Groundwater - groundwater of the nearest to the surface of the earth permanent aquifer, which does not have a continuous roof of water-proof soils on top.
Gravity water of the first permanent aquifer from the Earth's surface, located on the first water-bearing layer. It has a free water surface. Usually there is no continuous roof of waterproof rocks over it.
Formed mainly by infiltration (seepage) of atmospheric precipitation and water from rivers, lakes, reservoirs, irrigation canals; in some places, groundwater reserves are replenished by rising waters of deeper horizons (for example, waters of artesian basins), as well as due to condensation of water vapor.
Groundwater from above is usually not overlapped by waterproof rocks, and the water-permeable layer is not filled to its full capacity, so the surface of groundwater is free, unpressurized. In some areas, where there is a local watertight overlap, the groundwater gets a local head (the size of the latter is determined by the position of the water table in the adjacent areas without a watertight overlap). When a borehole or a digging well reaches the groundwater, its level is set at the depth where it was encountered. The areas of groundwater supply and distribution coincide. As a consequence, the conditions of their formation and regime have characteristic features that distinguish them from deeper artesian waters: groundwater is sensitive to all atmospheric changes. Depending on the amount of atmospheric precipitation their surface experiences seasonal fluctuations: in dry seasons it decreases, in wet seasons it increases, the flow rate, chemical composition and temperature also change. Near rivers and reservoirs, changes in level, flow rate and chemical composition are determined by the nature of their hydraulic connection with surface waters and the regime of the latter. The amount of runoff over a multi-year period is approximately equal to the amount of water brought in by infiltration. In conditions of humid climate, intensive processes of infiltration and groundwater runoff develop, accompanied by leaching of soils and rocks. At the same time, easily soluble salts - chlorides and sulfates - are removed from rocks and soils; as a result of long-term water exchange, fresh groundwater, mineralized only by relatively insoluble salts (mainly calcium hydrocarbonates), is formed. In conditions of arid warm climate (in dry steppes, semi-deserts and deserts) due to short duration of rainfall and small amount of atmospheric precipitation, as well as weak drainability of area groundwater runoff is not developed; in the flowing part of groundwater balance evaporation prevails and salinization occurs.
Differences in groundwater formation conditions cause zoning of its geographical distribution, which is closely connected with zonality of climate, soil and vegetation cover. In forest, forest-steppe and steppe areas fresh (or slightly mineralized) groundwater is spread; within dry steppes, semi-deserts and deserts on plains saline waters prevail, among which fresh waters are found only in separate areas.
The most significant reserves of groundwater are concentrated in alluvial deposits of river valleys, in the cones of withdrawal of piedmont areas, as well as in shallow arrays of fractured and karst limestone (less often in fractured erupted rocks).
Groundwater due to its relatively easy accessibility is of great importance for the national economy as a source of water supply for industrial enterprises, cities, towns, settlements in rural areas, etc.