An aquifer is an underground layer of
water-bearing permeable rock or unconsolidated materials (
gravel,
sand,
silt, or
clay) from which
groundwater can be usefully extracted using a
water well. The study of water flow in aquifers and the characterization of aquifers is called
hydrogeology. Related terms include: an aquitard, which is an impermeable layer along an aquifer, and an aquiclude (or aquifuge), which is a solid, impermeable area beneath an aquifer.
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An aquifer containing water that is not under pressure; the water level in a well is the same as the water table outside the well.