The term aquifer refers to the various rock formations, rock fractures, and other materials of the Earth’s mantle through which water moves easily and within which ground water is contained. The term aquifer encompasses the volume of the water found in these different formations.  Factors for classification of aquifers include water flow, saturation, accessibility, and location within the Earth’s crust.  Hydrogeology is the study of water flow through aquifers.
Aquifers exist all over the world and are not confined by national borders.  As of June 2015, 37 of the largest global aquifers were in danger of irreversible depletion.  Many of these span across the boundary lines between nations. A single nation may have multiple aquifers providing water to different regions. 
Aquifers occur at different depths within the crust and as a result are impacted by rainfall or accessed for human use at different rates.  Aquifers located deeper in the Earth’s subsurface are more difficult and costly to access and do not naturally refill as quickly as those closer to the surface. Whether and where an aquifer forms is based on of the types of materials through which the water flows.
Formation and geology
When surface water seeps into the ground, called infiltration , it flows through and around different materials in the Zone of Aeration. Groundwater occupies the void spaces found in rocks, soil, and sediment.  Aquifers can exist throughout every region of the Earth’s crust.
Porosity is the term used to explain the total volume of water these unoccupied spaces can accommodate.  Porosity varies in sediment and sedimentary rocks based on the size and shape of the granules, whether granules of similar shape and size are located near like granules (or are mixed with other sized and shaped granules), and the degree to which cementation (the near-complete filling-in of spaces between pores) has occurred. 
Larger, well-rounded granules have a higher porosity while smaller and/or irregular shaped sediment have a lower porosity. Lower porosity occurs in these cases because the granules do not fit well together, failing to provide spaces between pores (pore spaces) through which water flows easily.
Sediment in which many different sizes of granules are mixed together have reduced porosity due to clogs in the pore spaces caused by the interruption of smooth water flow by the finest granules.
Cementing of granules, in which the tiniest particles of sediment fill in pore spaces, leads to low porosity due to the near-complete blocking of the flow of water.
Unlike sedimentary rock, porosity is typically low in igneous and metamorphic rock because the minerals in the rock bind together with nearby minerals leaving virtually no pore space. Igneous and metamorphic rock that has many cracks (know as a fissures) have high porosity because pathways for water flow exist.
Low porosity is generally a good indicator of permeability (the measure of both the size of pore space interconnections and the degree to which pore spaces are interconnected). High porosity, however, does not always mean high permeability. Some types of rock (such as pumice) are highly porous but have no interconnection between pores, resulting in low permeability. 
Materials with high permeability (gravel, sand, and fractured rock) are ideal for aquifer formation. A material with low permeability is known as an aquitard. Examples of good aquitard materials include tightly packed clay and igneous or metamorphic rocks without fracturing. 
Zones of classification
Aquifers are divided into two classifications based on the degree of saturation of the region. These two regions are the saturated zone (also referred to as the phreatic zone) and the unsaturated zone (or vadose zone). 
Saturated zones are the portions of the mantle in which water fills all otherwise unoccupied space (those spaces not filled by other solid substances, e.g. minerals, clay, etc.).  Saturated zones occur when the atmospheric pressure is less than that of the pressure head (the difference in pressure between the top of a body of water and the point at which that water begins to flow).
Unsaturated zones are portions of the crust that contain some water but that have empty space filled with gas e.g. air, methane, etc.  Simply, unsaturated zones have room for more water. Unsaturated zones occur when water is prevented from flowing because the atmospheric pressure is greater than that of the pressure head. 
Types of aquifers
There are two types of aquifer: confined and unconfined. 
Aquitards are key to the formation of confined aquifers. A confined aquifer forms when it is sandwiched between the impermeable layers of more than one aquitard. Wells drilled in a confined aquifer are called “Artisanal Wells”.
The second and most common type of aquifer is the unconfined aquifer in which the water in the aquifer (water table) is exposed to the Earth’s atmosphere through permeable materials in the Zone of Aeration. 
Water reaches the atmosphere through evaporation or transpiration and falls on the Earth as precipitation. This precipitation may be reabsorbed by plants and re-cycled through the atmosphere, may enter greater bodies of water, such as creeks, rivers, lakes, or oceans thorough the process of runoff. Or, this precipitation may soak (infiltrate) into the mantle and through layers of porous and/or permeable materials (Zone of Aeration) and become part of the aquifer. This process of filtration which creates human accessed freshwater may take days, weeks, months, or even thousands of years. 
Recharge is the slow process through which water that has filtered through the Zone of Aeration begins to replace water that has be used in the water table. Aquifers located deeper in the Earth's mantel take longer to refill. An aquifer that is over-used can take years to recharge. 
Global aquifer degradation
Contamination of fresh water sources occurs both because of human activity and through naturally occurring processes. Water quality is gauged by the quantity of biological or toxic pollutants, the temperature of the water, and/or the amount of dissolved solids (often various minerals)  found in a source of water. These aquifers do not only contain fresh water. Aquifers around the world are deteriorating due to over use and contamination.
Human induced sources of contamination might include  seepage from gasoline storage tanks, agricultural pollutants, sewers and septic tanks, as well as residential and industrial waste dumps.
Nature itself contributes contaminants to groundwater often in the form of geologic constituents.  Salt-water intrusion is another natural source of contamination, in which sea rise causes salt water to flow into areas that have historically held fresh water.  An example of salt-water intrusion is the occurrence of previously fresh water wells used for human consumption become tainted with salt water and are then rendered non-potable.
Water scarcity occurs when the available freshwater is not enough to meet the needs of humans and animals. Water scarcity comes about as a result of many factors including naturally occurring fluctuations in water tables, climate change, and rapid depletion by humans.  Naturally occurring water table fluctuations can cause periodic water shortages or droughts. However, in recent years there is growing concern that humans over-reliance on aquifers, in addition to natural and human induced climate change is leading to more frequent and less recoverable water shortages worldwide. Adding to the drought and the depletion of aquifers around the world  are water-bottling companies that have not ceased packaging water from drought stricken areas. 
Humans rely heavily on aquifers and the fresh water they provide.  Human uses of water include industrial, agricultural, fire fighting, drinking, bathing, and swimming. The greatest human use of freshwater comes from the irrigation of crops. 
Management & conservation
Management of our water resources is taking on ever more importance as climate change and human consumption places stressors on the water supply. 
There are many fresh water management and conservation efforts are being utilized to address the growing pressures on access to water for human needs. 
Remediation occurs once water contamination has occurred. It is the expensive process of removing contamination and the source of contamination of freshwater. Often this process requires water to be pumped out in order for it to be treated.
Human over-reliance on aquifers, especially in the last 50 years , is leading some to find ways to conserve water.
Among the many methods individuals employ to conserve (or use less) water are low-flow shower heads, faucet aerators, and low-flush toilets. Some conservation can be accomplished by changes in behavior such as only running dishwashers when they are full or turning off the faucet when brushing one's teeth. 
Industrial and municipal efforts at conservation include the reuse and recycling of water (such as the application of grey water on landscaping) or the recirculation (rather than disposal) of water used for the cooling of heat-generating equipment. There are many other ways to conserve fresh water, such as rainwater harvesting and universal metering. 
One ingenious method being used, in concert with other methods, by a California municipality, are shade balls, which are dark and prevent evaporation in water reservoirs. 
Other closely related articles in this wiki include:
- Idaho Museum of Natural History. (2002, April 24). What is an Aquifer? Accessed October 29, 2015, from http://imnh.isu.edu/digitalatlas/hydr/concepts/gwater/aquifer.htm
- Nelson, S. A. (2012, April 9). Groundwater. Accessed October 29, 2015, from http://www.tulane.edu/~sanelson/eens1110/groundwater.htm
- igrac. (2014, November 17). Transboundary Aquifers of the World. Accessed October 29, 2015, from https://ggis.un-igrac.org/ggis-viewer/viewer/tbamap/public/default
- Frankel, T. C. (2015, June 16). New NASA data show how the world is running out of water. Accessed October 29, 2015, from http://www.washingtonpost.com/news/wonkblog/wp/2015/06/16/new-nasa-studies-show-how-the-world-is-running-out-of-water/
- Oskin, B. (2015, January 14). Aquifers: Underground Stores of Freshwater. Accessed October 29, 2015, from http://www.livescience.com/39625-aquifers.html
- U.S. Department of Interior, U. S. G. S. (January, 11, 2013). Ground Water and Surface Water A Single Resource. Reston, VA: U.S. Department of Interior, U.S. Geological Survey. Accessed October 29, 2015 from http://pubs.usgs.gov/circ/circ1139/
- Future management of aquifer recharge. Hydrogeology Journal, 13(1), 313–316. http://doi.org/10.1007/s10040-004-0413-6
- U.S. Department of Interior, U. S. G. S. (2014). Water Quality in Principal Aquifers of the United States, 1991-2010 - circ1360report.pdf (Circular No. Circular 1360) (p. 161). Reston, VA: U.S. Department of Interior, U.S. Geological Survey. Accessed October 29, 2015 from http://pubs.usgs.gov/circ/1360/pdf/circ1360report.pdf
- Spatafora, J. (2008, May 6). Saltwater Intrusion. Accessed October 29, 2015, from http://kanat.jsc.vsc.edu/student/spatafora/setup.htm
- NASA Jet Propulsion Laboratory. (2015, June 16). Study: Third of Big Groundwater Basins in Distress. Accessed October 29, 2015, from http://www.jpl.nasa.gov/news/news.php?feature=4626
- Meyer, R. (2015, June 17). Earth’s Aquifers Are Drying Up: Just How Bad Is It? Accessed October 29, 2015, from http://www.theatlantic.com/technology/archive/2015/06/earth-running-out-water-aquifiers/396152/
- Mohan, G. (2015, October 13). Nestle drawing millions of gallons of California water on expired permit, suit claims. Latimes.com. Accessed October 29, 2015 from http://www.latimes.com/business/la-fi-nestle-water-lawsuit-20151013-story.html
- USGS. (2005). Groundwater use in the United States. Accessed November 9, 2015, from http://water.usgs.gov/edu/wugw.html
- Dillon, P. (2005). Future management of aquifer recharge. Hydrogeology Journal, 13(1), 313–316. http://doi.org/10.1007/s10040-004-0413-6
- Foster, S. S. D., & Chilton, P. J. (2003). Groundwater: the processes and global significance of aquifer degradation. Philosophical Transactions of the Royal Society B: Biological Sciences, 358(1440), 1957–1972. http://doi.org/10.1098/rstb.2003.1380
- EPA. (2012, March 6). How to Conserve Water and Use It Effectively. http://water.epa.gov/polwaste/nps/chap3.cfm
- Howard, B. C., 12, N. G. P. A., & 2015. (n.d.). Why Did L.A. Drop 96 Million “Shade Balls” Into Its Water? Accessed October 29, 2015, from http://news.nationalgeographic.com/2015/08/150812-shade-balls-los-angeles-California-drought-water-environment/
- Hydrologic cycle - by the Encyclopedia of the Earth.
- Aquifer Characteristics - depth, important, types, effect - by the Water Encyclopedia.
- Aquifer - by the Earth Encyclopedia.
- Global Groundwater Information System (GGIS) - by the International Groundwater Resources Assessment Centre.
- If You Think the Water Crisis Can’t Get Worse, Wait Until the Aquifers Are Drained - from National Geographic.
- Regional Assessments of Principal Aquifers - from USGS NAWQA.
- Study: Impact of warming climate doesn’t always translate to streamflow - from Oregon State University.
- Importance of Water - from Longwood University.
- Operational Stage - from Sustainable Aggregates.
- Groundwater - from Oceanography in the 21st Century, an online textbook
- Offshore fresh groundwater reserves as a global phenomenon - from Nature, 504(7478), 71–78.
- Principal Aquifers of the United States - from USGS.
- Aquifers and Groundwater - by the USGS Water-Science School.
- Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004 - by the USGS and Department of the Interior