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An aquiclude is a water-bearing layer in which both the horizontal and vertical flow components are so small that they can be neglected. The groundwater flow in an aquiclude is assumed to be zero.

Impervious rock in the unsaturated zone may retain water infiltrating from the surface to produce a perched water table that is above the main water table and from which water may be extracted. However, the amounts of water that can be extracted from such a formation are limited and the water is vulnerable to contamination.

An aquitard is a water-bearing layer in which the horizontal flow component is so small with respect to the vertical flow component that it can be neglected. The groundwater flow in an aquitard is assumed to be predominantly vertical. An aquifuge is a geological formation with low permeability and porosity. This does not transmit any groundwater and does not contain groundwater in appreciable quantities.

See also: Aquifer, Geohydrology, Groundwater Aquifer.