All that water which occurs below the surface of the earth in one from or another, that is , as water moving down to greater depths under the influence of gravity, or moving in columns and thickness of saturated rocks, the aquifers under definite hydro-static pressure of flowing as underground streams etc, is broadly grouped as subsurface water. The water occurring in definite rock bodies and fully saturating them below a level is further distinguished as Groundwater.
Groundwater, like surface waters, is also very powerful natural agent responsible not only for modifying the existing features but also for creating many other geological features on and below the surface of the earth.
Most of the Groundwater occurring below the surface is derived from the following three sources:
(a) Meteoric Water
(b) Connate Water
(c) Magmatic water
METEORIC WATER– It is derived from recent or modern atmospheric precipitation by mainly rain and snow, which provide surface and sub-surface runoff rivers, streams and creeks. A considerable part of precipitation gradually infiltrates into the ground. This infiltrated water continues its journey till it reaches the zone of Saturation to become a part of the groundwater in the aquifer. Almost entire water obtained from the ground water belongs to this category. Depending upon the continuity and /or discontinuity of the aquifers based on stratigraphy, sedimentology, structural dip, directions of cleat and fracture systems, groundwater develops either local or regional flow.
Water may also be contributed to the groundwater by surface water bodies (which themselves are supplied by precipitation) such as rivers, lakes and seas. In the case of streams this happens when the water table is lower than the water level in the stream. Such streams are often referred as Influent Streams. A reverse condition is also common when groundwater nourishes a stream; in this case water table is higher than the level of water in the stream and such streams are called as Effluent streams.
CONNATE WATER– This is the water present in the rocks right from the time of their deposition in an aqueous environment. During the process of formation of sedimentary rocks in a lake or sea or river, deposition is followed by compaction, which leads to the squeezing out of most of the water present between the sediments. Sometimes however incomplete compaction may cause retention of some water by these rocks this is the connate water and may be encountered in sedimentary rocks like Limestones, sandstones and gravels. This water is highly saline and mineralised and is of no importance as a source for exploitable groundwater.
JUVENILE WATER– Also called Magmatic Water . Water formed in the cracks or crevices or pores of rocks due to condensation of stram emanating from hot molten masses or magmas existing below the surface of the earth.
The water that goes below the surface of the land may be found to exist in two main zones namely as Vadose zone and Saturation zone . The Vadose zone itself is classified into three main sub zones, namely Soil moisture zone, Intermediate Vadose zone and the capillary zone.
Soil Moisture zone – The soil moisture zone forms a thin layer of depth 1to 9 metres held up by the root zone of vegetable cover and soil chemicals. This water is very important for the life and growth of the vegetable cover of the globe. It is lost to the atmosphere by transpiration and evaporation.
Intermediate Vadose Zone- Also known as the zone of unsaturation, water is moving downwards under the influence of gravity. This zone is of small thickness and is also absent in some cases.
Capillary zone– This zone of capillary fringe comprises of soil and rocks of fine particle size underlying the vadose zone. It is absent in the coarse sediments. In the fine particle size zone, groundwater is drawn upwards by capillary action, sometimes to heights of 2-3 metres above the saturated zone lying underneath. The cause of rise of water in the capillaries of fine sediments is well known force of surface tension.
SATURATION ZONE: The water held up in this zone is called groundwater in real sense. The upper surface of water in the zone marks the water table in the area. The layers or bodies of rocks which are porous and permeable, have all their open spaces such as pores, cavities, cracks, etc. completely filled with groundwater. All these openings are thoroughly interconnected so that a well dug into this zone receives water from the entire saturated rock mass, called the aquifer .