When water reaches the surface of the earth in the form of rain or snow-melt, part of it percolates through the surface soil into the bedrocks. It occupies pores, cavities and other spaces in the crustal rocks.
Thus, water present in the cracks and pores of rocks of the lithosphere below the surface of the land is called ground-water. Some other terms are used for the ground-water such as underground-water and subsurface water.
There are mainly three principal sources of ground-water. First and most important is the rain water. It is called meteoric water. It may be in the form of rain or snow,. This water percolates into the rocks through their joints, pores and cavities.
Some of rain water sinks into the ground along the bedding planes of folded sedimentary or meta-sedimentary rocks. The second source of groundwater is the connate water which is fresh or salt water caught up in the sediments during their deposition.
It occurs mainly in different types of sandstones. Connate water is locked in these porous beds by surrounding impervious rocks-like shale’s. This water when heated up during metamorphism or by igneous activity constitutes the hydrothermal solutions which make up the mineral veins, and are the source of hot springs.
The third source of groundwater is called magmatic water. There are some minerals which contains water. When due to internal earth processes rocks become liquid and rise to the earth’s surface, the magmatic water escapes from the magma and spreads out into the surrounding rocks.
This water is highly mineralized. Such hot and mineral-rich water which reaches the surfaces from great depths is known as juvenile water.
Some scientists exclude vadose water because it occurs above the water table. Vadose water moves freely under the influence of gravity above the level of saturation or water table in permeable rocks.
Depth to which ground water sinks:
The depth to which ground water sinks is different at different places. Actually it depends on the presence of pores and openings in rocks through which water can infiltrate. The limit of depth in this case depends upon the nature of rocks. Along with increase in depth, the pressure of the overlying rocks also increases. At a depth of about 9 kilometers the pressure of rocks is sufficient to close all the pores and cavities in the rocks. Beyond this depth the downward movement of water is almost impossible. However, most ground water occurs within a thousand meters of the surface of the earth.
Modes of occurrence of ground water:
The following are the three modes of occurrence of ground water:
(i) In most humid regions regolith and soil are saturated with water to about 25 meters below the surface. Water for wells is available from this supply zone.
(ii) A layer of sand or gravel holding considerable quantities of water bounded above and below by impervious rocks is known as aquifer. Along such aquifers water moves at a slow rate for long and short distances. An aquifer acts as a container and transmitter of water.
(iii) The hard bedrocks contain very little ground water. Whatever little water is there, it occurs in joint cracks and fault fractures.
It may be pointed out that materials that are dense and compact and not very permeable cannot hold much water and tend to restrict the movement of such ground water. A relatively impermeable and impervious (non-porous) layer that restricts the flow of water and limits its storage is called an aquiclude. If a permeable layer-an aquifer-is found between two aquicludes, water cannot pass upward or downward.
Below a certain level, all the pores and fissures of rocks are saturated with water. The upper surface of this ground water is known as the water-table. Thus, water table may be defined as “the surface or undulating plane below which the ground for some distance is saturated with water.”
The water table is not regular, and it follows the relief of the ground. It is much lower under the surfaces of hills than those of valleys. The water table is lowered during dry season and rises again during wet periods. Seasonal changes in the height of the water-table are of great significance in connection with water-supply. Besides, the influence of water-table on stream flow is considerable. In humid regions, ground water flows into major stream channels through out the year, so that the streams flow continuously. Where there is a dry season, the water table may drop below the stream bed with the result that the stream dries up until the next wet season.
Aquicludes do not allow water to pass through it. Water percolating downward may be prevented from reaching the zone of saturation. So an accumulation of such water above an aquiclude is called a perched water table. It is separated from the normal water table by layers of unsaturated rocks.
Immediately below the land’s surface there are three successive zones:
(i) Zone of aeration:
This zone lies immediately below the surface and does not retain the percolating surface water. This zone, however, allows water to percolate to lower zones. It is through this zone that most water not used by plants penetrates to the underlying zones. It is also known as the zone of non-saturation. It may be pointed out that the excess surface water percolates through this zone where soil and rocks are less than saturated and some pore spaces contain air. Zone of aeration is also called Vadose Zone. Beneath this zone, water fills all openings in the soil and rock. This is the phreatic zone, the domain of ground water.
(ii) Zone of intermittent saturation:
This zone extends from the highest level reached by ground water after a period of prolonged wet weather, down to the lowest level to which the water table recedes after a period of drought.
(iii) Zone of permanent saturation:
This zone extends as far down as the limit beneath which there is no trace of ground-water. Wherever the zone of permanent saturation rises above the ground level, one finds seepages, swamps and lakes or rivers. In this zone pores contain only water and no air. Like a sponge made of sand, gravel and rock, this zone stores water within its structures filling all available pores and cavities. The depth at which rocks are found to be dry varies considerably according to the rock types and structures. But it is commonly of the order of 600 to 900 meters. Juvenile and expelled connate water may ascend from much greater depths.