Manganese is the one that usually stains black.
What it is
Manganese is a metal dissolved out of the same rock that gives up iron, and the two usually appear together in Michigan wells. It stays dissolved underground where there is no oxygen and converts to a black oxide once it reaches air. Labs report it in milligrams per liter, and it is usually present at much lower levels than iron.
What it usually does
Very little manganese goes a long way on a stain. It usually leaves gray-black smears in toilets and dishwashers, black grit in aerator screens, and laundry marks darker than iron leaves. It also oxidizes more slowly than iron, which is why a filter sized only for iron usually lets manganese through.
What it does not prove
A manganese number does not describe the rest of the water, and it does not by itself say what treatment will hold. Michigan has a secondary standard for manganese set on staining rather than on health, so a result above it usually means a laundry and fixture problem rather than anything else.
When to test
Usually worth testing with iron and pH as a set whenever black staining shows up, and before anybody sizes an oxidizing filter. Testing manganese alone usually leaves out the two numbers needed to size anything.
See what this usually means for your water.
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