Two classes may be distinguished: shallow or ordinary wells, sunk through a permeable stratum until an impermeable stratum is reached; or deep and artesian wells, the latter named from Artois in France, which are sunk through an impermeable stratum down into a water-bearing stratum which overlies an impermeable stratum.
Obviously ordinary wells can supply water very cheaply, but, since impurities readily reach them, there is great risk of contamination. The same does not apply to deep wells, such water being usually free from organic impurities. In ordinary wells, and in deep wells, the water requires pumping to the surface; in artesian wells, on the other hand, the water usually spouts up to a greater or less height above it.
See also: oil well