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An aquifer is a layer of water-bearing permeable rock, sand, or gravel capable of providing significant amounts of water (see also groundwater). The upper boundary of the topmost aquifer is known as the water table. Some areas have several aquifers, each capped on top by an impervious layer. If the recharge area is elevated higher that the capping layer, the water may be under considerable pressure, and flowing or Artesian wells may be likely. Most land areas on Earth have some form of aquifer underlying them, though often at significant depth. Aquifers in coastal regions do not necessarily contain fresh water, especially when they have been pumped at rates exceeding the input of fresh water from the recharge area. Such reversal of normal flow toward the ocean leads to intrusion of sea water backward into the acquifer, which is a serious problem in many coastal areas.

Aquifers are critically important in human habitation and agriculture. Deep aquifers in arid areas have long been water sources for irrigation. Many villages and even large cities draw their water supply from wells in aquifers.

The most sustainable aquifers, and those most often used for city water supplies, are riparian aquifers. These are present in unconsolidated deposits of sand and gravel along river corridors, and are usually rapidly replenished.

Aquifer depletion is a global problem, and is especially critical in northern Africa.

The Ogallala Aquifer of the central United States is one of the world's great aquifers, but is being rapidly depleted. This huge aquifer, present in around eight states, comprises fossil water from the time of the last glaciation.

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