Aquifer Glossary of Terms

Aquifers and Aquitards

Aquifer and aquitard are terms used to characterize hydrogeologic systems. A geologic unit that is highly permeable and can store and transmit a significant amount of groundwater is called an aquifer. When an aquifer is bounded by the water table on the top, the aquifer is called an unconfined aquifer. When an aquifer is confined between two much less permeable units, it is called a confined aquifer. Water pressure in confined aquifers is usually higher than pressure in unconfined aquifers. An aquitard, also known as a confining bed, is a much less permeable geologic unit. Because no naturally occurring porous material is completely impermeable, aquifers and aquitards are identified to distinguish their relative degree of high and low permeability, respectively. In general, gravel, sandy materials, limestone, or highly fractured rocks make good aquifers, whereas clay-rich, poorly sorted sediments, and unfractured rocks often form aquitards. The term aquiclude has been used for describing an impermeable unit, but this term has become obsolete. (1)

Aquitard

An aquitard is any geological formation of a rather semipervious nature that transmits water at slower rates than an aquifer. Freeze and Cherry (1979) describe an aquitard as the less-permeable beds in a stratigraphic sequence. These beds may be permeable enough to transmit water in quantities that are significant in the study of regional groundwater flow, but their permeability is not sufficient to allow the completion of production wells. In an interlayered sand-silt sequence, the sand may be considered as aquifer, whereas in the silt-formation aquitard clays, shale, and silty clays are the stratigraphic units, which can be considered as aquitards. (2)

Aquifers

The term aquifer can have many different definitions. For the most part, an aquifer can be described as an underground geological formation capable of receiving, storing, and transmitting large quantities of water. (1)

Semi-confined Aquifer

An aquifer that is partially overlain by a rock formation which has low permeability, through which water can pass only slowly to recharge the aquifer. (4)

Water Table

The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. (3)

1. S. Ge, S.M. Gorelick, in Encyclopedia of Atmospheric Sciences (Second Edition), 2015

2. Zekâi Şen, in Practical and Applied Hydrogeology, 2015

3. "What is the Water Table?". imnh.isu.edu. Retrieved 2016-11-25.

4. https://www.oxfordreference.com/view/10.1093/acref/9780198609957.001.0001/acref-9780198609957