Iron occurs in natural waters in both ferrous and ferric condition,
depending on the source of the sample. In ground waters the iron is
usually in an unoxidized and soluble condition, sometimes combined with
carbonic or sulfuric acid, and also in combination with organic matter.
Many waters, especially those that have been exposed to the air, contain
the iron in the form of a colloidal hydroxide. Silt-bearing waters often
contain much iron in suspension, usually in an oxidized form. Sewages
and sewage effluents, particularly those receiving manufacturing wastes,
contain various forms of iron of different degrees of solubility,
oxidation, and coagulation.