Occurrence of ground Water
Groundwater is the water present beneath the ground surface. Groundwater occurs in abundance under the ground surface, which constitutes more than 30% of all fresh water on the globe. Groundwater has been percolated all through the top crustal surface of the earth. Beneath most of the land areas the groundwater occurs in three zones: (i) Zone of Aeration, (ii) Capillary Fringe, and (iii) Zone of Saturation.
- Zone of Aeration: The zone of aeration extends from the ground surface down up to the capillary fringe. This is the unsaturated zone in which the pore spaces of the rock are partly filled with water and partly with air. The rainwater moving vertically downwards to the water table passes through this zone. A certain amount of water is always held in smaller pore spaces due to molecular attraction. All the water which exists in the zone of aeration is called ”vadose water”.
- Capillary Fringe: The position of the capillary is in between the zone of aeration and the zone of saturation. The water in this zone is connected with the zone of saturation and is raised above it by capillary forces. In coarse-grained soils, the capillary water rises only a few centimeters above the water table, but in fine-grained soils or clays, it may rise to a height of 10 to 15 meters.
- Zone of Saturation: This zone extends from the water table downwards. In this zone, all the pore spaces of rocks are completely filled with water. As the openings in the rocks decrease with depth, the lower limit of the zone of saturation is commonly found within a few hundred meters of the earth’s surface.
The upper limit of the zone of saturation is known as the ”water table”. The water table may be defined as a gently curved surface below the ground at which the ”vadose zone” ends and the saturation zone begins. It represents the level to which a well would fill with water.
The water table surface is obtained by connecting the level of water in wells. It is not a horizontal surface. It generally follows the surface topography in a subdued form. The water table fluctuates with the amount of precipitation. The outcrops of the water table are springs and beds of the rivers. In these places, the water drains out of the land. Swamps occur when the water table is right at the surface. In desert regions, the water table may lie hundreds of meters below the surface.
The depth of water table at a place represents a balance between the rate of infiltration (recharge) and the rate of discharge at rivers, springs, or pumped wells. Any imbalance such as seasonal fluctuation of rainfall raises or lowers the water table.
Perched Water Table:
At some places, a local impervious bed (aquiclude), such as a lens-shaped bed of clay occurs in a permeable rock formation above the main water table. An isolated body of groundwater may occur above this impervious layer in the zone of aeration. Such groundwater is called ”perched groundwater”. The local water table, thus formed is called the ”perched water table” and the water-bearing rock is called the ”perched aquifer”.
Water Table Maps:
The water table is not a horizontal surface. Its form and slope vary depending upon: (i) the area of recharge and discharge, (ii) pumpage from the wells, and (iii) permeability of the aquifers. The configuration of the water table surface may be shown by contour lines. In this case, all points on a given contour will represent an equal elevation of the water table. Such a map is called a ”water table contour map”. These maps are used in analyzing the direction and the rate of groundwater movements.
Permeable rock formations which store groundwater and transmit a sufficient quantity of it to a pumping well, are called ”aquifers”. The impermeable beds that do not yield water and hinder or prevent water movement are called ”aquicludes”. The aquifers are divided into two groups: (i) Unconfined Aquifers, (ii) Confined Aquifers.
- Confined Aquifers: A confined aquifer is that in which aquicludes present both above and below it. The aquicludes restrict the movement of the groundwater and as a result, in the confined aquifers the groundwater moves under pressure. The water pressure in an aquifer depends on the difference in height between it and the recharge area. If the difference is enough, the water will readily flow out a well drilled into it. Such wells are called ”artesian wells”. A region supplying water to a confined aquifer is called the ”recharge area”. In the recharge area, the rainwater infiltrates underground through the soil.
- Unconfined Aquifers: The aquifers in which groundwater occurs under atmospheric pressure are called ”unconfined aquifers”. If a well is drilled in an unconfined aquifer, the water level in it will represent the water table.
In confined aquifers, because the groundwater exists under pressure, the ”pressure surface” is found in place of the water table. The pressure surface is an imaginary surface, which coincides with the hydrostatic pressure level of the groundwater in the aquifer. If the pressure surface lies above the ground surface, the well will be a ”flowing artesian well”, and if the pressure surface is below the ground level, the wells will be artesian but non-flowing, that is water will rise above the level where it is initially encountered but it will not rise up to the ground surface.
When a well is drilled in a confined aquifer, the groundwater in it rises towards the surface. Such a well is called ”artesian well”. The conditions necessary for the artesian flow are as follow:
- Presence of an aquifer which must be confined between two impermeable beds. These beds prevent the water from escaping.
- The aquifer must be inclined so that one end of it is exposed at the surface from where it can receive water.
When such an aquifer is tapped, the pressure created due to the difference between the height of the water table in the recharge area and the bottom of the well will cause the water to rise. The friction reduces the height of the pressure surface. The greater the distance from the recharge area, the greater the friction and less the rise of water.