It's from copper plumbing.
The stain can be removed with ammonia or ammonia and soapsuds. It can
be removed with salt and vinegar. Many acidic cleaners advertised for
rust stains will work on copper stains. Zud is an example.
Too much copper in drinking water can cause nausea, diarrhea, pains,
inflammation, gas, and brain lesions.
If your water is picking up copper, it may also be picking up lead, from
brass fittings and solder. Lead causes anemia, kidney damage, behavior
changes, and retardation.
In America, state, county, and municipal governments will often check
household water for free and tell a resident how to correct a problem.
On Mon, 12 Jan 2004 16:24:38 +0000 (UTC), "Edward Martin"
Close the drain. Soak the area for an hour using a tablespoon of each
of the following: household bleach, vinegar, and warm salt water. If
the stain is not near the drain, dampen the area (with water) and
plaster a paper towel over the stain before applying the mentioned
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