An aquifer is an underground layer of water-bearing permeable rock,
rock fractures or unconsolidated materials (gravel, sand, or silt).
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.
2.1 Saturated versus unsaturated
2.2 Aquifers versus aquitards
2.3 Confined versus unconfined
2.4 Isotropic versus anisotropic
Groundwater in rock formations
4 Human dependence on groundwater
6 Saltwater intrusion
9 See also
11 External links
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
Mountains in North Africa, the
Anti-Lebanon ranges between
Syria and Lebanon, the Jebel Akhdar in Oman, parts of the Sierra
Nevada and neighboring ranges in the United States' Southwest, have
shallow aquifers that are exploited for their water. Overexploitation
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
Libya and Israel, increased water usage
associated with population growth has caused a lowering of the water
table and the subsequent contamination of the groundwater with
saltwater from the sea.
A beach 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 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
Saturated versus unsaturated
Water content and
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
Water content in a capillary fringe decreases
with increasing distance from the phreatic surface. The capillary head
depends on soil pore size. In sandy 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 1.80 m (six feet) but can
range between 0.3 and 10 m (one and 30 ft).
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.
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
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.
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
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
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
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
Deccan Traps (a basaltic 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 Cretaceous)
Chalk 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.
Human dependence on groundwater
Center-pivot irrigated fields in
Kansas covering hundreds of square
miles watered by the Ogallala Aquifer
Most land areas on
Earth 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 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
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
Occasionally, sedimentary or "fossil" aquifers are used to provide
irrigation and drinking water to urban areas. In Libya, for example,
Great Manmade River
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
Libya money over the alternative,
desalination, the aquifers are likely to run dry in 60 to 100
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.
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
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
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.
List of aquifers and Aquifers in the United States
Great Artesian Basin
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
The Guarani Aquifer, located beneath the surface of Argentina, Brazil,
Paraguay, and Uruguay, is one of the world's largest aquifer systems
and is an important source of fresh water. Named after the Guarani
people, it covers 1,200,000 km², with a volume of about
40,000 km³, a thickness of between 50 m and 800 m and a maximum
depth of about 1,800 m.
Aquifer depletion is a problem in some areas, and is especially
critical in northern Africa, for example the Great Manmade River
project of Libya. 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.
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
An example of a significant and sustainable carbonate aquifer is the
Edwards Aquifer in central 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 lakes. The primary risk to this
resource is human development over the recharge areas.
Discontinuous sand bodies at the base of the
McMurray Formation in the
Athabasca Oil Sands
Athabasca Oil Sands region of northeastern Alberta, Canada, are
commonly referred to as the Basal
Sand (BWS) aquifers.
Saturated with water, they are confined beneath impermeable
bitumen-saturated sands that are exploited to recover bitumen for
synthetic crude oil production. Where they are deep-lying and recharge
occurs from underlying
Devonian formations they are saline, and where
they are shallow and recharged by meteoric water they are non-saline.
The BWS typically pose problems for the recovery of bitumen, whether
by open-pit mining or by in situ methods such as steam-assisted
gravity drainage (SAGD), and in some areas they are targets for
Aquifer storage and recovery
Seasonal thermal energy storage - aquifers may be used for storing
heat or cold between opposing seasons and for ecologically
heating/cooling greenhouses, buildings, and district systems
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^ "Huge reserves of freshwater lie beneath the ocean floor".
Gizmag.com. 2013-12-11. Retrieved 2013-12-15.
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M. (2013). "Offshore fresh groundwater reserves as a global
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Land Reclamation and
Improvement (ILRI), Wageningen, The Netherlands. On line :  .
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Land Reclamation and Improvement (ILRI), Wageningen, The
Netherlands. On line :  . Download "WellDrain" software
from :  , or from : 
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Food Crisis of 2011." Foreign Policy
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Groundwater Resources Assessment Centre
SahysMod aquifer model
Arkell Spring Grounds
Laurentian River System
Oak Ridges Moraine
Fox Hills Formation
San Diego Formation
Santa Clara valley aquifer
Snake River Aquifer
Southern Hills Aquifer
Great Artesian Basin
Bas Saharan Basin
Lotikipi Basin Aquifer
Upper Rhine Plain
List of aquifers
Aquifers in the United States
Aquifer storage and recovery
Sole Source Aquifer
Physical aquifer properties used in hydrogeology
Pollution / quality
Ambient standards (USA)
Clean Air Act (USA)
Fossil fuels (peak oil)
Non-timber forest products
Types / location
storage and recovery
Earth Overshoot Day
Renewable / Non-renewable
Agriculture and agronomy