Groundwater is the
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
present beneath
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's surface in rock and
soil pore spaces and in the
fracture
Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...
s of
rock formations. About 30 percent of all readily available
fresh water
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...
in the world is groundwater. A unit of rock or an unconsolidated deposit is called an ''
aquifer
An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
'' when it can yield a usable quantity of water. The depth at which
soil
Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
pore spaces or
fracture
Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...
s and voids in rock become completely saturated with water is called the ''
water table''. Groundwater is
recharged from the surface; it may discharge from the surface naturally at
springs and
seeps, and can form
oases or
wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction
wells. The study of the distribution and movement of groundwater is ''
hydrogeology'', also called groundwater
hydrology
Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...
.
Typically, groundwater is thought of as water flowing through shallow aquifers, but, in the technical sense, it can also contain
soil moisture,
permafrost
Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...
(frozen soil), immobile water in very low permeability
bedrock, and deep
geothermal or
oil formation water. Groundwater is hypothesized to provide
lubrication that can possibly influence the movement of
faults. It is likely that much of
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's subsurface contains some water, which may be mixed with other fluids in some instances.
Groundwater is often cheaper, more convenient and less vulnerable to
pollution
Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...
than
surface water. Therefore, it is commonly used for public
drinking water supplies. For example, groundwater provides the largest source of usable water storage in the
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, and California annually withdraws the largest amount of groundwater of all the states. Underground reservoirs contain far more water than the capacity of all surface reservoirs and lakes in the US, including the
Great Lakes
The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes spanning the Canada–United States border. The five lakes are Lake Superior, Superior, Lake Michigan, Michigan, Lake Huron, H ...
. Many municipal water supplies are derived solely from groundwater.
Over 2 billion people rely on it as their primary water source worldwide.
Human use of groundwater causes environmental problems. For example,
polluted groundwater is less visible and more difficult to clean up than pollution in rivers and lakes. Groundwater pollution most often results from improper disposal of wastes on land. Major sources include
industrial and
household chemicals and garbage
landfills, excessive
fertilizers and pesticides used in agriculture, industrial waste lagoons, tailings and process
wastewater from mines, industrial
fracking, oil field brine pits, leaking underground oil storage tanks and pipelines,
sewage sludge and
septic systems. Additionally, groundwater is susceptible to
saltwater intrusion in coastal areas and can cause
land subsidence when extracted unsustainably, leading to
sinking cities (like
Bangkok
Bangkok, officially known in Thai language, Thai as Krung Thep Maha Nakhon and colloquially as Krung Thep, is the capital and most populous city of Thailand. The city occupies in the Chao Phraya River delta in central Thailand and has an estim ...
) and loss in elevation (such as the
multiple meters lost in the Central Valley of California). These issues are made more complicated by
sea level rise
The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
and other
effects of climate change, particularly
those on the water cycle. Earth's
axial tilt has shifted 31 inches because of human groundwater pumping.
Definition
Groundwater is fresh water located in the subsurface
pore space of soil and
rocks. It is also water that is flowing within
aquifer
An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
s below the
water table. Sometimes it is useful to make a distinction between groundwater that is closely associated with
surface water, and deep groundwater in an aquifer (called "
fossil water
Fossil water, fossil groundwater, or paleowater is an ancient body of water that has been contained in some undisturbed space, typically groundwater in an aquifer, for millennia. Other types of fossil water can include subglacial lakes, such as An ...
" if it
infiltrated into the ground millennia ago
).
Role in the water cycle

Groundwater can be thought of in the same terms as
surface water: inputs, outputs and storage. The natural input to groundwater is
infiltration from surface water, which must then
percolate downward to reach the groundwater. The natural outputs from groundwater are
springs and seepage to the oceans. Groundwater storage can be much larger (in volume) compared to its inputs than surface water and have a slower turnover rate, though this depends on the features of the aquifer. This difference makes it easy for humans to use groundwater unsustainably for a long time without severe consequences. Nevertheless, over the long term the average rate of infiltration above a groundwater source plus input from streams is the upper bound for average consumption of water from that source.
Groundwater is naturally replenished by surface water from
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
,
stream
A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...
s, and
river
A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...
s when this
recharge reaches the water table.
Groundwater can be a long-term '
reservoir
A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation.
Reservoirs are created by controlling a watercourse that drains an existing body of wa ...
' of the natural water cycle (with
residence times from days to millennia), as opposed to short-term water reservoirs like the atmosphere and fresh surface water (which have residence times from minutes to years). Deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle.
The
Great Artesian Basin in central and eastern
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
is one of the largest confined aquifer systems in the world, extending for almost 2 million km
2. By analysing the trace elements in water sourced from deep underground,
hydrogeologists have been able to determine that water extracted from these aquifers can be more than 1 million years old.
By comparing the age of groundwater obtained from different parts of the Great Artesian Basin, hydrogeologists have found it increases in age across the basin. Where water recharges the aquifers along the
Eastern Divide, ages are young. As groundwater flows westward across the continent, it increases in age, with the oldest groundwater occurring in the western parts. This means that in order to have travelled almost 1000 km from the source of recharge in 1 million years, the groundwater flowing through the Great Artesian Basin travels at an average rate of about 1 metre per year.
Groundwater recharge
Location in aquifers
Characteristics
Temperature
The high
specific heat capacity of water and the insulating effect of soil and rock can mitigate the effects of climate and maintain groundwater at a relatively steady
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
. In some places where groundwater temperatures are maintained by this effect at about , groundwater can be used for controlling the temperature inside structures at the surface. For example, during hot weather relatively cool groundwater can be pumped through radiators in a home and then returned to the ground in another well. During cold seasons, because it is relatively warm, the water can be used in the same way as a source of heat for
heat pumps that is much more efficient than using air.
Availability
Groundwater makes up about thirty percent of the world's
fresh water
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...
supply, which is about 0.76% of the entire world's water, including oceans and permanent ice. About 99% of the world's liquid fresh water is groundwater.
Global groundwater storage is roughly equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles. This makes it an important resource that can act as a natural storage that can buffer against shortages of
surface water, as in during times of
drought
A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...
.
The volume of groundwater in an aquifer can be estimated by measuring water levels in local wells and by examining geologic records from well-drilling to determine the extent, depth and thickness of water-bearing sediments and rocks. Before an investment is made in production wells, test wells may be drilled to measure the depths at which water is encountered and collect samples of soils, rock and water for laboratory analyses. Pumping tests can be performed in test wells to determine flow characteristics of the aquifer.
The characteristics of aquifers vary with the geology and structure of the substrate and topography in which they occur. In general, the more productive aquifers occur in sedimentary geologic formations. By comparison, weathered and fractured crystalline rocks yield smaller quantities of groundwater in many environments. Unconsolidated to poorly cemented alluvial materials that have accumulated as
valley
A valley is an elongated low area often running between hills or mountains and typically containing a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...
-filling sediments in major river valleys and geologically subsiding structural basins are included among the most productive sources of groundwater.
Fluid flows can be altered in different
lithological settings by brittle deformation of rocks in
fault zones; the mechanisms by which this occurs are the subject of
fault zone hydrogeology.
Uses by humans

Reliance on groundwater will only increase, mainly due to growing water demand by all sectors combined with
increasing variation in rainfall patterns.
[United Nations (2022]
The United Nations World Water Development Report 2022: Groundwater: Making the invisible visible
UNESCO, Paris Text was copied from this source, which is available under a Creative Commons Attribution 3.0 International License Safe use of groundwater varies substantially by the elements present and use-cases, with significant differences between consumption for humans, livestocks and different crops.
Quantities
Groundwater is the most accessed source of freshwater around the world, including as
drinking water,
irrigation, and
manufacturing
Manufacturing is the creation or production of goods with the help of equipment, labor, machines, tools, and chemical or biological processing or formulation. It is the essence of the
secondary sector of the economy. The term may refer ...
. Groundwater accounts for about half of the world's drinking water, 40% of its irrigation water, and a third of water for industrial purposes.
Another estimate stated that globally groundwater accounts for about one third of all
water withdrawals, and surface water for the other two thirds. Groundwater provides drinking water to at least 50% of the global population.
[ Text was copied from this source, which is available under ]
Creative Commons Attribution 4.0 International License
/ref> About 2.5 billion people depend solely on groundwater resources to satisfy their basic daily water needs.
A similar estimate was published in 2021 which stated that "groundwater is estimated to supply between a quarter and a third of the world's annual freshwater withdrawals to meet agricultural, industrial and domestic demands."
Global freshwater withdrawal was probably around 600 km3 per year in 1900 and increased to 3,880 km3 per year in 2017. The rate of increase was especially high (around 3% per year) during the period 1950–1980, partly due to a higher population growth rate, and partly to rapidly increasing groundwater development, particularly for irrigation. The rate of increase is (as per 2022) approximately 1% per year, in tune with the current population growth rate.
Global groundwater depletion has been calculated to be between 100 and 300 km3 per year. This depletion is mainly caused by "expansion of irrigated agriculture in drylands".
The Asia-Pacific region is the largest groundwater abstractor in the world, containing seven out of the ten countries that extract most groundwater (Bangladesh, China, India, Indonesia, Iran, Pakistan and Turkey). These countries alone account for roughly 60% of the world's total groundwater withdrawal.
Drinking water quality aspects
Groundwater may or may not be a safe water source. In fact, there is considerable uncertainty with groundwater in different hydrogeologic contexts: the widespread presence of contaminants such as arsenic, fluoride and salinity
Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
can reduce the suitability of groundwater as a drinking water source. Arsenic and fluoride have been considered as ''priority contaminants'' at a global level, although priority chemicals will vary by country.
There is a lot of heterogeneity of hydrogeologic properties. For this reason, salinity of groundwater is often highly variable over space. This contributes to highly variable groundwater security risks even within a specific region. Salinity in groundwater makes the water unpalatable and unusable and is often the worst in coastal areas, especially due to Saltwater intrusion from excessive use, which are notable in Bangladesh, and East and West India, and many islan nations.
Due to climate change
Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
groundwater is warming. The temperature of Viennese groundwater has increased by .9 degrees Celsius between 2001 and 2010; by 1.4 degrees between 2011 and 2020. In a joint research project scientists at the Karlsruher Institut für Technologie and the University of Vienna
The University of Vienna (, ) is a public university, public research university in Vienna, Austria. Founded by Rudolf IV, Duke of Austria, Duke Rudolph IV in 1365, it is the oldest university in the German-speaking world and among the largest ...
have tried to quantify the amount of drinking water loss to be expected due to ground water warming up to the end of the current century. Stressing the fact that regional shallow groundwater warming patterns vary substantially due to spatial variability in climate change and water table depth these researchers write that we currently lack knowledge about how groundwater responds to surface warming across spatial and temporal scales. Their study shows, however, that following a medium emissions pathway, in 2100 between 77 million and 188 million people are projected to live in areas where groundwater exceeds the highest threshold for drinking water temperatures (DWTs) set by any country.
Water supply for municipal and industrial uses
Municipal 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 groundwater 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. In cases where the groundwater has unacceptable levels of salinity or specific ions, desalination is a common treatment,. However, for the brine, safe disposal or reuse is needed.
Irrigation
In general, the irrigation of 20% of farming land (with various types of water sources) accounts for the production of 40% of food production. Irrigation techniques across the globe includes canals redirecting surface water,[ Peterson 2016] groundwater pumping, and diverting water from dams. Aquifers are critically important in agriculture. Deep aquifers in arid areas have long been water sources for irrigation. A majority of extracted groundwater, 70%, is used for agricultural purposes. Significant investigation has gone into determining safe levels of specific salts present for different agricultural uses.
In India, 65% of the irrigation is from groundwater[PM Launches Rs 6,000 Crore Groundwater Management Plan](_blank)
NDTV, 25 December 2019. and about 90% of extracted groundwater is used for irrigation.
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, seawater desalination, the aquifers are likely to run dry in 60 to 100 years.
In developing countries
Challenges
First, flood mitigation schemes, intended to protect infrastructure built on floodplains, have had the unintended consequence of reducing aquifer recharge
Groundwater recharge or deep drainage or deep percolation is a Hydrology, hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually ...
associated with natural flooding. Second, prolonged depletion of groundwater in extensive aquifers can result in land subsidence, with associated infrastructure damageas well as, third, saline intrusion. Fourth, draining acid sulphate soils, often found in low-lying coastal plains, can result in acidification and pollution of formerly freshwater and estuarine streams.
Overdraft
Groundwater is a highly useful and often abundant resource. Most land areas on Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
have some form of aquifer underlying them, sometimes at significant depths. In some cases, these aquifers are rapidly being depleted by the human population. Such over-use, over-abstraction or overdraft can cause major problems to human users and to the environment. The most evident problem (as far as human groundwater use is concerned) is a lowering of the water table beyond the reach of existing wells. As a consequence, wells must be drilled deeper to reach the groundwater; in some places (e.g., California
California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
, Texas
Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
, and India
India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
) the water table has dropped hundreds of feet because of extensive well pumping. The GRACE satellites have collected data that demonstrates 21 of Earth's 37 major aquifers are undergoing depletion. In the Punjab region of India
India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
, for example, groundwater levels have dropped 10 meters since 1979, and the rate of depletion is accelerating. A lowered water table may, in turn, cause other problems such as groundwater-related subsidence and saltwater intrusion.
Another cause for concern is that groundwater drawdown from over-allocated aquifers has the potential to cause severe damage to both terrestrial and aquatic ecosystemsin some cases very conspicuously but in others quite imperceptibly because of the extended period over which the damage occurs. The importance of groundwater to ecosystems is often overlooked, even by freshwater biologists and ecologists. Groundwaters sustain rivers, wetlands, and lake
A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...
s, as well as subterranean ecosystems within karst
Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...
or alluvial aquifers.
Not all ecosystems need groundwater, of course. Some terrestrial ecosystemsfor example, those of the open desert
A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the la ...
s and similar arid environmentsexist on irregular rainfall and the moisture it delivers to the soil, supplemented by moisture in the air. While there are other terrestrial ecosystems in more hospitable environments where groundwater plays no central role, groundwater is in fact fundamental to many of the world's major ecosystems. Water flows between groundwaters and surface waters. Most rivers, lakes, and wetlands are fed by, and (at other places or times) feed groundwater, to varying degrees. Groundwater feeds soil moisture through percolation, and many terrestrial vegetation communities depend directly on either groundwater or the percolated soil moisture above the aquifer for at least part of each year. Hyporheic zones (the mixing zone of streamwater and groundwater) and riparian zones are examples of ecotones largely or totally dependent on groundwater.
A 2021 study found that of ~39 million investigated groundwater wells 6–20% are at high risk of running dry if local groundwater levels decline by a few meters, oras with many areas and possibly more than half of major aquiferscontinue to decline.
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.
Subsidence
Subsidence occurs when too much water is pumped out from underground, deflating the space below the above-surface, and thus causing the ground to collapse. The result can look like craters on plots of land. This occurs because, in its natural equilibrium state, the hydraulic pressure of groundwater in the pore spaces of the aquifer and the aquitard supports some of the weight of the overlying sediments. When groundwater is removed from aquifers by excessive pumping, pore pressures in the aquifer drop and compression of the aquifer may occur. This compression may be partially recoverable if pressures rebound, but much of it is not. When the aquifer gets compressed, it may cause land subsidence, a drop in the ground surface.
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.
The city of New Orleans, Louisiana
New Orleans (commonly known as NOLA or The Big Easy among other nicknames) is a Consolidated city-county, consolidated city-parish located along the Mississippi River in the U.S. state of Louisiana. With a population of 383,997 at the 2020 ...
is actually below sea level today, and its subsidence is partly caused by removal of groundwater from the various aquifer/aquitard systems beneath it. In the first half of the 20th century, the San Joaquin Valley experienced significant subsidence, in some places up to due to groundwater removal. Cities on river deltas, including Venice in Italy, and Bangkok
Bangkok, officially known in Thai language, Thai as Krung Thep Maha Nakhon and colloquially as Krung Thep, is the capital and most populous city of Thailand. The city occupies in the Chao Phraya River delta in central Thailand and has an estim ...
in Thailand, have experienced surface subsidence; Mexico City, built on a former lake bed, has experienced rates of subsidence of up to per year.
For coastal cities, subsidence can increase the risk of other environmental issues, such as sea level rise
The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
. For example, Bangkok is expected to have 5.138 million people exposed to coastal flooding by 2070 because of these combining factors.
Groundwater becoming saline due to evaporation
If the surface water source is also subject to substantial evaporation, a groundwater source may become saline. This situation can occur naturally under endorheic
An endorheic basin ( ; also endoreic basin and endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water (e.g. rivers and oceans); instead, the water drainage flows into permanent ...
bodies of water, or artificially under irrigated farmland. In coastal areas, human use of a groundwater source may cause the direction of seepage to ocean to reverse which can also cause soil salinization.
As water moves through the landscape, it collects soluble salts, mainly sodium chloride
Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...
. Where such water enters the atmosphere through evapotranspiration, these salts are left behind. In irrigation districts, poor drainage of soils and surface aquifers can result in water tables' coming to the surface in low-lying areas. Major land degradation problems of soil salinity
Soil salinity is the salt (chemistry), salt content in the soil; the process of increasing the salt content is known as salinization (also called salination in American and British English spelling differences, American English). Salts occur nat ...
and waterlogging result, combined with increasing levels of salt in surface waters. As a consequence, major damage has occurred to local economies and environments.
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 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 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.
Seawater 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 sea level
Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
. This relationship is called the Ghyben-Herzberg equation. If too much groundwater 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.
Seawater intrusion is the flow or presence of seawater into coastal aquifers; it is a case of saltwater intrusion. It is a natural phenomenon but can also be caused or worsened by anthropogenic factors, such as sea level rise
The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
due to climate change
Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
. In the case of homogeneous aquifers, seawater intrusion forms a saline wedge below a transition zone to fresh groundwater, flowing seaward on the top. These changes can have other effects on the land above the groundwater. For example, coastal groundwater in California would rise in many aquifers, increasing risks of flooding and runoff challenges.
Sea level rise causes the mixing of sea water into the coastal groundwater, rendering it unusable once it amounts to more than 2–3% of the reservoir. Along an estimated 15% of the US coastline, the majority of local groundwater levels are already below the sea level.
Pollution
Climate change
The impacts of climate change on groundwater may be greatest through its indirect effects on irrigation water demand via increased evapotranspiration. There is an observed declined in groundwater storage in many parts of the world. This is due to more groundwater being used for irrigation activities in agriculture, particularly in drylands. Some of this increase in irrigation can be due to water scarcity issues made worse by effects of climate change on the water cycle. Direct redistribution of water by human activities amounting to ~24,000 km3 per year is about double the global groundwater recharge each year.
Climate change causes changes to the water cycle
The water cycle (or hydrologic cycle or hydrological cycle) is a biogeochemical cycle that involves the continuous movement of water on, above and below the surface of the Earth across different reservoirs. The mass of water on Earth remains fai ...
which in turn affect groundwater in several ways: There can be a decline in groundwater storage, and reduction in groundwater recharge and water quality deterioration due to extreme weather events. In the tropics intense precipitation and flooding events appear to lead to more groundwater recharge.
However, the exact impacts of climate change on groundwater are still under investigation. This is because scientific data derived from groundwater monitoring is still missing, such as changes in space and time, abstraction data and "numerical representations of groundwater recharge processes".
Effects of climate change could have different impacts on groundwater storage: The expected more intense (but fewer) major rainfall events could lead to ''increased'' groundwater recharge in many environments. But more intense drought periods could result in soil drying-out and compaction which would ''reduce'' infiltration to groundwater.
For the higher altitudes regions, the reduced duration and amount of snow may lead to reduced recharge of groundwater in spring. The impacts of receding alpine glaciers on groundwater systems are not well understood.
Global sea level rise
The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
due to climate change has induced seawater intrusion into coastal aquifers around the world, particularly in low-lying areas and small islands. However, groundwater abstraction is usually the main reason for seawater intrusion, rather than sea level rise (see in section on seawater intrusion). Seawater intrusion threatens coastal ecosystems and livelihood resilience. Bangladesh is a vulnerable country for this issue, and mangrove forest of Sundarbans is a vulnerable ecosystem.
Groundwater pollution may also increase indirectly due to climate change: More frequent and intense storms can pollute groundwater by mobilizing contaminants, for example fertilizers, wastewater or human excreta from pit latrines. Droughts reduce river dilution capacities and groundwater levels, increasing the risk of groundwater contamination.
Aquifer systems that are vulnerable to climate change include the following examples (the first four are largely independent of human withdrawals, unlike examples 5 to 8 where the intensity of human groundwater withdrawals plays a key role in amplifying vulnerability to climate change):
# low-relief coastal and deltaic aquifer systems,
# aquifer systems in continental northern latitudes or alpine and polar regions
# aquifers in rapidly expanding low-income cities and large displaced and informal communities
# shallow alluvial aquifers underlying seasonal rivers in drylands,
# intensively pumped aquifer systems for groundwater-fed irrigation in drylands
# intensively pumped aquifers for dryland cities
# intensively pumped coastal aquifers
# low-storage/low-recharge aquifer systems in drylands
Climate change adaptation
Using more groundwater, particularly in Sub-Saharan Africa, is seen as a method for climate change adaptation in the case that climate change causes more intense or frequent droughts.[WaterAid and BGS (2022]
Groundwater: The world's neglected defence against climate change
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Groundwater-based adaptations to climate change exploit distributed groundwater storage and the capacity of aquifer systems to store seasonal or episodic water surpluses. They incur substantially lower evaporative losses than conventional infrastructure, such as surface dams. For example, in tropical Africa, pumping water from groundwater storage can help to improve the climate resilience of water and food supplies.
Climate change mitigation
The development of geothermal energy, a sustainable energy
Energy system, Energy is sustainability, sustainable if it "meets the needs of the present without compromising the ability of future generations to meet their own needs." Definitions of sustainable energy usually look at its effects on the e ...
source, plays an important role in reducing CO2 emissions and thus mitigating climate change. Groundwater is an agent in the storage, movement, and extraction of geothermal energy.
In pioneering nations, such as the Netherlands and Sweden, the ground/groundwater is increasingly seen as just one component (a seasonal source, sink or thermal 'buffer') in district heating and cooling networks.
Deep aquifers can also be used for carbon capture and sequestration, the process of storing carbon to curb accumulation of carbon dioxide in the atmosphere.
Groundwater governance
Groundwater governance processes enable groundwater management, planning and policy implementation. It takes place at multiple scales and geographic levels, including regional and transboundary scales.
Groundwater management is action-oriented, focusing on practical implementation activities and day-to-day operations. Because groundwater is often perceived as a private resource (that is, closely connected to land ownership, and in some jurisdictions treated as privately owned), regulation and top–down governance and management are difficult. Governments need to fully assume their role as resource custodians in view of the common-good aspects of groundwater.
Domestic laws and regulations regulate access to groundwater as well as human activities that impact the quality of groundwater. Legal frameworks also need to include protection of discharge and recharge zones and of the area surrounding water supply wells, as well as sustainable yield norms and abstraction controls, and conjunctive use regulations. In some jurisdictions, groundwater is regulated in conjunction with surface water, including rivers.
By country
Groundwater is an important water resource for the supply of drinking water, especially in arid countries.
The Arab region is one of the most water-scarce in the world and groundwater is the most relied-upon water source in at least 11 of the 22 Arab states. Over-extraction of groundwater in many parts of the region has led to groundwater table declines, especially in highly populated and agricultural areas.
See also
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* Hydrology
Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...
* List of aquifers
* List of aquifers in the United States
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References
External links
USGS Office of Groundwater
IAH, International Association of Hydrogeologists
The Groundwater Project
Online platform for groundwater knowledge
UGPRO
7-year research project on the "Potential of Groundwater for the Poor" (2013–2020)
{{Authority control
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Hydrology
Hydraulic engineering
Soil mechanics
Liquid water
Water and the environment
Water
Lithosphere
Subterranea (geography)