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Why are there black stains on my bathroom sink and when I rinse cups and turn over to air dry on paper towel a black ring mark is left and also the cup is dry but has black marks on the inside of the cup and wipe it is dried black also? Sink also slow drain.

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    Would put water in a clear glass and leave it sit, probably something in the water. Slow drain is another separate question, drain cleaner, plunging, and/or snake to fix.
    – crip659
    Apr 26 at 14:33
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    Does it also smell bad? Sounds somewhat like black sulfur, but there are other things that make dark stains in the presence of oxygen after being clear dissolved in water, such as iron or manganese. Are you on a well or municipal water? A water test seems advisable if you are on a well. Public water you can request to look at the suppliers required tests.
    – Ecnerwal
    Apr 26 at 14:40

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You probably have high iron and manganese in your water.

https://water.unl.edu/article/drinking-water-wells/iron-and-manganese-water

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