An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials
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, 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.
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 Atlas Mountains in North Africa, the Lebanon and Anti-Lebanon
ranges of Syria, Palestine 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. The 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. Classification
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 between them,
surrounded by the bedrock aquiclude, which is in contact with a gaining stream. 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, where all available spaces are filled with water, and the unsaturated
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. The definition of the water table is surface where
the pressure head is equal to atmospheric pressure.
Unsaturated conditions occur above the water table where the pressure head is negative
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 by capillary action to saturate a small zone above the
phreatic surface 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 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 but can range between 0.3 and 10 m. 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 that goes down into the aquifer and is open to the atmosphere.
Basal aquifer Basal aquifer or basal water sands aquifers
describes "the water-bearing sands, gravel or fractured rock that is found at the bottom
of a geological formation, underlying the bitumen-saturated sands." An example is the
McMurray basal water sands aquifer. The McMurray Formation in the Athabaska oil sands in northern
Alberta, consists of "sandstone and shale deposited in a transgressive geological sequence",
resulting in the "course grained texture of the basal deposits". Basal water sands aquifers
occur when the basal sand is low in bitumen. Although the water at this depth may be saline,
portions of the water-saturated McMurray basal water sand aquifer are non-saline.
Aquifers versus aquitards Aquifers are typically saturated regions of
the subsurface that produce an economically feasible quantity of water to a well or spring.
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 comprise layers of either clay or non-porous rock with low hydraulic
conductivity. In mountainous areas, the main aquifers are
typically unconsolidated alluvium, composed of mostly horizontal layers of materials deposited
by water processes, which in cross-section 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, whereas the fine-grained material will make it farther
from the source. Since there are less fine-grained deposits near the source, this is a place
where aquifers are often unconfined, 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. Unconfined aquifers are sometimes also called
water table or phreatic aquifers, because their upper boundary is the water table or
phreatic surface. Typically the shallowest aquifer at a given location is unconfined,
meaning it does not have a confining layer 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.
If the distinction between confined and unconfined is not clear geologically, the value of storativity
returned from an aquifer test can be used to determine it. Confined aquifers have very
low storativity values, 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 greater than 0.01; they release water from
storage by the mechanism of actually draining the pores of the aquifer, releasing relatively
large amounts of water.
Isotropic versus anisotropic In isotropic aquifers or aquifer layers the
hydraulic conductivity is equal for flow in all directions, while in anisotropic conditions
it differs, notably in horizontal and vertical 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.
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 Groundwater may exist in underground rivers.
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, provided
the rock has a hydraulic conductivity sufficient to facilitate movement of water. Porosity
is important, but, alone, it does not determine a rock's ability to act as an aquifer. Areas
of the Deccan Traps 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
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
Most land areas on Earth have some form of aquifer underlying them, sometimes at significant
depths. These aquifers are rapidly being depleted by the human population.
Fresh-water aquifers, especially those with limited recharge by 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 be contaminated by mineral poisons,
such as arsenic - see Arsenic contamination of groundwater.
Aquifers are critically important in human habitation and agriculture. Deep aquifers
in arid areas have long been water sources for irrigation. 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 water.
Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation
are typically close to the ground surface and have some recharge by fresh water. This
recharge is typically from rivers or meteoric water 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,
Muammar Gaddafi's 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 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 subsidence. Unfortunately, much
of the subsidence from groundwater extraction is permanent. 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 aquifers near the coast, the thickness of freshwater atop
saltwater is about 40 feet for every 1 ft 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.
Salination
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
or vertical. To estimate the drainage requirement, the use of a groundwater model with an agro-hydro-salinity
component may be instrumental, e.g. SahysMod. Examples
The Great Artesian Basin situated in Australia is arguably the largest groundwater aquifer
in the world. It plays a large part in water supplies for Queensland and remote parts of
South Australia. 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; see the Great Manmade River project of 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 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, the depletion between 2001–2008, inclusive, is about 32 percent of the cumulative
depletion during the entire 20th century." 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 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. See also
Aquifer storage and recovery Artesian aquifer
Cistern Fossil water
Groundwater model Hydraulic tomography
List of aquifers Overexploitation
Seasonal thermal energy storage - aquifers may be used for storing heat or cold between
opposing seasons, for ecologically heating/cooling greenhouses, buildings large or small, and
district systems. Surficial Aquifer
References
External links Falling Water Tables
Bibliography on Water Resources and International Law Peace Palace Library
IGRAC International Groundwater Resources Assessment Centre
SahysMod aquifer model