1. Those parts of the earth's crust in which all voids are filled with water under pressure greater than atmospheric [22]. 2. That part of the earth's crust beneath the regional water table in which all voids, large and small, are ideally filled with water under pressure greater than atmospheric [22]. When discussing a karst setting, it is preferable to use the term, phreatic zone, so as to avoid confusion regarding chemical saturation. Synonym: saturated zone. See also zone of saturation.