An artesian aquifer is a confined aquifer containing groundwater under positive pressure. This causes the water level in a well to rise to a point where hydrostatic equilibrium has been reached.
A well drilled into such an aquifer is called an artesian well. If water reaches the ground surface under the natural pressure of the aquifer, the well is called a flowing artesian well.
An aquifer is a geologic layer of porous and permeable material such as sand and gravel, limestone, or sandstone, through which water flows and is stored. An artesian aquifer is confined between impermeable rocks or clay which causes this positive pressure. Not all the aquifers are artesian, because the water table must reach the surface (not the case for underground groundwater such as, for example, the Nubian Sandstone Aquifer System). The recharging of aquifers happens when the water table at its recharge zone is at a higher elevation than the head of the well.
Fossil water aquifers can also be artesian if they are under sufficient pressure from the surrounding rocks. This is similar to how many newly tapped oil wells are pressurized.
Artesian wells were named after the former province of Artois in France, where many artesian wells were drilled byCarthusian monks from 1126.