizontal. The preferred direction of flow in the confining layers above and below is vertical.

An unconfined aquifer is not overlain by any confining layer, but it has a confining layer at its bottom and is recharged by water seeping down from above in the form of rainfall and snow melt. Thus, an unconfined aquifer is a partly saturated aquifer bounded below by an aquiclude and above by the free water table or phreatic surface. At the free water table, the groundwater is at atmospheric pressure. In general, the water level in a well penetrating an unconfined aquifer does not rise above the water table, except when there is vertical flow. The unconfined aquifer is normally partly saturated with water, and the top of the saturated surface is known as the water table, which is under atmospheric pressure – it is also known as a water table/phreatic aquifer. The water level in wells perpetrating this aquifer indicates the position of the water table in the surrounding aquifer.

A semi-unconfined aquifer (also known as leaky aquifer) is an aquifer which exhibits characters in between semi-confined and unconfined aquifers as the permeability of the fine grained overlying layers is more than in a semiconfined aquifer and the horizontal flow component in it cannot be neglected. This type of aquifer is a completely saturated aquifer that is bounded below by an aquiclude and above by an aquitard.

If the overlying aquitard extends to the land surface, the aquifer may be partly saturated, but if the aquifer is overlain by an unconfined aquifer that is bounded above by the water table, the aquifer will be fully saturated. If there is hydrological equilibrium, the piezometric level in a well tapping a leaky aquifer may coincide with the water table. In areas with upward or downward flow (i.e., in discharge or recharge areas) the piezometric level may rise above or fall below the water table.

Finally, a multi-layered aquifer is a succession of leaky aquifers sandwiched between aquitards. Systems of interbedded permeable and less permeable layers are common in deep sedimentary basins.

See also: Aquiclude, Aquitard, Aquifuge.