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.

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