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The groundwater constitutes a major source of water for drinking and other economic activity. The major source of ground water is rain. The rain water seeps into the Surface and collects in the various layers of the earth's crust. If a hole is bored in the crust, it will pass through the following three zones:
(i) Zone of Aeration. The open spaces of the regolith or the bed rock are filled with air. When water falls on the ground it is stopped in this zone by the presence of clay layer on its upper part. The particles of the clay prevent the percolation of water by attraction. This layer of the zone is called soil water.
(ii) Zone of Saturation. The soil water layer cannot prevent all the water from percolating to the lower layers, some water is held up and the rest reaches the zone of saturation by passing through the Intermediate belt. The pores in the rocks are filled with water. The water table is really the upper surface of this zone.
There are many small pores in the layers above the water-table. These pores resemble capillaries, which are small tubes with very small diameters. Water rises in the capillaries of their own. Hence the level of water in saturation zone is higher than the water table.
(iii) Zone of Rock Flowage. The pressure of upper layers on rocks 16 km. deep is so high that the pores in the rocks are closed. Hence water is not available below this zone.
It is usually seen that the underground water is not obtained below a depth of 600 meters. The flow of underground water depends upon the following two factors.
(I) Porosity of Rocks. The amount of water that is locked up in rocks depends upon the porosity of the rocks. The ratio of the total volume of the open spaces in rocks to the total volume of the rocks is called Porosity of the rocks. The porosity of sand and gravel ranges from 25 to 45% but the porosity of hard granite is hardly 1%. The porosity of a rock depends upon size, arrangement, closeness and compaction. The seeping water dissolves the salts of the rocks and increases their porosity.
(II) Permeability of Rocks. The water flows through rocks. Some rocks allow more flow of water than others. This capacity of rocks to transmit water through them is known as Permeability of Rocks. The presence of pores in rocks is not the only means of transmitting water. Joints and fissures are formed in rocks and are also responsible transmitting water. Even if a rock has porosity, i.e., it has many pores, but the deposition of mineral salts have separated the holes
There is a difference between porosity and permeability. A couple of examples will make it clear. Coarse grained sand and gravel are highly permeable. Clay and Shale are porous because they have very fine pores but are not permeable. Aquifers
A part of rock or sediment through which the underground water flows is called aquifer. Sand and gravel accumulations are good aquifers but clay is not a good aquifer. Some sand-stones are good aquifers provided the deposition of minerals has not closed their pores. Shale, igneous rocks and metamorphic rocks do not act as good aquifers. In case these rocks have developed fissures and joints, they act as good aquifers.
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