The 37 Great Aquifer Systems of Earth: Solution to Global Drought

Water exists in huge quantities below the surface of the earth. This subsurface water may or may not completely saturate the rock formation in which it exists. A saturated zone is fully wet (every space contains all the water it can hold) and cannot take on additional moisture. An unsaturated zone can take on more moisture.

Outwash deposits, which consist of well sorted and stratified sand and gravel deposited primarily by streams during the melting and retreat of the glacial ice, form productive valley-fill glacial aquifers. Photo by P.G. Olcott, U.S. Geological Survey. Available at http://www.nationalatlas.gov/articles/water/a_aquifer.html; accessed December 28, 2009.		Cape Cod (Massachusetts, USA) aquifer. Source: http://toxics.usgs.gov/photo_gallery/photos/capecod/cape_cod_aquifer_seds_lg.jpg; accessed December 28, 2009.
Pepin County, Wisconsin, USA, aquifer. Source: http://www.co.pepin.wi.us/Groundwater%20website/New%20Folder%20(2)/Pepin%20County%20Aquifer.html; accessed December 28, 2009.

The term aquifer refers to water-saturated rock formations. The word aquifer means “water-bearing formation,” i.e., Latin “aqua” (water) and Latin “ferre” (to bear). (1) One aquifer may refer to a single geologic formation, a group of formations, or a part of a formation; in all cases, the formation must transmit and yield a “significant” amount of water. (1)

A rock “formation” is a set of rock strata (layers) that share enough properties to suggest they have come into existence under the same prevailing environmental conditions, i.e., they have experienced a common geologic history. For example, consider layers of sedimentary rock, one sitting on top of another, like a layered cake--first a layer of shale, then a layer of limestone, then shale, then limestone (see photo below). Each layer is one formation, i.e., (from bottom to top) Vilas Shale formation, Captain Creek Limestone formation, Eudora Shale formation, and Stoner Limestone formation. The four layers together may comprise a group of formations, which receive another name.

Kansas, USA, geologic formations. Source: http://www.geospectra.net/lewis_cl/geology/geo06.jpg; accessed December 28, 2009.

A rock formation that transmits or absorbs water relatively slowly compared with aquifers is called an aquitard. The name of a formation that absorbs or transmits even less water than an aquitard is called an aquiclude. (1) Clay often behaves as an aquitard or aquiclude.

“Vertical joints and bedding planes in massive sandstone beds above and below the dark shale confining unit act as channels for the movement of the water that is visible as dark stains on this quarry face in New Jersey.” Photo by H. Trapp, Jr., U.S. Geological Survey. Available at http://www.nationalatlas.gov/articles/water/a_aquifer.html; accessed December 28, 2009.

Aquitards/aquicludes are also termed “confining units,” because they confine water to a certain region. For example, an aquitard may separate a lake (surface water) from an aquifer that resides beneath the lake; this means the aquifer cannot easily recharge or replenish the lake with water “from below.” The aquitard is the rock formation making up the bottom of the lake, e.g., clay. In this case, the aquitard (confining layer) lies above the aquifer. An aquitard can also lie below an aquifer. For example, another layer of clay lying under the aquifer just posited (which lies beneath the lake just posited) would qualify as an aquitard beneath an aquifer. 

A geologic formation that neither absorbs nor transmits any water through itself is an aquifuge. Crystalline bedrock is a good example of an aquifuge. The only way water can move in and about an aquifuge is through fractures of the rock caused, for example, by earthquake faulting.  

“In crystalline rocks, water moves through fractures. The dark spot in the photograph of this roadcut shows where water issues at the rock face from part of a nearly horizontal fracture. The water first moved downward through vertical fractures, then moved laterally to its point of discharge. The surrounding, lighter colored rock is dry.” Photo by J.A. Miller, U.S. Geological Survey. Available at http://www.nationatlas.gov/articles/water/a_aquifer.html; accessed December 28, 2009.		“Frozen seepage from joints and other fractures in metamorphosed volcanic rocks indicates how water moves through secondary openings in the rocks.” Source: P.G. Olcott, U.S. Geological Survey. Source: http://www.nationalatlas.gov/articles/water/a_aquifer.html; accessed December 28, 2009.

Thus, a smart continuum exists to name and communicate the degree to which rock formations are permeable to water, with aquifers exhibiting the greatest permeability, then aquitards, then aquicludes, and finally aquifuges, which transmit no water whatsoever (except through fractures in the rock). (1) Alternative terminology is “high hydraulic-conductivity aquifer,” “low hydraulic-conductivity confining unit,” and very low hydraulic-conductivity” bedrock.

Of note, “hydraulically [that is, pertaining to the mechanical properties of fluids], single aquifers seldom exist in nature. An aquifer is generally part of a system of aquifers,” explains Batu. (2) The first person to discover that aquifers are invariably part of a more complex hydrogeologic system was the Dutch hydrologist G.J. De Glee in 1930. (6)

Diagram of aquifer basin. Source:  http://www.americanwatersurveyors.com/images/glossary1.jpg; accessed December 28, 2009.

1. Earth’s 37 Very Large Aquifer Systems

French hydrogeologist Jean Margat posits 37 first-order aquifer systems, whose size ranges from between 100,000 (38,610 square miles) and 2,000,000 square kilometers (772,204 square miles) or more. These huge aquifer systems stretch across the continents and through all the climatic zones. The map below (by Margat) shows their locations. (3)

Diagram created by Jean Margat showing the world’s 37 very large aquifer systems. Source: Jean Margat: “Great aquifer systems of the world.” In Aquifer Systems Management: Darcy’s Legacy in a World of Impending Water. Chery Laurence and Ghislain de Marsily (Eds). Oxford, England: Taylor & Frances, 2007, p. 105.

The largest known aquifer system on earth is the Nubian Sandstone Aquifer System (NSAS), which underlies all or part of four countries in North Africa (Egypt, Libya, Chad, and Sudan). A very large aquifer system lies beneath Taklamakan Desert in Tarim Basin (Central Asia, as described elsewhere [4]), which explains where a lot of the water cascading out of the surrounding mountains via Tarim and other rivers is heading when it suddenly disappears beneath the desert floor.

2. Three Structural Types of Very Large Aquifer Systems

Margat declares that the large aquifer systems of the earth belong to one of three structural types: sedimentary, subsidence troughs and detrital accumulations at the foot of large mountain chains. The vast majority belong to the sedimentary structure type.

Jean Margat. Source: http://www.developpementdurablelejournal.com/spip.php?article4542; accessed December 28, 2009.

1. Sedimentary Type Large Aquifer System

The sedimentary structural type of aquifer is a multilayered basin of sedimentary rock formations ranging in age from Pre-Cambrian to the Quaternary. These layered formations may be as deep as 60,000 feet (11.2 miles straight down), “as on the Russian platform,” according to Margat and in the formations underlying Taklimakan Desert in Tarim Basin, Central Asia, according to Chai Guilin, et al. (4-5)

The sedimentary-type aquifer system is composed of very permeable layers made of sand, sandstone or carbonate rock, which alternate with semi-permeable confining layers (aquitards), such as clay or marl, or impervious layers (aquifuges), such as saline rock (halite). (1) The semipermeable confining layer permits vertical communication (leakage) between aquifers (“leaky aquifer”); the impervious layer does not. However, if fractures occur in the aquifuge, vertical communication can occur, as noted above.

The sedimentary-type aquifer system contains “one or several water-bearing layers with unconfined freshwater aquifers, and a varying number of layers with confined, partly inter-connected horizons.” (3)

An unconfined aquifer (also called water-table aquifer) is bounded by a free surface at the upper boundary, which is responsive to atmospheric pressure. (5)

The person who digs a hole in the ground that penetrates the water table has generated an unconfined aquifer. An overflowing spring is another type of unconfined aquifer.

Sedimentary-type aquifer systems may be predominantly fresh but with brackish or saline water deeper down where the recharge is weaker and the residence time longer. The majority of sedimentary aquifer systems have brackish water deeper down, says Margat. (2) Aquifer recharge is the hydrologic process where water moves downward from surface water to groundwater to resaturate the formation. 

Examples of the sedimentary-type aquifer system are the Great Artesian Basin in Australia, Sahara Basins, Northern Great Plains Aquifer System in North America, and the Western Siberian Basin.

Great artesian aquifer system of Australia. Source: http://www.abc.net.au/science/slab/groundwater/img/map.gif; accessed December 28, 2009.		Location of Paris, France, aquifer system. Source: http://maps.ihs.com/basin-monitor-ordering-service/europe/paris-basin.html; accessed December 28, 2009.

Sahara sedimentary basins, containing great aquifers. Source: http://www.idrc.ca/IMAGES/books/804/MAP-02.GIF; accessed December 28, 2009.		Northern Great Plains (North America) Aquifer system. Source: http://www.bigskyco2.org/files/maps/aquifer_uppz_600.jpg; accessed December 28, 2009.

Margat further categorizes sedimentary-type aquifer basins into the “Paris” model and the “Pannonian” model. The Paris model is the prototype, he says, and is exemplified by the Paris (France) Basin. “Outcropping water-bearing sedimentary rocks on the periphery of the [sedimentary basin] contain unconfined aquifers and overflowing springs that are in continuous connection with the deeper confined, sometimes artesian, horizons toward the centre.” (3) An artesian aquifer is a “confined aquifer containing groundwater that will flow upward through a well, called an artesian well, without the need for pumping.” (7)

Diagram of artesian well. Source: http://www.littledippers.com/geocaching/ArtesianWell.jpg; accessed December 28, 2009.		Roadside artesian well. Source: http://en.wikipedia.org/wiki/File:Artesianwell.jpg; accessed December 28, 2009.

The Hungarian Basin exemplifies the Pannonian model of sedimentary-type aquifer systems. “Upper layers of rock completely cover the deeper aquifers, which are only accessible by drilling and are less easy to identify, and where vertical transport by leakage or through faults predominates,” explains Margat. (3)

Hungarian (Carpathian) aquifer system. Source: http://member.melbpc.org.au/~tmajlath/etruscan.html; accessed December 28, 2009.

2. Subsidence Troughs

The second type of structural aquifer system is the subsidence trough, which is mainly “filled with alluvium, with an unconfined aquifer which may also be multilayered and contain several confined aquifers including artesian zones.” Examples are the Central Valley of California, the Northern China Plains, and the Indo-Ganga-Brahmaputra Basin, India. (3)

Central Valley, California, aquifer system. Source: http://pubs.usgs.gov/ha/ha730/ch_b/gif/b071.gif; accessed December 28, 2009.

3. Detrital Accumulation

The third type of structural aquifer system is detrital accumulation at the foot of large mountain chains. This type of system is comprised of mostly unconfined aquifers such as the High Plains (Ogallala) aquifer in the United States and the basins on the Russian platform, notes Margat. (3)

Ogallala High Plains Aquifer, USA. Source: http://en.wikipedia.org/wiki/File:Ogallala_saturated_thickness_1997-sattk97-v2.svg; accessed December 28, 2009.		USA aquifer map. Source: http://www.classzone.com/books/earth_science/terc/content/investigations/es1406/es1406page10.cfm; accessed December 28, 2009.

4. Thirsty Libya Accesses the Sahara Sedimentary-Type Aquifer System

Searing sand and gravel deserts, occasionally interrupted by uplifted massifs of pure bedrock, stretch from Egypt to Algeria in Northern Africa. Beneath the Sahara lie “three major aquifers, strata of saturated sandstones and limestones…The easternmost of these, extending over two million square kilometers, underlies all of Egypt west of the Nile, all of eastern Libya, and much of northern Chad and Sudan, and contains 375,000 cubic kilometers of water—the equivalent of 3750 years of Nile River flow. It is called the Nubian Sandstone Aquifer System (NSAS).” (8)

Diagram showing the Nubian Sandstone Aquifer System http://www.saudiaramcoworld.com/issue/200701/seas.beneath.the.sands.htm

“The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite captured this image (April 10, 2006) of Libya’s massive water project, known as the Grand Omar Mukhtar, near the city of Suluq. Water residing in reservoirs appears at the bottom of this image in dark blue. In this false-color image, vegetation appears red, and the brighter the red, the more robust the vegetation. In this arid place, the vegetation results from irrigated agriculture, so the areas of red appear in the crisp geometric shapes of carefully planned fields. The circular spots of red almost certainly result from center-pivot irrigation. Cityscape structures such as pavement and buildings appear in gray. Bare ground appears tan or beige.” Source: http://www.nasa.gov/multimedia/imagegallery/image_feature_562.html; accessed December 28, 2009.

Some water in NSAS is thought to be “one million years old, but most of it fell between 50,000 and 20,000 years ago at the time of the paleomonsoon.” (8) A monsoon (from the Arabic mausim, a season) is the name for seasonal winds, which bring rain. (9) Since the 50,000 to 20,000-year-ago monsoon, North Africa has slowly turned to desert, especially “during an acutely arid period from 20,000 to 12,000 years ago.” Little water was available to recharge the aquifer.

In the 1950s, oil exploration in Libya (population, 4 million) turned up huge aquifers in the south of the country. The Great Al-Fatah Revolution in 1969 emplaced Muammar Al Qadhafi in power (deposing King Idris), who pushed for rapid industrialization, which strained water supplies. (10) The Libyan government weighed the costs of bringing water up from the aquifers against transporting water from Europe and desalination of salt water, and chose the aquifers as the most cost-effective option.” (11)

Map showing extent of Grand Omar Mukhtar drilling and irrigation system in Libya. Source: “Man Made River, Libya.” New Dawn Magazine. Available at http://www.galenfrysinger.com/man_made_river_libya.htm; accessed December 28, 2009.		Pipes on way to extension system as part of Libya’s Grand Omar Mukhtar aquifer drilling and irrigation system. Source: http://www.galenfrysinger.com/man_made_river_libya.htm; accessed December 28, 2009.

Libya has moved “forward with big water-pumping projects that tap the Nubian Aquifer,” state Werner and Bubriski. (8) The name of the Libyan project is Grand Omar Mukhtar, which is essentially an irrigation project comprised of a network of underground pipes and aqueducts that supplies water from the Nubian Sandstone Aquifer System underlying Al-Kufrah, Libya, to coastal Libyan cities (e.g., Tripoli, Benghazi, Sirt) and farms, some 500 miles to the north. There are supposedly more than 1,300 wells, most more than 1,500 feet deep, which pump out 6,500,000 m³ of freshwater per day. Muammar al-Gaddafi calls the project the “Eighth Wonder of the World.” The sudden availability of water has vastly increased the amount of arable land in Libya. (12)

Drilling in the Sahara. Source: http://www.panoramio.com/photo/6823680; accessed December 28, 2009.

5. Summary

There are at least 37 great aquifer systems on earth. Some arid countries that overlay such aquifer systems, such as Libya, are beginning to tap them via mammoth drilling and irrigation projects to supply water for farming and industrialization.

Notes:

1. Vedat Batu: Aquifer Hydraulics: A Comprehensive Guide to Hydrogeologic Data Analysis. Wiley-Interscience, 1998, p. 22. 
2. Ibid, p. 24. 
3. Jean Margat: “Great aquifer systems of the world.” In Aquifer Systems Management: Darcy’s Legacy in a World of Impending Water. Chery Laurence and Ghislain de Marsily (Eds). Oxford, England: Taylor & Frances, 2007, pp. 105-116.
4. SEMP Biot Report #665: 4,000 Year-Old- Caucasian Mumies in Tarim Basin, Central Asia.” November 16, 2009. Available at http://www.semp.us/publications/biot_reader.php?BiotID=665; accessed December 28, 2009.
5. Chai Guilin, Wang Xiaomu and Jin Xuezheng: “Petroleum geology and oil potential of Tarim basin, West China.” #24102. 13th World Petroleum Congress, October 20-25, 1991, Buenos Aires, Brazil.
6. G.J. De Glee: Over Grondwaterstomingen Gij Wateronttrekking Door Middel Van Putten,” Thesis (in Dutch), J. Waltman, Delft, The Netherlands, 175 pp, 1930.
7. “Artesian aquifer.” Wikipedia. Available at http://en.wikipedia.org/wiki/Artesian_aquifer; accessed December 28, 2009.
8. Louis Werner and Kevin Bubriski: “Seas beneath the sands.” Saudi Aramco World, January/February 2007. Available at http://www.saudiaramcoworld.com/issue/200701/seas.beneath.the.sands.htm; accessed December 28, 2009.
9. “monsoon.” “Glossary of Meteorology.” American Meteorological Society. Available at http://amsglossary.allenpress.com/glossary/search?p=1&query=monsoon&submit=Search; accessed December 28, 2009.
10. John Watkins: “Libya’s thirst for fossil water.” BBC World Service. March 18, 2006. Available at http://news.bbc.co.uk/2/hi/science/nature/4814988.stm; accessed December 28, 2009.
11. “Fossil water in Libya.” NASA. Available at http://www.nasa.gov/multimedia/imagegallery/image_feature_562.html; accessed December 28, 2009.
12. “Man Made River, Libya.” New Dawn Magazine. Available at http://www.galenfrysinger.com/man_made_river_libya.htm; accessed December 28, 2009.