Artesian well water is water that is driven out of the earth by natural pressure.
The water will be in the ground in an “aquifer.” An aquifer is layers of earth, porous rock, shale and sand in which water is trapped. The water in the aquifer has flowed down into it from a higher elevation, either from rain or from snowfall.
These layers are under other layers or rock and clay. The weight from them puts pressure on the water, as well as the weight of the water in the elevations higher up trying to get down.
When you drill into this layer, the pressure suddenly finds an outlet and drives the water up and out.
The pressure is called “artesian pressure.” Sometimes the pressure can be great enough to bring the water up two or three stories in buildings without any pumping.
Sometimes, it is pumped as well to increase the capacity, but if an aquifer is overused, eventually the pressure will drop as too much water is taken out without the incoming water being able to keep up. And ground water pumping close by can deplete the water in an artesian aquifer, causing the water in it to lose the pressure it previously had on its own.
Artesian wells are just as prone to becoming clogged with sand as other types of wells.
The water that comes out can be of any quality. Some artesian wells, such as that in Shubuta, Mississippi even pump out red water which is potable.
There is a large artesian system under Queensland, Australia, and in North America, under the Great Plains.
There is an artesian well in the heart of Paris, in the Lamartine square in the residential area of Passy in the 16th arrondissement. It powers the fountain there. It was drilled in 1855 by a man named Saxon Kind to provide water for the Bois de Boulogne.
It is commonly believed that the various layers of earth act as filters, making the water purer than other forms of water, but the American Environment Protection Agency (EPA) says there’s no guarantee. In the Middle Ages, it certainly was safer than standing or river water. But now, man-made pollutants such as pesticides and other chemicals and waste from livestock can seep down into the aquifers.
“Artesian” comes from the name of the town of Artois, France (which was called Artesium by the Romans.) During the Middle Ages, the artesian wells drilled here were well known.
The first artesian well in Europe was dug in 1126 by Carthusian monks (i.e. of the Chartreuse order) at their convent in Lillers. It was drilled not by a rotating drill, but by “percussion drilling” — pounding the drill down in to force a hole. The hole was just a few inches wide.
The Chinese, though, were making artesian wells hundreds of years BC.