During the water cycle impurities get into the water from its environment. The water evaporates in the atmosphere, and after the water vapour is condensed in the upper atmospheric layers the water falls out to the Earth surface as precipitation, thus creating surface run-off and ground water flows and aquifers. The precipitation water infiltrates into more or less deeper soil layers where the water is collected above impermeable rock formations, flows over them and again comes out to the surface at the outcrop areas of such rock formations and then enters surface run-off.

Then water moves in rivers and springs to lakes, seas, water-storage reservoirs, thus completing the water cycle. Along with the natural there is a residential and industrial water cycle resulted from water consumption for various purposes (refrigeration, municipal water supply, etc.).

Impurities enter the water at all stages of above mentioned water cycles, thus dividing natural waters by their origin and impurities content to atmospheric waters (rain, fog, snow), surface waters (rivers, lakes, swamps), ground waters (artesian wells, pit wells), and sea waters (seas, oceans).

When the water vapour is condensed in the atmosphere, oxygen and carbon dioxide are dissolved in such condensate according to the partial pressures of these gases; besides significant amount of sulphur oxides and other products containing in flue gases are also dissolved in the condensate in the industrial areas. Total salt content of atmospheric precipitations amounts to 10 mg/dm3. While infiltrating through the ground the water interacts with various salts (NaCl, NaSO4, MgSO4, CaCO3, silicates, etc.) and organic substances, dissolves them or captures them mechanically. Simultaneous presence in water of oxygen, organic substances, and microorganisms (bacteria) permanently contained in the surface soil makes it possible to transfer the main components of a number of organic substances to the mineral acids (carbon to carbonic acid, nitrogen to nitric acid, sulphur to sulphuric acid, phosphor to phosphorus acid, etc.). These acids generated in ground waters interact with limestones widely spread in nature - calcium carbonate, ferrous carbonate, CaMg(CO3) dolomites, and other rocks, as a result highly soluble Ca, Mg, Fe hydrocarbonates enter the water.

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* The text is taken from:
A.S. Kopylov, V.M. Lavygin, V.F. Ochkov, "Water-treatment in power energetics"
(M. MEI Publishing House, 2003)

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