(subsurface water, underground water), water located in rocks in the upper portion of the earth’s crust in a liquid, solid or gaseous state. Depending on the type of openings in the water-bearing rock, subterranean water is divided into interstitial water, occurring in sand, gravel beds, and other fragmentai rock; fissure water in dense rock (granites, sandstones); and karst water (fissure-karst) in soluble rock (limestones, dolomites, gypsums).
The subterranean water that moves under the influence of the force of gravity is called gravity, or free, water in contrast to the bound water held by molecular forces and known as hygroscopic, film, capillary or crystallization water. The layers of rock saturated with gravity water form aquifers, or beds constituting water-bearing complexes. Subterranean water varies in its permeability and yield (the ability to flow out of the waterbearing rock in response to the force of gravity).
The first permanent nonartesian water-bearing stratum below the surface is called the groundwater layer. Directly above the groundwater surface (groundwater table) lies capillary water, which may be perched, that is, it may not be in contact with the groundwater table. The entire area from the surface of the earth to the groundwater table is called the aeration zone, within which water seeps from the surface. In the aeration zone, during periods of groundwater recharge, certain rock interlayers with a lower filtration capacity temporarily accumulate subterranean water known as leakage water.
The aquifers that lie beneath the groundwater are separated from it by strata of impermeable rock or semipermeable rock. These aquifers are called middle-water strata. They are usually under hydrostatic pressure, although occasionally they have a free surface and are nonartesian. The water-intake area of middle water occurs in places where the water-bearing rock outcrops or where it lies close to the surface. Middle water is also recharged by the flow of water from other aquifers.
Subterranean water is a natural solution containing more than 60 chemical elements, chiefly K, Na, Ca, Mg, Fe, Al, Cl, S, C, Si, N, O, H, as well as microorganisms, which oxidize and reduce various substances. As a rule, subterranean water is saturated with gases, including CO2, O2, N2, and C2 H2. In terms of its degree of mineralization, subterranean water is divided (V. I. Vernadskii’s classification) into fresh water (up to 1 g per liter), brackish water (from 1 to 10 g per/), saline water (from 10 to 50 g per l), and underground brine (more than 50 g per l). In later classifications, water with a mineralization exceeding 36 g per l was called underground brine. According to temperature, a distinction is made between supercooled subterranean water (below 0°C), very cold (from —41°C to —20°C), cold (from 0°C to —41°C), warm (from 2°C to 37°C), hot (from 37°C to 50°C), very hot (from 50°C to 100°C), and superheated (above 100°C).
Subterranean water is divided into several types based on its origin. Infiltration water is formed by the seepage of rain water, snowmelt, and river water from the surface of the earth. In terms of its composition, such water is predominantly calcium hydro-carbonate and magnesium water. As a result of the leaching of gypsum-bearing rock, calcium sulfate water is formed, and the dissolution of salt-bearing rock produces sodium chloride water. Condensation subterranean water is formed by the condensation of water vapor in rock pores or fissures. Sedimentation water is formed during geological sedimentation and is usually altered buried water of marine origin, usually sodium chloride water and calcium sodium chloride water. Such water includes the buried brines of halogen basins, as well as the ultrafresh waters of sand lenses in moraine deposits. Water formed during the crystallization of magma or during rock metamorphism is called magmato-genic, or juvenile water (E. Suess’ terms).
One of the indicators of the natural conditions under which subterranean water is formed is the composition of dissolved and freely released gases. The upper aquifers, where oxidation occurs, contain oxygen and nitrogen; the lower parts of the cross section, where a reduction medium predominates, have gases of biochemical origin, such as hydrogen sulfide and methane. In areas of intrusions and thermometamorphism, water is saturated with carbon dioxide, for example, the acidulous waters of the Caucasus, the Pamirs, and Transbaikalia. Around the craters of volcanoes, acid sulfate waters are found, known as fumarolic springs.
In many water-pressure systems, which are frequently large artesian basins, three zones have been identified that differ with respect to the intensity of water exchange with surface water and the composition of the subterranean water. The upper and marginal parts of the basins are usually occupied by infiltration fresh water of the zone of active water exchange (according to N. K. Ignatovich), or active circulation. In the deep central parts of the basins there is a zone of extremely slow water exchange, or stagnant regime, where highly mineralized waters are found. The intermediate zone of relatively slow or obstructed water exchange contains mixed waters of varying composition.
The distribution patterns of subterranean water depend on many geological and physicogeographic factors. Within platforms and foredeeps, artesian basins and slopes have developed. In the USSR, they include the Western Siberian, Moscow, and Baltic artesian basins. On platforms, there are large areas with an uplifted Precambrian crystalline basement where fissure waters are found, for example, the Ukrainian Crystalline Massif and the Anabar Massif. Folded-mountain areas also have subterranean water of the fissure type.
Specific hydrogeological conditions that determine the circulation and composition of subterranean water are created in areas where permafrost rock has developed. Here superperma-frost, interpermafrost, and infrapermafrost waters are formed.
Subterranean water is part of the earth’s resources. The total reserves of subterranean water exceed 60 million cu km. Subterranean water is regarded as a mineral. In contrast to other types of minerals, the reserves of subterranean water are replenished in the process of exploitation. Aquifers or their complexes that yield subterranean water of a composition that meets established requirements and in a quantity sufficient for economic use are called deposits of subterranean water.
In the USSR subterranean water is divided according to its use into household-drinking, technical, industrial, mineral, and thermal water. Household-drinking water is fresh water that has certain taste qualities and does not contain substances and microorganisms harmful to man. Industrial water with large amounts of such chemical elements as I, Br, B, and Li are of interest to various industries. Subterranean water containing specific components, either gases or trace constituents, is used for medicinal purposes and as beverages.
In certain instances, subterranean water causes the formation of swamps, flooding, landslides, or ground subsidence under engineering works. It may also hinder mining excavation and mining operations in shafts and open pits. To reduce the flow of subterranean water into an area that has industrial installations, such techniques as drainage and the draining of mineral deposits are employed.
Many of the qualitative and quantitative indicators for the parameters of subterranean water (level, head, flow rate, chemical and gas composition, temperature) are subject to brief or seasonal changes or to changes over many years or centuries. These changes determine the regime of the subterranean water. The regime reflects the formation of subterranean water over time and within a certain area under the influence of various natural factors (climatic, hydrological, geological, and hydrogeological) and factors arising from human activity. The greatest fluctuations in the elements of the regime are observed in subterranean water close to the surface.
In the USSR, there are more than 100 hydrogeological stations, including some 25,000 observation points, which provide information on the regime of subterranean water. The regime is studied in planning construction, in working out measures to prevent or eliminate salinization and the formation of swamps, in compiling forecasts of the water-salt balance in irrigated areas, and in assessing and predicting the influx of water into mining excavations.
Each year in the USSR forecasts are issued of the prespring minimum, the maximum, and the autumn position of the water level in the zone of intensive water exchange. The forecasts are issued in the form of charts showing changes in the level of subterranean water.
Hydrogeology is the study of subterranean water.
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A. A. KONOPLIANTSEV