The term phreatic is used in hydrology and the earth sciences to refer to matters relating to ground water (an aquifer) below the water table (the word originates from the Greek phrear, phreat- meaning "well" or "spring"). The term 'phreatic surface' indicates the location where the pore water pressure is under atmospheric conditions (i.e. the pressure head is zero). This surface normally coincides with the water table. The slope of the phreatic surface is assumed to indicate the direction of ground water movement in an unconfined aquifer.
The phreatic zone, below the phreatic surface where rock and soil is saturated with water, is the counterpart of the vadose zone, or unsaturated zone, above. Unconfined aquifers are also referred to as phreatic aquifers because their upper boundary is provided by the phreatic surface.
In speleogenesis, a division of speleology, 'phreatic action' forms cave passages by dissolving the limestone in all directions, as opposed to 'vadose action', whereby a stream running in a cave passage erodes a trench in the floor.
Phreatic action usually takes place when the passage is below the water table (although it may happen if the passage is full of water and not saturated with calcium carbonate or calcium magnesium carbonate). A cave passage formed in this way is characteristically circular or oval in cross-section as limestone is dissolved on all surfaces.
Many cave passages are formed by a combination of phreatic followed by vadose action. Such passages form a keyhole cross section: a round-shaped section at the top and a rectangular trench at the bottom.
See also 
- New Mexico: Bureau of Mines & Mining Bulletin 117 (Part I: Discussion of Deposits and Events)
- "Glossary of Cave-Related Terms". Upper Cumberland Grotto Home Cave. Retrieved 13 November 2010.
- John A. Webb & Stanley Lithco (September 2001). "Use of water chemistry to identify flow conduits in the porous Gambier Limestone, southeast Australia" (PDF). 7th Conference on Limestone Hydrology and Fissured Media. France: Universite de Franche-Comte, Sciences & Techniques de l'Environnement. pp. 333–336. ISBN 2-905226-14-5. Retrieved 13 November 2010. "Passages are usually narrow vertical fissures, but phreatic tubes, circular or oval in cross-section, are present in some caves..."
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