or water-table) aquifer: An aquifer in which the water table is at the upper boundary of the ground-water flow system that is at atmospheric pressure.

Water that is in direct contact vertically with the atmosphere through open spaces in permeable material

in reference to aquifers, it describes a condition whereby water-bearing rocks are at atmospheric pressure, i.e., water table conditions.

To present participles which come from intransitive verbs, or are themselves employed as adjectives, to mark the absence of the activity, disposition, or condition implied by the participle;

not confined

free from confinement or physical restraint