Because water dissolves numerous substances in large amounts, pure water rarely occurs in nature. Precipitation absorbs carbon dioxide and other gases, as well as traces of organic and inorganic material from the atmosphere. Because water reacts with minerals in the soil and rocks, surface and groundwater may contain many different dissolved substances. Surface waters may also contain domestic sewage and industrial wastes. Groundwater from shallow wells may contain nitrogen compounds and chlorides, but water from deep wells generally contains only dissolved minerals.
Seawater contains many soluble compounds in addition to salt.
Impurities are removed from water by screening, sedimentation, filtration, chlorination, or irradiation. See also Sewage Disposal. Aeration saturates water with air, usually by spraying fountains of water into the air. Aeration removes odors and tastes caused by decomposing organic matter, industrial wastes, and some gases. Various salts and metals cause hardness in water.