In modern usage, an artesian well is any well in which the water level rises above the top of the confined aquifer to which the well is open. In both flowing and non-flowing artesian wells, the water is under higher pressure than would be created by a column of water extending from the depth of the open well to the top of the aquifer. This excess pressure can be explained by the fact that the recharge area, where water enters the aquifer system, is at a higher elevation than the confined aquifer at the well. In the recharge area the hydraulic head, a measure of the mechanical energy of the groundwater, is determined by the potential energy associated with the elevation of the water-table. As water flows downwards into and within a confined aquifer, friction causes some of the potential energy to be converted into heat. The frictional losses are, however, smaller than the potential energy loss associated with loss of elevation. Much of the potential energy is therefore converted into elastic energy, which is associated with an increase in pressure.
Flowing wells are often assumed to be tapping aquifers that are isolated from shallower groundwater by a confining unit of low permeability. This is not necessarily the case. Flowing wells can occur where there is no confining unit between the open portion of the well and the land surface. All that is required is that the hydraulic head at the depth tapped by the well must be higher than the land surface. This is generally the case in groundwater discharge areas located in topographic ‘lows’. Groundwater flow in these areas is upwards, which requires that the hydraulic head must increase with depth since, according to Darcy's law, groundwater flows from a higher to a lower head.
Artesian wells, and confined aquifers in general, were once thought to be virtually inexhaustible sources of water. Continued extraction of water from an artesian well can, however, lead to a decline in pressure, causing a once-flowing well to require pumping. In extreme cases, the water levels can eventually drop below the top of the confining unit. At this point the confined aquifer becomes a water-table aquifer. The extent to which the water level declines depends on the balance between recharge to the aquifer system and the combined discharge through wells and in natural discharge areas
PAUL HANCOCK and BRIAN J. SKINNER. "artesian well." The Oxford Companion to the Earth. 2000. Encyclopedia.com. (August 10, 2011). http://www.encyclopedia.com/doc/1O112-artesianwell.html
This spring was caused by a 1094 foot hole drilled in 1903 by the Oliver Mining Company in the search for iron ore. The hole cuts several steeply dipping porous strata that trap water at higher elevations to the north. The difference in elevation causes pressure: this pressure is released by the drilled hole, demonstrating the principle of the artesian well.
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