All water occurring beneath the ground surface is known as ”Ground Water”. It has been estimated that the volume of water in the upper 0.8 km of the continental crust is nearly 20 times greater than the combined volume of water in all lakes and rivers. The chief source of groundwater is the downward percolation of rainwater (meteoric water). Some groundwater may also be derived from Juvenile and connate water. Juvenile water is the term used for magmatic water, while connate water is entrapped in the sedimentary rocks during their deposition.
The hydrological cycle is a simplified description of the ways in which water moves from one place to another and how much is transported. The water in the atmosphere condenses to form clouds and then falls as rain and snow. The rainwater divides itself into four parts:
- Much of the rain soaks into the ground by infiltration and forms groundwater.
- Another part flows over the surface as ”runoff” and finds its way into the rivers. Ultimately, it reaches the sea.
- Some of the water in the ground returns directly to the atmosphere by evaporation.
- Another part is absorbed by the plant roots, which transport it up to the leaves by a process known as ”ascent of sap”. The ascent water is thus transpired through the pores of the leaves to the atmosphere.
Some of the groundwater may come out upon the surface again as springs or through wells but a large part of it reaches the sea through underground circulation. From the sea, the water is again drawn into the atmosphere by evaporation thereby completing the hydrological cycle.
Erosion carried out by the groundwater:
The groundwater does a lot of erosional work mainly by solution. The physical erosion is negligible as the groundwater moves very slowly through bedrocks. The erosive action of the groundwater is most conspicuous in regions where easily soluble rocks, especially limestones, underlie the surface. The water charged with the carbon dioxide dissolves limestone very easily. The common solution structures found in limestone are as follows:
- Sinkholes: These are funnel-shaped hollows of varying sizes, which are made in the carbonate rocks by the solvent action of groundwater. A sinkhole may form when a cavern roof collapses or it may develop slowly as the material is dissolved and carried away in solution. In regions with many sinkholes, streams are often absent as following a rainfall most of the runoff is funneled below the ground.
- Caverns: The tunnels and the underground chambers, which are formed in limestone by the solution of rock by groundwater are called ”Caverns”. These stand without collapsing the roofs. Joints are the natural avenues through which groundwater moves. The dissolution of rocks gradually enlarges the joints and produces a complex system of caverns. The caverns are believed to have formed at or just below the water table. The stream’s water may sometimes flow through these underground caverns.
- Solution Valleys: With the continued solution of limestone, the closely spaced sinks and solution basin are enlarged into a big valley called ”solution valley”.
- Karst topography: The irregular terrain produced largely by the underground solution of limestone, is called ”karst topography”. The karst topography is characterized by numerous sinkholes, caverns and underground channels. This topography develops in a humid climate.
- Stylolites: When the groundwater percolates through bedding planes of limestone, the more soluble parts are dissolved easily leaving the less soluble parts as ridges. In adjacent beds, these ridges project into each other forming a zigzag line along the junction of the bedding plane. Such a structure is known as ”stylolite”.
Transportation and deposition carried out by the Groundwater:
The materials dissolved by the groundwater are carried in the solution until they are deposited. Some materials may also reach the sea through underground percolation thereby increasing the salinity of the seawater. The deposition of the dissolved materials from the groundwater takes place by (i) loss of carbon dioxide from water, (ii) evaporation, (iii) decrease of temperature, (iv) fall of pressure, or (v) chemical reaction.
Depositional features of the groundwater:
- Stalactite and Stalagmite: These features are found in caverns. As the water drips from the roof of the cavern, evaporation leaves a small deposition of Calcite behind. Gradually a cone-shaped pillar of Calcium Carbonate hanging from the roof of the cavern develops. Such a deposit is called ”stalactite”. Similar deposits also grow from the floor of the cavern, where water drops. Such pillar-like forms, which grow upward from the floor of the cavern is called the Stalagmite. When a stalactite and stalagmite meet, a column is formed.
- Geode: Sometimes in the cavities of rocks the groundwater deposits crystals of quartz, calcite, and other minerals. These deposits are called ”geodes”.
- Replacement: The groundwater generally contains silica or calcium carbonate in solution. This solution sometimes replaces a portion of the country-rock or any other substance it may meet. The replacement involves the solution of pre-existing substance and deposition of an equal volume of silica or calcium carbonate from the solution. In replacement, the original structure of the dissolved substance is commonly preserved. For example, in petrified wood, the cellulose is replaced by silica and its woody structure remains intact.