Aquifer

Aquifers are geologic formations capable of yielding significant amounts of groundwater. The ground-water in most aquifers is stored in the open spaces, known as pores, between solid particles comprising the skeletal framework of the sediment or rock of the aquifer. In some cases, groundwater accumulates in open cracks within rock or, more rarely, in cave-like voids in limestone or dolostone. The definition of an aquifer in terms of its ability to yield significant amounts of water makes the definition relative. Whereas a formation may yield enough water to supply a few households in a rural setting and thereby be classified as an aquifer, the same formation may be incapable of supplying the amount of water needed to support a large city or industrial complex. In the second case the formation would not be considered an aquifer.

The ability of an aquifer to transmit groundwater is known as its hydraulic conductivity or transmissivity (transmissivity is the hydraulic conductivity of an aquifer multiplied by its thickness). Hydrogeologists also use several different measures of the capacity of an aquifer to store and yield water. The simplest of these measures is specific yield, which is the ratio of the volume of water that will flow freely from an aquifer to the total volume of the aquifer. Typical values of specific yield range from 5% to 30%.

Aquifers occurring in layered sediments or sedimentary rocks are in many cases separated by less conductive layers known as aquitards. Aquitards can store large amounts of water but, because of their low hydraulic conductivity, yield water very slowly when pumped. Taken together, a series of aquifers and aquitards comprise an aquifer system. Some older books and reports use the term aquiclude to denote a geologic formation that is impermeable, but that word has fallen out of use.

Aquifers can be classified according to their connection with the atmosphere. Water within an uncon-fined aquifer (sometimes referred to as a phreatic or water-table aquifer) is in direct contact with the atmosphere through open pore spaces of the soil or rock above the water. Water at the top of the saturated portion of an unconfined aquifer, commonly referred to 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 aquitard separates the water within a water-bearing formation from the atmosphere, the aquifer is said to be confined. The aquitard 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, giving rise to artesian conditions. Perched aquifers are unconfined aquifers that occur above aquitards that are underlain by unsaturated soil or rock.

If water is pumped from a confined aquifer system at a rate greater than it can be replenished by infiltration of rain or melted snow, water will be removed from storage in the aquitards. If the amount withdrawn is small, the aquifer system will respond elastically and return to its initial condition when the rate of withdrawal decreases. If the amount withdrawn is large (a condition often referred to as groundwater overdraft), the pore space within the aquifer can permanently collapse and reduce the ability of the aquifer system to store or transmit water in the future even if pumping is curtailed. Permanent collapse of deformation of an aquifer system often gives rise to land subsidence and the development of large cracks known as earth fissures. The term safe yield is often used to refer to the amount of water that may be pumped without significant consequences but, like the definition of an aquifer, the inclusion of significance makes safe yield a relative term.

Aquifers can be susceptible to contamination that may render the water unusable. The susceptibility of an aquifer to contamination depends on whether it is confined or unconfined, its depth, the kind of soil or rock above the aquifer, the kind of sediments or rock comprising the aquifer, the amount of infiltration or recharge to the aquifer, and the topography above the aquifer. Common sources of groundwater contamination include septic systems, leaking gasoline storage tanks, poorly designed landfills or dumps, and illegal disposal of hazardous waste. Aquifers in coastal areas are also susceptible to seawater intrusion if pumping allows buoyant fresh water to be replaced by dense saltwater.

See also Groundwater.

Aquifer

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.

Aquifer

An aquifer is a body of sand or porous rock capable of storing and producing significant quantities of water . An aquifer may be a layer of loose gravel or sand, a layer of porous sandstone , a limestone layer, or even an igneous or metamorphic body of rock. An aquifer may be only a few feet to hundreds of feet thick. Aquifers occur near the surface or buried thousands of feet below the surface. It may have an aerial extent of thousands of square miles or a few acres. The key requirements are that the layer or body has sufficient porosity to store the water, sufficient permeability to transmit the water, and be at least partly below the water table . The water table is the elevation of the top of the completely saturated (phreatic) zone. Above the water table is the vadose or unsaturated zone where the pore spaces are only partially saturated and contain a combination of air and water.

Porosity and permeability are important measures of producibility in aquifers. Porosity is the ratio of the volume of voids in a rock or soil to the total volume. Porosity determines the storage capacity of aquifers. In sand or sedimentary rocks , porosity is the space between grains and the volume of open space (per volume) in fractures. In dense rocks such as granite , porosity is contained largely within the crack and/or fracture system. Permeability is the capacity of a rock for transmitting a fluid, and is a measure of the relative ease with which a fluid can be produced from an aquifer.

A rock that yields large volumes of water at high rates must have many interconnected pore spaces or cracks. A dense, low porosity rock such as granite can be an adequate aquifer only if it contains an extensive enough system of connected fractures and cracks to be permeable. In the shallow subsurface, this is common because nearly all (indurate) rocks are fractured, often heavily. For that reason, caution should be exercised before assuming a low porosity rock will be an aquitard (impermeable body) and not an aquifer.

Fluid pressure, measured in pounds per square inch (psi), in an aquifer depends on whether it is unconfined or confined. An unconfined aquifer is one that is hydraulically open or connected to the surface. Examples would include sand bodies on or near the surface and more deeply buried layers of rock or sand connected to the surface by fractures and/or faults. The fluid pressure in unconfined aquifers is equivalent to what one would measure at a point in a standing body of water and would increase linearly (at a constant rate) with depth. The elevation of the top surface of an unconfined aquifer is free to fluctuate with rainfall.

A confined aquifer is one that is surrounded on all sides by an aquitard, a formation that does not transmit fluid. The pressure in a confined aquifer can be different from that of an unconfined aquifer at the same elevation. A body of sand surrounded on all sides by a soft, impermeable clay or shale serves as a typical example.

See also Hydrogeology; Saturated zone; Water table

aquifer A body of permeable rock (e.g. unconsolidated gravel or a sand stratum) that is capable of storing significant quantities of water, that is underlain by impermeable material, and through which groundwater moves. An unconfined aquifer is one in which the water-table defines the upper water limit. A confined aquifer is sealed above and below by impermeable material. A perched aquifer is an unconfined groundwater body supported by a small impermeable or slowly permeable unit. See artesian water.

aquifer A body of permeable rock, for example, unconsolidated gravel or sand stratum, that is capable of storing significant quantities of water, is underlain by impermeable material, and through which groundwater moves. An unconfined aquifer is one in which the water table defines the upper water limit. A confined aquifer is sealed above and below by impermeable material. A perched aquifer is an unconfined groundwater body supported by a small impermeable or slowly permeable unit. See PERMEABILITY.