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Zones of Underground Water
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The various zones are as follows:

a) Zone of Aeration, b) Zone of saturation

  • Zone of Aeration: -
  1. The zone of aeration is layer of Earth where the pores and cracks are filled with air and water. It is sub-earthen region that exists between the Earth's surface and water table.
  2. This zone mainly comprises porous soil & rock. The pores in this layer are usually partially filled with water, while rests of them are filled with air.
  3. Thus, in this layer, air and water are able to interact with each other, leading to aeration of water. Therefore, this zone is known as zone of aeration.
  4. The main source of water in this zone is the rainwater that has percolated through the layer of soil above, in a process known as infiltration. In some cases, water from the water table is also able to rise into the aeration zone, in a process known as capillarity.
  5. Capillarity results from the capillary action of the aeration layer where water is able to climb against gravity from a wet particle to a dry one.
  6. The formation and location of the zone of aeration largely depends on the topography, geology, climate, and drainage. These factors decide the depth of the water table underneath the surface.
  7. Soil water zone: extends from the ground surface down through the major root zone, varies with soil type and vegetation but is usually a few feet in thickness
  8. Vadose zone (unsaturated zone): extends from the surface to the water table through the root zone, intermediate zone, and the capillary zone
  9. Capillary zone: extends from the water table up to the limit of capillary rise, which varies inversely with the pore size of the soil and directly with the surface tension
  10. Water table: the level to which water will rise in a well drilled into the saturated zone

    • Zone of Saturation: -
  11. The zone of saturation is a layer of permeable rock in which the cracks and pores are completely filled with water. As the rainwater keeps penetrating the soil under the influence of gravity through the pores of the aeration layer, it finally reaches the impermeable layer deep below the Earth's surface.

  12. The composition of this layer is such that further penetration is restricted and water is effectively trapped inside the pores of the rocks present there.
  13. As more and more water percolates, down from the aeration layer to the impermeable layer, over a period of time, the accumulated water saturates all the pores up to a certain height above the impermeable layer, forming a new zone. This zone, where all the pores are completely filled with water, is known as the zone of saturation.
  14. The uppermost portion of the zone of saturation is known as the water table or the phreatic surface. It forms the dividing line between the zone of aeration and the zone of saturation.
  15. As the depth of the saturation layer increases, the interconnected openings filled with water become so few, that they are almost non-existent.
  16. The width of the zone of saturation may be very small in areas underlying consolidated rocks having joints tapering at shallow depths or may be thousands of meters in areas underlying thick sedimentary formation.

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$$\text{Figure 35. Zones of underground water}$$

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