Mumbai University > Civil Engineering > Sem 7 > Irrigation Engineering

Marks: 10M

Year: May 2016

1. The main source of ground water is precipitation. A portion of rain falling on the earth’s surface infiltrates into ground travels down and, when checked by impervious layer to travel further down, form ground water.
2. The ground water reservoir consists of water held in voids within a geological stratum. Other sources of ground water reservoir consists water from deep in the earth which is carried upward in intrusive rocks and water which is trapped in sedimentary rocks during their formation.
3. The quantities of such water are small and they are often so highly mineralized as to be unsuited for use.
4. Water bearing formations of the earth’s crust act as conduits for transmission and as reservoirs of storage of ground water.

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.

Zone of Saturation:-

1. 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.
2. 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.
3. 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.
4. 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.
5. As the depth of the saturation layer increases, the interconnected openings filled with water become so few, that they are almost non-existent.
6. 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.