The USGS Water Science School
Artestian water and artesian wells
Maybe you've heard advertisements by water companies wanting to sell you "artesian-well drinking water." Is this water different from other bottled water taken from springs?
The water may not be different, but it comes to the earth's surface a bit differently. Groundwater in aquifers between layers of poorly permeable rock, such as clay or shale, may be confined under pressure. If such a confined aquifer is tapped by a well, water will rise above the top of the aquifer and may even flow from the well onto the land surface. Water confined in this way is said to be under artesian pressure, and the aquifer is called an artesian aquifer. The word artesian comes from the town of Artois in France, the old Roman city of Artesium, where the best known flowing artesian wells were drilled in the Middle Ages. The level to which water will rise in tightly cased wells in artesian aquifers is called the potentiometric surface.
Deep wells drilled into rock to intersect the water table and reaching far below it are often called artesian wells in ordinary conversation, but this is not necessarily a correct use of the term. Such deep wells may be just like ordinary, shallower wells; great depth alone does not automatically make them artesian wells. The word artesian, properly used, refers to situations where the water is confined under pressure below layers of relatively impermeable rock. The picture to the right shows an artesian well with the potentiometric surface being just above the land surface, but, as the picture above shows, artesian pressure can be very strong!
Example of an aquifer system with artesian wells
The diagram below shows the aquifer system near Brunswick, Georgia, as it was before development of the Floridan aquifer system in the 1880's. The aquifer system was under artesian conditions and the pressure in the aquifer system was great enough that wells flowed at land surface throughout most of the coastal area. In some areas, pressure was high enough to elevate water to multi-story buildings without pumping. The artesian water level (potentiometric surface) was about 65 feet above sea level at Brunswick. Groundwater discharged naturally to springs, rivers, ponds, wetlands, and other surface-water bodies and to the Atlantic Ocean. Nowadays, groundwater pumping has caused the water level in the aquifer to decline throughout the entire coastal area, with the result that some artesian aquifers no longer have enough pressure to cause a well to naturally flow to the land surface.
Sources and more information