Editing Aquifer (section)

=== Confined versus unconfined ===
An unconfined aquifer has no impermeable barrier immediately above it, such that the water level can rise in response to recharge. A confined aquifer has an overlying impermeable barrier that prevents the water level in the aquifer from rising any higher. An aquifer in the same geologic unit may be confined in one area and unconfined in another. ''Unconfined'' aquifers are sometimes also called ''water table'' or ''phreatic'' aquifers, because their upper boundary is the [[water table]] or phreatic surface (see [[Biscayne Aquifer]]). Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller). Confined aquifers are aquifers that are overlain by a confining layer, often made up of clay. The confining layer might offer some protection from surface contamination.

If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an [[aquifer test]] can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low [[Specific storage|storativity]] values (much less than 0.01, and as little as {{10^|-5}}), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically called [[Specific storage|specific yield]]) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable [[Hydrogeology#Porosity|porosity]] of the aquifer material, or the minimum volumetric [[water content]]).
{{See also|Porosity|Storativity}}