Groundwater(redirected from subsurface water)
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the underground water in the first permanent water-bearing stratum below the earth’s surface. It is formed chiefly as a result of the seepage of atmospheric precipitation and water from rivers, lakes, reservoirs, and irrigation ditches. In places groundwater reserves are replenished by water moving upward from deeper strata (for example, by water from artesian basins), as well as by the condensation of water vapor.
Since groundwater is not usually covered with rocks impermeable to water and since the water-permeable layer is not always filled to capacity, the surface of the groundwater is free and not under pressure. In some places where there is a local water-resistant cover, the groundwater acquires local pressure (the magnitude of the pressure is determined by the groundwater level in the adjacent areas that do not have a water-resistant cover). When a borehole or an excavated well reaches the groundwater, its level (the so-called water table) is set at the depth where the groundwater was found. The areas of the feeding and spread of groundwater coincide. As a result, the conditions under which groundwater is formed and its regime possess features that distinguish it from deeper artesian water. Unlike the latter, groundwater is sensitive to all atmospheric changes. The surface of groundwater varies from season to season, rising in wet seasons and falling in dry periods. The yield, chemical composition, and temperature of groundwater also vary. Near rivers and other bodies of water, changes in the level, rate of flow, and chemical composition of groundwater are determined by the hydraulic relation between the groundwater and the surface water and the letter’s regime. The flow of groundwater over a long period of time is roughly equal to the amount of water entering by percolation. Intensive processes of percolation and of underground flow, accompanied by leaching of the soil and rocks, develop in a moist climate. Readily soluble salts-chlorides and sulfates-are removed from the soil and rocks. Prolonged water exchange results in the formation of fresh groundwater mineralized only by the relatively poorly soluble salts (mostly calcium hydrocarbonate). No underground flow occurs in arid, warm climates (in dry steppes and semidesert and desert regions) owing to the short duration and small amount of atmospheric precipitation, as well as to the poor drainage of the region. In these regions evaporation is dominant on the expenditure side of the groundwater balance, and the water becomes saline.
The differences in the conditions under which groundwater is formed determine the zonation of its geographical distribution, which is closely related to the zonation of the climate and soil and plant cover. Fresh (or slightly mineralized) groundwater is common in wooded, forest-steppe, and steppe regions. Salty groundwater predominates on plains in regions where there are dry steppes, semideserts, and deserts; here, fresh water occurs only sporadically.
The most substantial reserves of groundwater are concentrated in the alluvial deposits of river valleys, in the alluvial cones of piedmont regions, and in the shallow masses of fissured and karstic limestones (less commonly in fissured igneous rock).
Because of its relative availability, groundwater is extremely important in the economy as a source of water supply for industries, cities, villages, and populated rural areas.
REFERENCESSavarenskii. F. P. Gidrogeologiia. Moscow, 1935.
Lange. O. K. Gidrogeologiia. Moscow, 1969.
P. P. KLIMENTOV