Aquitards are often saturated, but due to low permeability do not yield water freely to wells. However, aquitards may transmit appreciable water quantities to and from adjacent aquifers1, and depending on thickness may constitute a significant groundwater storage zone.

The usage of the term aquitard is also relative. What is considered an aquitard in one area of the world could be considered an aquifer in other parts. For example, in The Netherlands a stratum of clay is almost always considered an aquitard, because it is an order of magnitude less permeable than the (often) adjacent strata of sand. In areas with predominantly hard, rocky subsurface strata, a layer or formation of clay would probably be considered an aquifer, because here it is an order of magnitude more permeable than the surrounding rock.

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1 This is because the flow between strata passes through the total area of contact between the strata, thereby balancing out the low conductivity per surface area.

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