aquifer ‘Aquifer’ is the term applied to a geological formation or group of formations with sufficient permeability and water-saturated porosity to transmit and store significant quantities of subsurface water under normal hydraulic gradients. In this context, ‘significant’ is usually related to potential water yield from wells or flow to springs. Aquifers can be considered as reservoirs for groundwater resources and thus are also defined, somewhat imprecisely, on the basis of their economic potential.

Geological materials that are generally considered to be good aquifers include unconsolidated sands and gravels, sedimentary rocks such as sandstones or carbonates with abundant primary pores, and sedimentary rocks containing significant secondary porosity resulting from fractures or dissolution, or both. Certain igneous rocks, such as basalt containing abundant cooling fractures or interconnected vesicles, can also be productive aquifers. Geological materials with lower porosity and permeability, such as fractured granitic rock, would not be classified as aquifers in regions where they are overlain by coarse sediments or sedimentary rocks. Such materials of lower permeability could, however, be considered as aquifers in regions where the only other formations were less-fractured crystalline rocks.

Confining units and confined aquifers

Aquifers are distinguished from adjacent formations with lower permeability or storage potential, which are called confining units. Aquifers containing the top of the saturated zone, or water-table, are called water-table or unconfined aquifers. Aquifers that are overlain by confining units are known as confined aquifers. The water level in a well drilled into a confined aquifer will rise above the top of the aquifer, in some instances even rising above the land surface so that the well flows freely without pumping. The imaginary surface defined by the stable, or static, water levels measured in wells completed in a confined aquifer is called the potentiometric surface. Directions and rates of groundwater flow in a confined aquifer are determined by the slope of the potentiometric surface. Early drilling through a confining unit in the Artois region of France produced flowing wells from the underlying confined aquifer. These flowing wells were subsequently called artesian from Artesium, the Latin name for the Artois region. Although ‘artesian’ is still commonly used to refer to flowing wells, according to modern technical usage the terms ‘artesian’ and ‘confined’ are synonymous; confined aquifers are therefore equivalent to artesian aquifers.

Geological materials that commonly form confining units include fine-grained sediments such as silts and clays, sedimentary rocks of low permeability, such as shales or evaporites, and relatively unfractured igneous or metamorphic rocks. Confining units are further subdivided by some workers into aquicludes and aquitards. In contrast to an aquifer, an aquiclude is not capable of transmitting significant quantities of water. Aquitards retard, but do not prevent, flow of groundwater to or from adjacent aquifers. Both aquicludes and aquitards may contain significant amounts of water in storage. During the exploitation of groundwater in basins containing thick sequences of alternating aquifers and confining units, much of the water removed from storage may ultimately come from draining of the confining units. Removal of water from compressible materials such as silt and clay can lead to compaction of the confining unit and subsidence of the land surface. Confining units such as evaporite deposits that neither transmit nor store significant quantities of water are sometimes called aquifuges.

Perched aquifers

In cases where large variations in permeability and pore size exist in the sediments or rocks of the unsaturated zone, infiltrating water can sometimes accumulate within and above low-permeability layers such as clays, forming a zone of water-saturated pores above the regional water-table. The upper boundary of this saturated zone is called a perched water-table and the saturated zone itself is called a perched aquifer. Perched aquifers may be of limited or large areal extent and they may be stable features or only transient phenomena that develop during periods of intense recharge. Unlike confined or unconfined aquifers, perchd aquifers are rarely reliable and economic reservoirs for water supply. They can, however, be significant to problems of groundwater contamination. Contaminants that infiltrate to a perched aquifer can spread laterally before resuming downward transport to the regional water-table, resulting in a larger area of initial water-table contamination.

Hydrostratigraphic units and regional aquifer systems

Geological formations are frequently divided and combined by stratigraphers into lithostratigraphic or biostratigraphic units on the basis of similarities either in lithological properties or fossil assemblages. The major geological units in a region can also be divided and combined on the basis of similarities in hydrological properties. The resulting units, which can range from low-permeability confining units to highly productive aquifers, are known as hydrostratigraphic units. Important hydrostratigraphic units, some of which extend over very large regions, are commonly given names similar to those assigned to lithostratigraphic units. Examples include the Floridan, a carbonate aquifer in Florida; the Ogalalla aquifer, consisting of sands and gravel that extend from South Dakota to Texas in the United States; and the fissured, highly permeable Chalk aquifer of the United Kingdom.

Patterns of groundwater flow are controlled, in part, by the spatial arrangement of hydrostratigraphic units that make up the regional aquifer system. Early mathematical analyses of regional flow fields were based on assumptions that the aquifer system was essentially homogenous. Later work, facilitated by the advent of computer modelling techniques, demonstrated striking effects on groundwater flow fields caused by contrasts in permeability associated with regional hydrostratigraphy (Fig. 1). A major component of most modern groundwater studies is the identification and characterization of the major hydrostratigraphic units.

J. Bahr

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.

aquifer A body of rock, unconsolidated gravel, or sand stratum that is capable of storing significant quantities of water, that is underlain by impermeable material, and through which ground water 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 ground-water body supported by a small impermeable or slowly permeable unit. See ARTESIAN WATER.

aquifer Rock, often sandstone or limestone, which is capable of both storing and transmitting water owing to its porosity and permeability. Much of the world's human population depends on aquifers for its water supply. They may be directly exploited by sinking wells.

aq·ui·fer / ˈäkwəfər; ˈak-/ • n. a body of permeable rock that can contain or transmit groundwater.

aquifer : see artesian well .

aquifer •chaffer, gaffer, Jaffa, kafir, Staffa •alfalfa, alpha, Balfour, Wadi Halfa •camphor, chamfer •Luftwaffe •laugher, staffer •heifer, zephyr •chafer, trefa, wafer •cockchafer •feoffor, reefer •differ, sniffer •pilfer • titfer • umbellifer • Jennifer •conifer • apocrypha • thurifer •crucifer, Lucifer •Potiphar • aquifer •cipher, encipher, fifer, Haifa, knifer, lifer •coffer, cougher, Offa, offer, proffer, quaffer, scoffer •golfer • phosphor • Forfar • Altdorfer •chauffeur, gofer, goffer, gopher, loafer, Nuku'alofa, Ophir, shofar, sofa •Fraunhofer •hoofer, loofah, opera buffa, roofer, spoofer, tufa, woofer •waterproofer •bluffer, buffer, duffer, puffer, snuffer, suffer •sulphur (US sulfur) • telegrapher •calligrapher, serigrapher •autobiographer, bibliographer, biographer, cartographer, choreographer, cinematographer, crystallographer, geographer, Hagiographa, hagiographer, iconographer, lexicographer, lithographer, oceanographer, palaeographer (US paleographer), photographer, pornographer, radiographer, stenographer, topographer, typographer •philosopher, theosopher •metaphor • Christopher • surfer •Bonhoeffer • windsurfer