Water that lies beneath the earth’s surface, and fills in the pore spaces, cracks, and fractures in rocks and sediments found beneath the earth’s surface are termed as underground water, groundwater, subsurface water, or subterranean water. Water can be termed as groundwater when it moves beneath the earth’s surface. Groundwater occurs everywhere beneath the earth’s surface and is usually restricted to a depth of about 750 meters or less.
The definition of groundwater in practice is quite complex. The groundwater includes water that is contained in the soil, which is in the intermediate unsaturated zone. It also includes water that is in the capillary fringe (zone above the water table) and which is below the water table. The soil comprises of the broken down and weathered rock along with the decaying plant debris at the surface.
Groundwater or phreatic water is subsurface water that absolutely saturates the pore spaces above an impermeable layer.
Water found in the pore spaces, cracks, tubes, and crevices beneath the surface has been termed as underground water, groundwater, subsurface water, and subterranean water.
Sources of Groundwater:
Diagram illustrating relationships of genetic types of water (Courtesy The Geological Society of America, 1957).
1. Meteoric Water
Water derived from atmosphere. Includes water from lakes, rivers, Rain, Snow etc.
It originates in the atmosphere, falls as rain and ultimately becomes groundwater by infiltration. It forms the major part of the groundwater.
It is the water which is derived from precipitation i.e. snow and rain. It includes water in lakes, rivers, and ice melts, which has fallen from precipitation indirectly. It originates in the atmosphere by evaporation which falls as rain and ultimately becomes groundwater by the process of infiltration. This infiltrating water continues its downward journey to the zone of saturation to become a part of the groundwater in aquifers, thus forming the major part of underground water.
2. Connate Water
It is the water which includes in the rock itself, at the time of rock formation when water gets trapped in rock strata. Because rock containing connate water is typically formed from ocean sediments, it is normal saline. In geology and sedimentology, connate waters are liquid molecules which got trapped in the pores of sedimentary rocks at their time of deposition. These waters also contain many mineral components in the form of ions in solution.
The connate liquids are expelled as rocks get buried and undergo lithification. If the escape route for these fluids blocks, the pore fluid pressure builds up in the rocks leading to overpressure.
Water that was out of contact with the atmosphere for at least an appreciable part of geological period. Consists of water derived from fossils characterized by high mineral content. May have originated from either fresh or oceanic water.
(Water trapped in Sediments)
a. Syngenetic Water: Trapped in sediments containing it.
b. Epigenetic Water: Enters from basin to rocks that had formed earlier.
3. Magmatic Water
Magmatic water, also known as juvenile water, is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth’s crust and is released to the atmosphere during a volcanic eruption.
Water that derived from magma.
a. Volcanic water: If they are originated at shallow depth (e.g., 3-5 km)
b. Plutonic water: If they are originated at deeper depth.
4. Juvenile Water
It originates in the earth’s interior and reaches the upper layers of the earth’s surface as magmatic water.
It is also known as magmatic water as it originates in the earth’s interior and is associated with the magmatic activities below the crust. Magmatic water rises from great depth accompanying magma intrusion. With the cooling of magma, its contents, gaseous and vapor etc. separate from it. The water vapors gets condensed into heated water and gradually move upward from the area of high temperature and pressure to that of low temperature and pressure. This is also called virgin water.
5. Metamorphic Water
Metamorphic waters are defined as waters that have been associated with rocks undergoing metamorphism (and/or diagenesis) and dehydration. Such types of groundwater were identified in many sedimentary basins in the world.
Water that is or has been associated with rocks during their metamorphism
6. Condensational Water
It is the prime source which replenishes groundwater in deserts and semi-desert areas where precipitation is scanty, and there is rapid evaporation. During summers, the air over land is warmer than the air trapped in the soil, which lead to pressure difference between the two. Because of the pressure gradient, the atmospheric water vapor penetrates the rocks and gets converted into water through condensation due to falling temperature of the water vapor below. This process may lead to the accumulation of a certain amount of water in rocks in arid and desert regions.
7. Fossil Water
Fossil water or paleowater is an ancient body of water that has been contained in some undisturbed space, typical groundwater in an aquifer, for millennia. Other types of fossil water can include subglacial lakes, such as Antarctica’s Lake Vostok, and even ancient water on other planets.
Water becomes dry.
(May be originated from the fact of climate change phenomenon, i.e., some areas used to have wet weather and the aquifers of that area were recharged and then the weather of that area becomes dry.)
Importance of Groundwater
Groundwater is critical in supplying fresh water to streams and wetlands. The water is used mainly for irrigation, manufacturing, domestic and other purposes. In the United States, 80 to 90 percent of the fresh water is extracted from groundwater. Due to limited availability of surface water, there is an increased dependence on groundwater. Initially it was thought to be of an unlimited quantity and naturally protected by the soils above it. Later it was found to be of overuse and improper use with disposal of chemicals at the land surface. The proper use and protection of groundwater require an insight and an understanding of the groundwater system.
Out of the total earth’s water, 97% is salt water which is in the oceans; the remaining 3% being fresh water. Groundwater is a significant part of the hydrological cycle, containing 21 percent of Earth’s freshwater. Groundwater makes up about 1% of the water on the Earth (most water is in oceans). It makes up to 35 times the amount of water in lakes and streams. Groundwater is a component of the hydrologic cycle, which tells about the occurrence of water and the processes by which it moves.
Water covers 71% of the Earth’s surface. On Earth, 96.5% of the planet’s crust water is found in seas and oceans, 1.7% in groundwater, 1.7% in glaciers, and 0.001% in the air as vapor, clouds (formed of ice and liquid water suspended in air), and precipitation.
- Professor Dr. SUMANT DANDPATH SIR (Professor Shivaji University Kolhapur)