An underground layer of rock holding usable groundwater.
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An aquifer is an underground layer of water-bearing rock, sand, gravel, or silt from which groundwater can be extracted for drinking, irrigation, and industry. Aquifers form where permeable material allows water to accumulate and move, bounded by impermeable layers that trap it. They are classified as unconfined, where the water table is open to the surface, or confined, where a low-permeability layer sits above. The Ogallala Aquifer beneath the US Great Plains, the Guarani Aquifer in South America, and the North China Plain aquifer are among the world's largest and most agriculturally important. Groundwater supplies roughly one-third of global freshwater withdrawals and irrigates a large share of cropland, particularly in India, the United States, and China. Over-extraction causes water tables to fall, land subsidence, and salinisation, while pollution from nitrates, pesticides, and industrial chemicals threatens quality. Managed aquifer recharge, conjunctive use of surface and groundwater, and improved irrigation efficiency are key strategies for sustainability.
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