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In unconfined aquifers , the ground water only partially fills the aquifer and the upper surface of the ground water (the water table) is free to rise and decline. The ground water is at atmospheric pressure. The height of the water table will be the same as the water level in a well constructed in that unconfined aquifer. The water table typically mimics, in a subdued way, the topography of the land surface above, resulting in a water table with hills, valleys, or flat areas. It is important to note that unconfined aquifers, especially those close to the surface, can be vulnerable to contamination from activities on the land surface.
Confined aquifers may also be referred to as artesian aquifers.
A confined aquifer is sandwiched between confining beds (layers of impermeable materials such as clay which impede the movement of water into and out of the aquifer). Because of the confining beds, ground water in these aquifers is under high pressure. Because of the high pressure, the water level in a well will rise to a level higher than the water level at the top of the aquifer. The water level in the well is referred to as the potentiometic surface or pressure surface.
Even in a confined aquifer, water seeks its own level. Geological strata are not perfectly horizontal. At some point the lithological unit that comprises the confined aquifer is exposed to the surface. This is the aquifer's recharge zone, and it may be miles away from where one hopes to construct a well. The "confined" aquifer is actually unconfined at the recharge zone. In order for pressure to build, the water level in the recharge zone must be at a higher elevation than the base of the confining unit. When a well is drilled through the confining unit, usually far from the recharge zone, the water in this well will rise to the level of the water at the recharge zone. In some instances this may be above the surface of the ground, in which case the well is called a flowing artesian well. This same situation, where the level of the water at the recharge zone is above the base of the confining unit, leads to the appearance of springs or seeps where the confining unit is penetrated by a hillside.
It is important to note that confining beds not only serve to hamper the movement of water into and out of the aquifer, they also serve as a barrier to the flow of contaminants from overlying unconfined aquifers. For this same reason, however, contaminants that reach a confined aquifer through a poorly constructed well or through natural seepage, can be extremely difficult and expensive to remove.
Fiction: Bottled water is often advertised as having come from artesian aquifers. These waters are often more pure because confining beds impede the movement of contamination. There is no guarantee, however, that artesian waters are any cleaner than ground water from an unconfined aquifer.
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