Aquitard

From Citizendium, the Citizens' Compendium

An aquitard or aquiclude is a zone of earth material that will hold water but not transmit it at a useful rate or fast enough to be pumped from a well. Aquitards often form a confining layer through which little water moves. Clay soils, shale, and igneous or metamorphic rocks with little interconnected porosity or fractures are likely to form aquitards.