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Tuesday, February 1, 2011

Aquifers and Aquitards

kind of aquifer

An aquifer is a layer of unconsolidated or consolidated rock that is able to transmit and store enough water for extraction. Aquifers range in geology from unconsolidated gravels such as the Yogyakarta aquifer in the Indonesia to distinct geological formations (e.g. chalk underlying Gunung Kidul
and much of south Java). An aquitard is a geological formation that transmits water at a much slower rate than the aquifer. This is an oddly loose definition, but reflects the fact that an aquitard only becomes so relative to an aquifer. To borrow from a popular aphorism, ‘one man’s aquifer is another man’s aquitard’. The aquitard becomes so because it is confining the flow over an aquifer. In another place the same geological formation may be considered an aquifer. The term aquifuge is sometimes used to refer to a totally impermeable rock formation (i.e. it could never be considered an aquifer).

There are two forms of aquifer that can be seen: confined and unconfined. A confined aquifer has a flow boundary (aquitard) above and below it that constricts the flow of water into a confined area. Geological formations are the most common form of confined aquifers, and as they often occur as layers the flow of water is restricted in the vertical dimension but not in the horizontal. Water and if intersected by a borehole will rise up higher than the constricted boundary. If the water reaches the earth’s surface it is referred to as an artesian well. The level that water rises up to from a confined aquifer is dependent on the amount of fall (or hydraulic head) occurring within the aquifer. This is analogous to a hose pipe acting as a syphon. If the syphon has a long vertical fall between entry of the water and exit then water will exit the hose pipe at a high velocity (i.e. under great pressure). If there is only a short vertical fall there is far less hydraulic head and the water exits at a much slower velocity. To continue the analogy further: if you could imagine that the end of the hose pipe was blocked off but that you punctured the hose, then you would expect a jet of water to shoot upwards. This jet is analogous to an artesian well.

An unconfined aquifer has no boundary above it and therefore the water table is free to rise and fall dependent on the amount of water contained in the aquifer. The lower boundary of the aquifer may be impervious but it is the upper boundary, or water table, that is unconfined and may intersect the surface. It is possible to have a perched water table or perched aquifer where an impermeable layer prevents the infiltration of water down to the regional water table. Perched water tables may be temporary features reflecting variable hydraulic conductivities within the soil and rock, or they can be permanent features reflecting the overall geology.

Source : Tim Davie. 2002. Fundamental of Hydrology.New York : Routledge 270 Madison Avenue.
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