Aquifers, Aquicludes and Aquitards

An aquifer is the term given to a rock or soil mass that not only contains water but from which water can be abstracted readily in significant quantities. The ability of an aquifer to transmit 

Map of part of Nottinghamshire showing the water table in the Bunter Sandstone (now the Sherwood Sandstone). 

By contrast, a formation with a permeability of less than 10- m s- is one that, in engineering terms, is regarded as impermeable and is referred to as an aquiclude. For example, clays and shales are aquicludes. Even when such rocks are saturated, they tend to impede the flow of water through stratal sequences. 

An aquitard is a formation that transmits water at a very slow rate but that, over a large area of contact, may permit the passage of large amounts of water between adjacent aquifers that it separates. Sandy clays provide an example. 

An aquifer is described as unconfined when the water table is open to the atmosphere, that is, the aquifer is not overlain by material of lower permeability (Fig. 4.2a). Conversely, a confined aquifer is one that is overlain by impermeable rocks (Fig. 4.2a). Confined aquifers may have relatively small recharge areas as compared with unconfined aquifers and, therefore, may yield less water. A leaky aquifer is one which is overlain and/or underlain by aquitard(s) (Fig. 4.2b). 

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More complex arrangements of aquifers, aquiclude, and aquitards, notably in deep sedimentary basins, are systems of interbedded geologic units of variable permeability. These systems are referred to as a multilayered aquifer system. Such systems are considered more of a succession of semiconfined aquifers separated by aquitards. 

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