An aquifer is an underground layer of <a water-bearing <a permeable rock or unconsolidated materials (<a gravel, <a sand, or <a silt) from which <a groundwater can be extracted using a <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.
 Aquifer depth
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. Parts of the sea.
The water table.
The above diagram indicates typical flow directions in a hydraulic conductivity.
 Saturated versus unsaturated
vadose zone), where there are still pockets of air with some water, but can be filled with more water.
Saturated means the pressure head of the water is greater than 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 
The capillary rise of water in a small diameter tube is this 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.
 Aquifers versus aquitards
Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a bedrock often make good aquifer materials).
An aquitard is a zone within the earth that restricts the flow of 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.
 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 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).
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 <a 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 <a 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 <a porosity of the aquifer material, or the minimum volumetric water content).
 Isotropic versus anisotropic
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 resistance).
When calculating  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
pumped out for agricultural, industrial, or municipal uses.
If a rock unit of low Chalk of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with much of its good water-yielding characteristics being due to micro-fracturing and fissuring.
 Human dependence on groundwater
Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths.
Fresh-water aquifers, especially those with limited recharge by Arsenic contamination of groundwater.
Aquifers are critically important in 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 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.
Aquifer depletion has been cited as one of the causes of the food price rises of 2011.
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.
 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.
Aquifers in surface <a irrigated areas in semi-arid zones with reuse of the unavoidable irrigation water losses <a percolating down into the underground by supplemental irrigation from wells run the risk of <a 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.
 Examples of aquifers
The <a 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.
Aquifer depletion is a problem in some areas, and is especially critical in northern Libya for an example. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States.
The glaciation. Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals.
An example of a significant and sustainable carbonate aquifer is the <a Edwards Aquifer in central <a Texas. This carbonate aquifer has historically been providing high quality water for nearly 2 million people, and even today, is full because of tremendous recharge from a number of area streams, rivers and <a lakes. The primary risk to this resource is human development over the recharge areas.
 See also
- Hamza River
- Aquifer storage and recovery
- Artesian aquifer
- Groundwater model
- List of aquifers
- Seasonal thermal energy storage (STES) – aquifers may be used for storing heat or cold between opposing seasons, for ecologically heating/cooling greenhouses, buildings large or small, and district systems.
- “aquitard: Definition from”. Answers.com. Archived from the original on 29 September 2010. http://www.answers.com/topic/aquitard. Retrieved 2010-09-06.
- “Morphological Features of Soil Wetness”. Ces.ncsu.edu. http://www.ces.ncsu.edu/plymouth/programs/vepras.html. Retrieved 2010-09-06.
- Brown, Lester. “The Great Food Crisis of 2011.” <a Foreign Policy Magazine, 10 January 2011.
- “The Great Artesian Basin” (PDF). Facts: Water Series. Queensland Department of Natural Resources and Water. http://www.nrw.qld.gov.au/factsheets/pdf/water/w68.pdf. Retrieved 2007-01-03.
- Falling Water Tables
- Bibliography on Water Resources and International Law Peace Palace Library
- IGRAC International Groundwater Resources Assessment Centre
- SahysMod aquifer model
This article uses material from the Wikipedia article Aquifer, which is released under the Creative Commons Attribution-Share-Alike License 3.0.