Although groundwater exists beneath most land surfaces, it is frequently limited in its availability to human users by local hydrogeologic conditions. Those portions of the water-bearing subsurface that are capable, within their hydrogeologic constraints, of yielding significant amounts of that water are called aquifers. Aquifers can store large amounts of water within pore spaces throughout the rock or sediment. These same voids are the means by which water is transported into and out of the aquifer, and ultimately, to the user. An aquifer might also be known as a groundwater reservoir. By contrast, aquicludes are capable of groundwater storage, but their internal structure is such that movement of the water through the rock is severely limited, making them unsatisfactory for water supply. The term aquitard is applied to a unit of rock that restricts the movement of water through it but to a lesser degree than an aquiclude. In the extreme case, rock that neither transmits nor stores any water is called an aquifuge. This represents a rock that either contains no voids at all or the existing voids have no interconnection, thereby prohibiting both the storage and transmission of water.
The aforementioned terms are used in a relativistic manner and most have no strict definition associated with them. The hydrologic context of the aquifer, i.e., the relative abundance of water, will frequently be the determining factor as to which of the terms are used in defining a particular aquifer. For example, in an arid environment, the lack of a more productive unit might lead one to refer to a restrictive layer as an aquifer, while the same layer in an area of more plentiful groundwater and free-flowing rock types would be classified as an aquitard. This imprecise usage leads many hydrogeologists to define an aquifer as a subsurface zone capable of producing water in sufficient quantity to make it economically useful.
Aquifers can occur in a variety of forms. The classic representation is a uniform sandy horizon with well-sorted sand grains and an ample percentage of void space that permits substantial storage and transport. Aquifers can frequently be found in unconsolidated valley sediments, i.e., sand and gravels through which the water can readily flow. In more dense rocks, such as granite, groundwater might flow readily only through fractures. The most important factor in the classification of an aquifer is the presence of sufficient void volume and the degree to which the openings allow movement of the water.
Aquifers can be further classified on the basis of contact with the atmosphere. Water within an unconfined aquifer is in direct contact with the atmosphere through the open pore spaces of the material that overlies the aquifer. Water at the top of the saturated portion of an unconfined aquifer, known as the water table, is at atmospheric pressure and is free to move vertically in response to water level changes within the aquifer. When an impermeable material, such as a clay layer, separates the water within a water-bearing formation from the atmosphere, the aquifer is known as a confined aquifer. The overlying layer restricts the upward movement of the water within the aquifer and causes the pressure at the top of the aquifer to be at levels greater than atmospheric pressure.
See also Groundwater.