While tannins in drinking water are not harmful, they can cause yellowing of glasses and fixtures and poor water taste. Tannins (humic acid) appear as a result of water passing through decomposing vegetation and are sometimes accompanied by iron or manganese. Water high in iron or manganese may appear red, rusty, brown, tan, black, or greenish in color.

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If you have a shallow well, you may have noticed water turning brown during rainy periods. This happens because surface water containing high levels of tannins often seeps into shallow wells.