# SOME IMPORTANT TERMS

Following are some important terms which are frequently use in the exploitation of ground water reservoir.

1. Aquifer.

• The permeable formations which permit appreciable water to move through them under ordinary field conditions are know as aquifers.
• These are geological formations made of gravel or sand, which possess lot of ground water.

2. Aquicludes.

• These are such formations which contain water in them but are not capable of transmitting.
• Clay formations are the examples of such formations.

3. Aquifuge.

• These are such formations which neither contain any water nor allow any transmission of water through them.
• Rock formations are the examples of such formations.

4. Porosity.

• Porosity is defined as the percentage of the voids present in a given volume of aggregate.
• Mathematically it can be expressed

Porosity $= \frac{Total- volume -of -voids }{Total -volume -of-the-soil}$

$= \frac{V}{V}\times 100$

• The porosity of rock and unconsolidated material may vary considerably.
• Generally porosity does not exceed 40% except in very poorly compacted materials.
• For uniform loose sand and dense sand it is 45% and 35% respectively. For clays, it may vary from 20% to as much as 75%. Porosity as worked out for soils

5. Specific Yield.

• The amount of ground water that can be extract by gravity drainage from a saturat water bearing material is know the yield.
• When it is expresse ratio of the volume of the total material drain,  then it is know specific yield.

∴ Specific yield $= \frac{Volume -of -water- obtain- by- gravity- drainage}{Total- volume -of -the -material -drain}\times 100$

6. Specific Retention.

• The quantity of water retain by the material against the pull of the gravity is term as specific retention or field capacity.
• Specific yield or field capacity

$= \frac{Volume -of- water- held -against -gravity -drainage}{Total -volume- of- the -material -draine}\times 100$

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