underground waters enclosed between impermeable strata and under hydraulic pressure. They occur for the most part in pre-Anthropogenic deposits in large geological structures forming artesian basins.
Artesian waters uncovered artificially rise above the roof of the aquiferous layer. When there is sufficient pressure, they flow out onto the surface of the land and sometimes even gush. The line joining marks of the established pressure level in boreholes is known as the piezometric level.
As opposed to groundwaters, which are involved in the contemporary water exchange with the surface of the land, many artesian waters are ancient, and their chemical composition usually reflects the conditions of their formation.
Initially, artesian waters were believed to be linked to troughlike structures. However, the conditions under which the waters have formed are quite varied: artesian waters may frequently be encountered where there is foldlike asymmetrical monoclinal stratification of layers. In many regions they are related to complex systems of joints and breaks.
Three areas are discernible in an artesian basin: feeding, pressure, and discharge areas. In the feeding area, the aquiferous layer is usually raised somewhat and drained, so that the waters have a free table. In the area of pressure, the level up to which the water can rise is located above the roof of the aquiferous layer. The vertical distance from the roof of the aquiferous layer to this level is called the head.
As opposed to the area of feeding, where the thickness of the aquiferous layer changes as a function of meteorological factors, the thickness of the artesian layer in the area of pressure is constant over time. Depending on the amount of atmospheric water which enters, during different seasons there may be a temporary transition of free-table waters into head waters on the boundary between the areas of feeding and pressure. In the area of discharge, waters emerge on the land surface in the form of rising sources. When there are several aquiferous layers, each may have its own level, determined by the conditions of feeding and flow of the water. When synclinal stratification of layers corresponds to depressions in the relief, pressures in lower layers rise; where there are elevations in the relief, the piezometric level of the lower strata is located at lower elevations. If, because of a borehole or well, two aquiferous strata are in communication, then where the relief is inverted artesian waters from the upper layer flow into the lower.
Artesian basins and artesian slopes can be differentiated. In the artesian basin, the area of feeding is located alongside the area of pressure; the area of discharge of the pressure stratum is located farther along in the direction of underground drainage. In the artesian slope, the area of discharge is located near the area of feeding.
Every large artesian basin includes waters of different chemical composition—from highly mineralized brines of the chloride type to fresh, weakly mineralized waters of the hy-drocarbonate type. The former usually occur in deep areas of a basin, while waters of the latter type occur in upper layers (in various artesian basins of the USSR, at depths of 100 to 1000 meters).
Fresh waters of upper aquiferous layers form as a result of the seepage of atmospheric precipitation and processes of leaching of rocks. Deep, highly mineralized artesian waters are linked with converted waters of ancient marine basins which were located, during various geological eras, on the sites of contemporary artesian basins.
In the USSR, artesian basins are sometimes called water-pressure systems because of the great diversity of hy-drogeological conditions. The largest water-pressure system in the USSR is the West Siberian artesian basin, which has an area of 3 million sq km. There are large basins of head waters abroad in North Africa and in eastern Australia.
REFERENCESOvchinnikov, A. M. Obshchaia gidrogeologiia, 2nd ed. Moscow, 1954.
Kamenskii, G. N., M. M. Tolstikhina, and N. I. Tolstikhin. Gidrogeologiia SSSR. Moscow, 1959.
A. M. OVCHINNIKOV