A type of ferric iron which never settles out is colloidal iron. Radon is considered carcinogenic when inhaled by humans, usually when it breaks free of the water during aeration such as in a shower. Radon can be removed from water by aeration vented to the outside or activated carbon. Radium can be removed from water by cation exchange softening. Tannins - Tannins occur in water in almost any location where large quantities of vegetation have decayed.
Tannins can impart a faintly yellowish to brown color to water which never settles out.
They make coffee and tea taste bad.