An illustration showing groundwater in aquifers (in blue) (1, 5 and 6) below the water table (4), and three different wells (7, 8 and 9) dug to reach it.

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.[1] A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures 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 Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells.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.

Typically, groundwater is thought of as water flowing through shallow aquifers, but, in the technical sense, it can also contain soil moisture, permafrost (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'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 than surface water. Therefore, it is commonly used for public water supplies. For example, groundwater provides the largest source of usable water storage in the United States, and California annually withdraws the largest amount of groundwater of all the states.[2] Underground reservoirs contain far more water than the capacity of all surface reservoirs and lakes in the US, including the Great Lakes. Many municipal water supplies are derived solely from groundwater.[3] Over 2 billion people rely on it as their primary water source worldwide.[4]

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) 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 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.[5][6][7]

  1. ^ "What is Groundwater? | International Groundwater Resources Assessment Centre". Retrieved 2022-03-14.
  2. ^ National Geographic Almanac of Geography, 2005, ISBN 0-7922-3877-X, p. 148.
  3. ^ "What is hydrology and what do hydrologists do?". The USGS Water Science School. United States Geological Survey. 23 May 2013. Retrieved 21 Jan 2014.
  4. ^ Famiglietti, J. S. (November 2014). "The global groundwater crisis". Nature Climate Change. 4 (11): 945–948. Bibcode:2014NatCC...4..945F. doi:10.1038/nclimate2425. ISSN 1758-6798. Retrieved 2 March 2022.
  5. ^ Weisberger, Mindy (2023-06-26). "Humans pump so much groundwater that Earth's axis has shifted, study finds". CNN. Retrieved 2023-08-15.
  6. ^ Castelvecchi, Davide (2023). "Rampant Groundwater Pumping Has Changed the Tilt of Earth's Axis". Nature. doi:10.1038/d41586-023-01993-z. PMID 37328564. S2CID 259183868. Retrieved 2023-08-15.
  7. ^ "Humans Have Shifted Earth's Axis by Pumping Lots of Groundwater". Smithsonian Magazine. Retrieved 2023-08-15.

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