An AQUIFER is an underground layer of water -bearing permeable rock , rock fractures or unconsolidated materials (gravel , sand , or silt ) from which groundwater can be extracted using a water well . The study of water flow in aquifers and the characterization of aquifers is called hydrogeology . Related terms include AQUITARD, which is a bed of low permeability along an aquifer, and AQUICLUDE (or _aquifuge_), which is a solid, impermeable area underlying or overlying an aquifer. If the impermeable area overlies the aquifer, pressure could cause it to become a confined aquifer.
* 1 Depth
* 2 Classification
* 2.1 Saturated versus unsaturated * 2.2 Aquifers versus aquitards * 2.3 Confined versus unconfined * 2.4 Isotropic versus anisotropic
Aquifers may occur at various depths. Those closer to the surface are
not only more likely to be used for water supply and irrigation, but
are also more likely to be topped up by the local rainfall. Many
desert areas have limestone hills or mountains within them or close to
them that can be exploited as groundwater resources. Part of the Atlas
In 2013 large freshwater aquifers were discovered under continental shelves off Australia, China, North America and South Africa. They contain an estimated half a million cubic kilometers of "low salinity" water that could be economically processed into potable water. The reserves formed when ocean levels were lower and rainwater made its way into the ground in land areas that were not submerged until the ice age ended 20,000 years ago. The volume is estimated to be 100x the amount of water extracted from other aquifers since 1900.
The above diagram indicates typical flow directions in a cross-sectional view of a simple confined or unconfined aquifer system. The system shows two aquifers with one aquitard (a confining or impermeable layer) between them, surrounded by the bedrock _aquiclude_, which is in contact with a gaining stream (typical in humid regions). The water table and unsaturated zone are also illustrated. An _aquitard_ is a zone within the earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an _aquiclude_ or _aquifuge_. Aquitards are composed of layers of either clay or non-porous rock with low hydraulic conductivity .
SATURATED VERSUS UNSATURATED
Groundwater can be found at nearly every point in the Earth's shallow subsurface to some degree, although aquifers do not necessarily contain fresh water . The Earth's crust can be divided into two regions: the _saturated zone_ or _phreatic zone_ (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the _unsaturated zone_ (also called the vadose zone ), where there are still pockets of air that contain some water, but can be filled with more water.
SATURATED means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of the water table is the surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0).
UNSATURATED conditions occur above the water table where the pressure
head is negative (absolute pressure can never be negative, but gauge
pressure can) and the water that incompletely fills the pores of the
aquifer material is under suction . The water content in the
unsaturated zone is held in place by surface adhesive forces and it
rises above the water table (the zero-gauge-pressure isobar ) by
capillary action to saturate a small zone above the phreatic surface
(the capillary fringe ) at less than atmospheric pressure. This is
termed tension saturation and is not the same as saturation on a
The capillary rise of water in a small-diameter tube involves the
same physical process. The water table is the level to which water
will rise in a large-diameter pipe (e.g., a well) that goes down into
the aquifer and is open to the atmosphere. See also:
AQUIFERS VERSUS AQUITARDS
Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials).
An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another. A completely impermeable aquitard is called an AQUICLUDE or AQUIFUGE. Aquitards comprise layers of either clay or non-porous rock with low hydraulic conductivity .
In mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium , composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), whereas the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas—sometimes called the pressure area). Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface. See also: Hydraulic conductivity and Storativity
CONFINED VERSUS UNCONFINED
There are two end members in the spectrum of types of aquifers; _confined_ and _unconfined_ (with semi-confined being in between). UNCONFINED aquifers are sometimes also called _water table_ or _phreatic_ aquifers, because their upper boundary is the water table or phreatic surface. (See Biscayne Aquifer .) Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller). Confined aquifers are aquifers that are overlain by a confining layer, often made up of clay. The confining layer might offer some protection from surface contamination.
If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as 10−5), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically then called specific yield ) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable porosity of the aquifer material, or the minimum volumetric water content ). See also: Porosity and Storativity
ISOTROPIC VERSUS ANISOTROPIC
In isotropic aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal (Kh) and vertical (Kv) sense.
Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because the compound Kh and Kv values are different (see hydraulic transmissivity and hydraulic resistance ).
When calculating flow to drains or flow to wells in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty.
GROUNDWATER IN ROCK FORMATIONS
Map of major US aquifers by rock type
Groundwater may exist in _underground rivers_ (e.g., caves where water flows freely underground). This may occur in eroded limestone areas known as karst topography , which make up only a small percentage of Earth's area. More usual is that the pore spaces of rocks in the subsurface are simply saturated with water—like a kitchen sponge—which can be pumped out for agricultural, industrial, or municipal uses.
If a rock unit of low porosity is highly fractured, it can also make
a good aquifer (via fissure flow), provided the rock has a hydraulic
conductivity sufficient to facilitate movement of water.
important, but, _alone_, it does not determine a rock's ability to act
as an aquifer. Areas of the
HUMAN DEPENDENCE ON GROUNDWATER
Most land areas on
Fresh-water aquifers, especially those with limited recharge by snow or rain, also known as meteoric water , can be over-exploited and depending on the local hydrogeology , may draw in non-potable water or saltwater intrusion from hydraulically connected aquifers or surface water bodies. This can be a serious problem, especially in coastal areas and other areas where aquifer pumping is excessive. In some areas, the ground water can become contaminated by arsenic and other mineral poisons.
Aquifers are critically important in human habitation and agriculture. Deep aquifers in arid areas have long been water sources for irrigation (see Ogallala below). Many villages and even large cities draw their water supply from wells in aquifers.
Municipal, irrigation, and industrial water supplies are provided through large wells. Multiple wells for one water supply source are termed "wellfields", which may withdraw water from confined or unconfined aquifers. Using ground water from deep, confined aquifers provides more protection from surface water contamination. Some wells, termed "collector wells," are specifically designed to induce infiltration of surface (usually river) water.
Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface (within a couple of hundred metres) and have some recharge by fresh water. This recharge is typically from rivers or meteoric water (precipitation) that percolates into the aquifer through overlying unsaturated materials.
Occasionally, sedimentary or "fossil" aquifers are used to provide
irrigation and drinking water to urban areas. In Libya, for example,
Great Manmade River project has pumped large
amounts of groundwater from aquifers beneath the Sahara to populous
areas near the coast. Though this has saved
In unconsolidated aquifers, groundwater is produced from pore spaces between particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress from the weight of overlying geologic materials. In severe cases, this compression can be observed on the ground surface as subsidence . Unfortunately, much of the subsidence from groundwater extraction is permanent (elastic rebound is small). Thus, the subsidence is not only permanent, but the compressed aquifer has a permanently reduced capacity to hold water.
Main article: Saltwater intrusion
Aquifers near the coast have a lens of freshwater near the surface and denser seawater under freshwater. Seawater penetrates the aquifer diffusing in from the ocean and is denser than freshwater. For porous (i.e., sandy) aquifers near the coast, the thickness of freshwater atop saltwater is about 40 feet (12 m) for every 1 ft (0.30 m) of freshwater head above sea level. This relationship is called the Ghyben-Herzberg equation. If too much ground water is pumped near the coast, salt-water may intrude into freshwater aquifers causing contamination of potable freshwater supplies. Many coastal aquifers, such as the Biscayne Aquifer near Miami and the New Jersey Coastal Plain aquifer, have problems with saltwater intrusion as a result of overpumping and sea level rise.
Diagram of a water balance of the aquifer
Aquifers in surface irrigated areas in semi-arid zones with reuse of the unavoidable irrigation water losses percolating down into the underground by supplemental irrigation from wells run the risk of salination .
Surface irrigation water normally contains salts in the order of 0.5 g/l or more and the annual irrigation requirement is in the order of 10000 m³/ha or more so the annual import of salt is in the order of 5000 kg/ha or more.
Under the influence of continuous evaporation, the salt concentration of the aquifer water may increase continually and eventually cause an environmental problem.
For salinity control in such a case, annually an amount of drainage water is to be discharged from the aquifer by means of a subsurface drainage system and disposed of through a safe outlet. The drainage system may be _horizontal_ (i.e. using pipes, tile drains or ditches) or _vertical_ (drainage by wells ). To estimate the drainage requirement, the use of a groundwater model with an agro-hydro-salinity component may be instrumental, e.g. SahysMod .
The Great Artesian Basin situated in Australia is arguably the largest groundwater aquifer in the world (over 1.7 million km²). It plays a large part in water supplies for Queensland and remote parts of South Australia.
Guarani Aquifer , located beneath the surface of
The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted by growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contains primarily fossil water from the time of the last glaciation . Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals. According to a 2013 report by research hydrologist Leonard F. Konikow at the United States Geological Survey (USGS), the depletion between 2001–2008, inclusive, is about 32 percent of the cumulative depletion during the entire 20th century (Konikow 2013:22)." In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction. "Cumulative total groundwater depletion in the United States accelerated in the late 1940s and continued at an almost steady linear rate through the end of the century. In addition to widely recognized environmental consequences, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet the Nation’s water needs."
An example of a significant and sustainable carbonate aquifer is the
Edwards Aquifer in central
Discontinuous sand bodies at the base of the
McMurray Formation in
Athabasca Oil Sands region of northeastern
* Book: Aquifers
Aquifer storage and recovery
* ^ "aquitard: Definition from". Answers.com. Archived from the
original on 29 September 2010. Retrieved 2010-09-06.
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Gizmag.com. 2013-12-11. Retrieved 2013-12-15.
* ^ Post, V. E. A.; Groen, J.; Kooi, H.; Person, M.; Ge, S.;
Edmunds, W. M. (2013). "Offshore fresh groundwater reserves as a
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* ^ "Morphological Features of
* ^ ILRI (2000), _Subsurface drainage by (tube)wells: Well spacing
equations for fully and partially penetrating wells in uniform or
layered aquifers with or without anisotropy and entrance resistance_,
9 pp. Principles used in the "WellDrain" model. International