A layer of rock or sediment with very little permeability is termed an Aquitard in hydrogeologic contexts.
Permeability is defined as the property of a solid to allow water to percolate through it.
Any geological formation with a semipermeable character that conducts water more slowly than an aquifer is called an aquitard. An aquitard is defined as a stratigraphic sequence's less permeable beds by Freeze and Cherry (1979). In the study of regional groundwater flow, these beds may be permeable enough to convey water in volumes that are considerable, but they are not permeable enough to allow the drilling of production wells.
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