perched water[′pərcht ′wȯd·ər]
free underground waters that lie the closest to the earth’s surface and have incomplete propagation. Such waters form as a result of the infiltration of atmospheric and surface waters held back by impenetrable or slightly penetrable interfingering layers and lenses or as a result of the condensation of water vapor in rock. They are characterized by seasonal existence. In dry periods they frequently disappear, reappearing in periods of rain or intensive snow melting. They are subject to sharp fluctuations depending on hydrometeorological conditions (the quantity of atmospheric precipitation, the humidity, temperature, and others). Waters that appear at times in swampy formations as a result of excessive feeding of the swamp are also called perched waters. Frequently such waters arise as a result of leaks from water pipes, sewers, swimming pools, and other water-carrying structures, as a result of which the area can become swampy and foundations and basements can flood. In areas where permanetly frozen strata occur, perched water is part of waters above the permafrost. Perched water is usually fresh and weakly mineralized, but it is often polluted by organic substances and has a high content of iron and silicic acid. As a rule, perched water is not a good source for water supplies. However, in cases of necessity measures are taken to artificially preserve it: ponds are built; water is diverted from rivers, guaranteeing a constant supply to the wells being used; vegetation is planted to hold snow runoff; and water-pressure dams are built. In desert regions atmospheric water is drawn off by means of ditches on areas of clayey land (takyry) into nearby sand plots, where a perched-water lens is created, thus forming a certain reserve of fresh water.
REFERENCELebedev, A. F. Pochvennye i gruntovye vody, 4th ed. Moscow, 1936.
A. M. OVCHINNIKOV