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Have been noted.
A layer of rock that is sufficiently porous to store water, and permeable enough to allow water to flow through it, is called an aquifer.
Consolidated porous and permeable rocks, for example, sandstone and limestone, can form important and extensive aquifers.
Unconsolidated sands and gravels may also be good aquifers because they are relatively porous and very permeable, but in the UK their saturated thickness is usually quite small and they have limited storage, so they are not important aquifers.
Although the porosity of an aquifer is a measure of the amount of water stored within the pores or fissures, it does not provide a direct measure of the amount of water that may be recovered by pumping or drainage. This is because a proportion of the water is always retained around the individual grains by surface tension, and this is known as the specific retention.
The specific yield is the maximum amount of water that can be recovered. The diagram illustrates how specific yield, specific retention and porosity vary with grain size for unconsolidated sediments. The relationship between specific yield, specific retention and porosity is expressed by the equation:
All three terms in the equation are expressed as percentages of the total volume of the rock. The specific retention decreases with increasing grain size in unconsolidated sediments.(A few large particles would have a smaller total surface area than a lot of smaller particles occupying the same volume, and a smaller surface area retains less water by surface tension.)
This means that less water is retained in coarse-grained sediments. However, the specific yield is greatest for medium-grained sediments (sands), rather than for coarse-grained sediments, because porosity decreases with increasing grain size.
The exploitable storage of water in an aquifer is the volume of water it will yield:
where V is the volume of the aquifer that is being exploited, and Y the specific yield.
It is important to distinguish between the specific yield (a percentage of the volume of the rock) and the exploitable storage (a volume of water).
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