An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials such as gravel, sand, or silt. It is a body of rock and/or sediment that holds groundwater. Aquifers must be both permeable and porous and include rock types such as sandstone. Groundwater from aquifers can be extracted using a water well. Aquifers occur from near-surface to deeper than 9,000 meters (30,000 ft) . Those closer to the surface are not only more likely to be used for water supply and irrigation, but are also more likely to be replenished by local rainfall. Although aquifers are sometimes characterized as "underground rivers or lakes," they are actually porous rock saturated with water.
Aquifers can be categorized into three main types: unconfined, confined, and perched. Unconfined aquifers are piles of sand and gravel typically found within a hundred feet or less of the surface. These shallow aquifers can provide an abundant supply of groundwater but may also be susceptible to contamination. Confined aquifers are those that are bounded above and below by less permeable materials, such as clay, which restrict the flow of water into and out of the aquifer. Perched aquifers are small, localized bodies of water that occur above the regional water table in areas where an impermeable layer, such as clay, is present above the main aquifer.
Aquifers are natural filters that trap sediment and other particles, like bacteria, and provide clean water. Groundwater recharge is the process by which water enters an aquifer. An aquitard is a zone within the Earth that restricts the flow of groundwater from one aquifer to another. Aquitards are composed of layers of either clay or non-porous rock with low hydraulic conductivity.
