Manganese?

[Terrierman]

Manganese in drinking water causes problems - mostly with causing more or less permanent black spots on laundry and also staining porcelain sinks and toilet bowls. Just guessing that's what the City of Branson was concerned about as Taneycomo is the primary water supply for the City.  Not toxic in the levels- normally well under .2 mg/l - found in drinking water. 

It's more soluble in it's reduced state so water with oxygen levels that support trout will normally oxidize that manganese and it will precipitate out.