aquifer
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An aquifer is an underground layer of -bearing , rock fractures or unconsolidated materials (, , or ). can be extracted using a water . The study of water flow in aquifers and the characterization of aquifers is called . 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, the pressure of which could create a confined aquifer.


Depth

Aquifers occur from near-surface to deeper than . 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 replenished by 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 in North Africa, the and ranges between Syria and Lebanon, the in Oman, parts of the and neighboring ranges in the , have shallow aquifers that are exploited for their water. can lead to the exceeding of the practical sustained yield; i.e., more water is taken out than can be replenished. Along the coastlines of certain countries, such as and Israel, increased water usage associated with population growth has caused a lowering of the and the subsequent from the sea. A provides a model to help visualize an aquifer. If a hole is dug into the sand, very wet or saturated sand will be located at a shallow depth. This hole is a crude well, the wet sand represents an aquifer, and the level to which the water rises in this hole represents the water table. 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 . 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 ended 20,000 years ago. The volume is estimated to be 100 times the amount of water extracted from other aquifers since 1900.


Classification

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 or non-porous rock with low .


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 . The Earth's crust can be divided into two regions: the '' zone'' or '' zone'' (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the ''unsaturated zone'' (also called the ), 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 (it has a gauge pressure > 0). The definition of the water table is the surface where the 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 in the unsaturated zone is held in place by surface and it rises above the water table (the zero- ) by to saturate a small zone above the phreatic surface (the ) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water-content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In y soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than but can range between . The capillary rise of water in a small- 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.


Aquifers versus aquitards

Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or (e.g., sand and or fractured 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 . In mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated , 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 or phreatic surface. (See .) 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 can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low values (much less than 0.01, and as little as ), 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 ) 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 of the aquifer material, or the minimum volumetric ).


Isotropic versus anisotropic

In aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in 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 and ). When calculating or in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty.


Porous, karst, or fractured

To properly manage an aquifer its properties must be understood. Many properties must be known to predict how an aquifer will respond to rainfall, drought, pumping, and . Where and how much water enters the groundwater from rainfall and snowmelt? How fast and what direction does the groundwater travel? How much water leaves the ground as springs? How much water can be sustainably pumped out? How quickly will a contamination incident reach a well or spring? can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance. Environmental regulations require sites with potential sources of contamination to demonstrate that the has been .


Porous

Porous aquifers typically occur in sand and . Porous aquifer properties depend on the and later natural cementation of the sand grains. The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the and of sandy aquifers. Sandy deposits formed in and in have moderate to high permeability while sandy deposits formed in have low to moderate permeability. Rainfall and snowmelt enter the groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from maps of water levels in wells and springs. s and s can be used with flow equations to determine the ability of a porous aquifer to convey water. Analyzing this type of information over an area gives an indication how much water can be pumped without and how contamination will travel. In porous aquifers groundwater flows as slow seepage in pores between sand grains. A groundwater flow rate of 1 foot per day (0.3 m/d) is considered to be a high rate for porous aquifers, as illustrated by the water slowly seeping from sandstone in the accompanying image to the left. Porosity is important, but, ''alone'', it does not determine a rock's ability to act as an aquifer. Areas of the (a ic lava) in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly, the micro-porous (Upper ) of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring.


Karst

aquifers typically develop in . Surface water containing natural moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging the fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings create a conduit system that drains the aquifer to springs. Characterization of karst aquifers requires field exploration to locate , , and in addition to studying s. Conventional hydrogeologic methods such as aquifer tests and potentiometric mapping are insufficient to characterize the complexity of karst aquifers. These conventional investigation methods need to be supplemented with , measurement of spring discharges, and analysis of water chemistry. U.S. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers. Linear alignment of surface features such as straight stream segments and sinkholes develop along . Locating a well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. Voids in karst aquifers can be large enough to cause destructive collapse or of the ground surface that can create a catastrophic release of contaminants. Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers as shown in the accompanying image to the left. For example, in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3 km/d). The rapid groundwater flow rates make to groundwater contamination than porous aquifers. In the extreme case, groundwater may exist in ''underground rivers'' (e.g., s underlying .


Fractured

If a rock unit of low is highly fractured, it can also make a good aquifer (via flow), provided the rock has a hydraulic conductivity sufficient to facilitate movement of water.


Transboundary aquifer

When an aquifer transcends international boundaries, the term ''transboundary aquifer'' applies. Transboundariness is a concept, a measure and an approach first introduced in 2017. The relevance of this approach is that the physical features of the aquifers become just additional variables among the broad spectrum of considerations of the transboundary nature of an aquifer: * social (population); * economic (groundwater productivity); * political (as transboundary); * available research or data; * water quality and quantity; * other issues governing the agenda (security, trade, immigration and so on). The discussion changes from the traditional question of “is the aquifer transboundary?” to “how transboundary is the aquifer?”. The socio-economic and political contexts effectively overwhelm the aquifer's physical features adding its corresponding geostrategic value (its transboundariness) The criteria proposed by this approach attempt to encapsulate and measure all potential variables that play a role in defining the transboundary nature of an aquifer and its multidimensional boundaries.


Human dependence on groundwater

Most land areas on have some form of aquifer underlying them, sometimes at significant depths. In some cases, these aquifers are rapidly being depleted by the human population. Fresh-water aquifers, especially those with limited recharge by snow or rain, also known as , can be over-exploited and depending on the local , 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 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 aquifers are used to provide irrigation and drinking water to urban areas. In Libya, for example, project has pumped large amounts of groundwater from aquifers beneath the Sahara to populous areas near the coast. Though this has saved Libya money over the alternative, desalination, the aquifers are likely to run dry in 60 to 100 years. Aquifer depletion has been cited as one of the causes of the food price rises of 2011.


Subsidence

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 . 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 for every of freshwater head above . This relationship is called the . 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 near Miami and the New Jersey Coastal Plain aquifer, have problems with saltwater intrusion as a result of overpumping and sea level rise.


Salination

Aquifers in surface areas in semi-arid zones with reuse of the unavoidable irrigation water losses down into the underground by supplemental irrigation from wells run the risk of . Surface irrigation water normally contains salts in the order of or more and the annual irrigation requirement is in the order of or more so the annual import of salt is in the order of or more. Under the influence of continuous evaporation, the salt concentration of the aquifer water may increase continually and eventually cause an problem. For in such a case, annually an amount of drainage water is to be discharged from the aquifer by means of a subsurface and disposed of through a safe outlet. The drainage system may be ''horizontal'' (i.e. using pipes, or ditches) or ''vertical'' (). To estimate the drainage requirement, the use of a with an agro-hydro-salinity component may be instrumental, e.g. .


Examples

The situated in is arguably the largest groundwater aquifer in the world (over ). It plays a large part in water supplies for Queensland, and some remote parts of South Australia. The , located beneath the surface of , , , and , is one of the world's largest aquifer systems and is an important source of . Named after the , it covers , with a volume of about , a thickness of between and a maximum depth of about . Aquifer depletion is a problem in some areas, and is especially critical in northern , for example the project of . 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 of the central United States is one of the world's great aquifers, but in places it is being rapidly by growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contains primarily from the time of the last . 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 (USGS), the depletion between 2001 and 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 in central . 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 s. The primary risk to this resource is human development over the recharge areas. Discontinuous sand bodies at the base of the in the region of northeastern , Canada, are commonly referred to as the . Saturated with water, they are confined beneath impermeable -saturated sands that are exploited to recover bitumen for oil production. Where they are deep-lying and recharge occurs from underlying they are saline, and where they are shallow and recharged by they are non-saline. The BWS typically pose problems for the recovery of bitumen, whether by or by ''in situ'' methods such as (SAGD), and in some areas they are targets for waste-water injection.Barson, D., Bachu, S. and Esslinger, P. 2001. Flow systems in the Mannville Group in the east-central Athabasca area and implications for steam-assisted gravity drainage (SAGD) operations for in situ bitumen production. Bulletin of Canadian Petroleum Geology, vol. 49, no. 3, pp. 376–92.


See also

* * * * * * * * * * * - aquifers may be used for storing heat between opposing seasons and for ecologically heating/cooling greenhouses, buildings, and district systems * * *


References


External links


Falling Water Tables

IGRAC International Groundwater Resources Assessment Centre


{{Authority control Water supply