Fossil water or paleowater is an ancient body of water that has been contained in some undisturbed space, typically groundwater in an

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

, for millennia. Other types of fossil water can include

subglacial lake A subglacial lake is a lake that is found under a glacier, typically beneath an ice cap or ice sheet. Subglacial lakes form at the boundary between ice and the underlying bedrock, where gravitational pressure decreases the pressure melting point ...

s, such as Antarctica's

Lake Vostok Lake Vostok (russian: озеро Восток, ''ozero Vostok'') is the largest of Antarctica's almost 400 known subglacial lakes. Lake Vostok is located at the southern Pole of Cold, beneath Russia's Vostok Station under the surface of the cent ...

, and even ancient water on other planets.

UNESCO The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture. It ...

defines ''fossil

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...

'' as Determining the time since water infiltrated usually involves analyzing

isotopic signature An isotopic signature (also isotopic fingerprint) is a ratio of non-radiogenic ' stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample m ...

s. Determining "fossil" status—whether or not that particular water has occupied that particular space since the distant past—involves modeling the flow, recharge, and losses of aquifers, which can involve significant uncertainty. Some aquifers are hundreds of meters deep and underlie vast areas of land. Research techniques in the field are developing quickly and the scientific knowledge base is growing. In the cases of many aquifers, research is lacking or disputed as to the age of the water and the behavior of the water inside the aquifer.

Renewability

Large, prolific aquifers (notably the

Nubian Sandstone Aquifer System The Nubian Sandstone Aquifer System (NSAS) is the world's largest known fossil water aquifer system. It is located underground in the Eastern end of the Sahara desert and spans the political boundaries of four countries in north-eastern Africa. ...

and the

Ogallala Aquifer The Ogallala Aquifer () is a shallow water table aquifer surrounded by sand, silt, clay, and gravel located beneath the Great Plains in the United States. One of the world's largest aquifers, it underlies an area of approximately in portio ...

) containing fossil water are of significant socio-economic value. Fossil water is extracted from these aquifers for many human purposes, notably,

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...

industry Industry may refer to: Economics * Industry (economics), a generally categorized branch of economic activity * Industry (manufacturing), a specific branch of economic activity, typically in factories with machinery * The wider industrial sector ...

, and consumption. In arid regions, some aquifers containing available and usable water receive little to no significant recharge, effectively making groundwater in those aquifers a

non-renewable resource A non-renewable resource (also called a finite resource) is a natural resource that cannot be readily replaced by natural means at a pace quick enough to keep up with consumption. An example is carbon-based fossil fuels. The original organic mat ...

. Extraction rates greater than recharge rates result in lowering of the water table and can lead to

groundwater depletion Overdrafting is the process of extracting groundwater beyond the equilibrium yield of an aquifer. Groundwater is one of the largest sources of fresh water and is found underground. Groundwater depletion is comparable to a bank account in which mor ...

. Extraction of non-renewable groundwater resources is referred to as groundwater "mining" because of their finite nature.

General geology

Aquifers are typically composed of semi-

porous rock Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...

or unconsolidated material whose pore space has been filled with water. In the relatively rare cases of confined aquifers, an impermeable geologic layer (e.g.

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

calcrete Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions, ...

) encloses an aquifer, isolating the water within, sometimes for millennia. More commonly, fossil water is found in arid or semi-arid regions where the climate was significantly more humid in recent geologic history. In some semi-arid regions, the majority of precipitation evaporates before it can infiltrate and result in any significant

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 ...

. Most fossil groundwater has been estimated to have originally infiltrated within the

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

and

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

(10,000–40,000 years ago). Some fossil groundwater is associated with the melting of ice in the time since the

last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...

. Dating of groundwater relies on measuring concentrations of certain stable isotopes, including (

tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...

) and ( "heavy" oxygen), and comparing values with known concentrations of the geologic past. Fossil water can potentially dissolve and absorb a number of ions from its host rock.

Salinity Salinity () is the saltiness or amount of 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 dimensionless and equal ...

in groundwater can be higher than seawater. In some cases, some form of treatment is required to make these waters suitable for human use. Saline fossil aquifers can also store significant quantities of

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

and

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...

Notable bodies of fossil water

Ogallala Aquifer

The Ogallala or High Plains Aquifer sits under 450,000 km² of 8 states of the United States of America. It is one of the largest

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

deposits in the world. The aquifer is composed of unconsolidated

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

deposits. Groundwater in this aquifer has been dated to have been deposited in the humid time following the last glacial maximum. In much of the aquifer's area, an impermeable layer of calcrete prevents precipitation from infiltrating. In other regions of the aquifer, some relatively small rates of recharge have been measured. The aquifer supplies water for the many people who live above it and for widespread agricultural uses. In many areas, the water table has dropped drastically due to heavy extraction. Depletion rates are not stabilizing; in fact, they have been increasing in recent decades. ManMadeRiverLibya-7A

Nubian Sandstone Aquifer System

The Nubian Sandstone Aquifer System is located in northeastern Africa, under the nations of Sudan, Libya, Egypt, and Chad, covering about 2,000,000 km². It is largely composed of many hydraulically interconnected

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

aquifers. Some parts of the system are considered to be confined, if somewhat leaky, due to impermeable layers such as marine shales. The water was deposited between 4,000 and 20,000 years ago, varying by specific locality. The water in the Nubian Sandstone Aquifer System is of high importance to the people living above it, and has been for millennia. In modern times, as demand increases, avoiding rapid depletion and international conflict will depend on careful cross-boundary monitoring and planning. Libya and Egypt are currently planning development projects to withdraw significant amounts of the aquifer's fossil water for use. Other fossil aquifers have been identified throughout Northern Africa as well.

Kalahari Desert aquifers

The

Kalahari Desert The Kalahari Desert is a large semi-arid sandy savanna in Southern Africa extending for , covering much of Botswana, and parts of Namibia and South Africa. It is not to be confused with the Angolan, Namibian, and South African Namib coastal de ...

is in central southern Africa (Botswana, Namibia, and South Africa). Geology of the area includes significant

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

formations. Most of the precipitation in the region evaporates before it can contribute to significant recharge of the aquifers below. Whether or not the region's aquifers receive any significant recharge has long been the subject of debate and research. In the northern region of the Kalahari, a deep aquifer in Cave sandstone was found to have isotopic signatures that suggested it had been confined with little to no leakage for long periods of time.

References

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