Phreatic

Phreatic is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to a type of volcanic eruption.

Hydrology
The term phreatic (the word originates from the Greek phrear, phreat- meaning "well" or "spring") is used in hydrology and the earth sciences to refer to matters relating to ground water (an aquifer) below the water table. 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.

Speleology
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. It occurs when the passage is full of water, and therefore normally only when it is below the water table, and only if the water is 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.

Volcanology
A phreatic eruption or steam-blast eruption occurs when magma heats ground or surface water.

Biology
Animals living within the phreatic zone of groundwater aquifers can be referred to as phreatobites. They are usually isopod or amphipod crustaceans such as species of Stygobromus, though there is also a genus of snails (Phreatodrobia) and Phreatobius are a genus of catfish living within flooded leaf litter. Alternative descriptions for such animals include stygobite, troglobite or Interstitial animals, as they live in water between the particles of the flooded substrate. They usually exhibit troglomorphism, with a loss of colour and eyesight, like the familiar blind cavefish which may also be referred to as 'phreatic fish'.

Such animals can often be found in wells, where human pursuit of water has dug down into their habitat.