Is Sweetwater, Missouri, named due to water contamination with lead compounds?
According to The State Historical Society of Missouri:


  Sweetwater is the name of a little trading-point established on Sweetwater Branch just after the Civil War. It was named for the branch.


and


  Sweetwater Branch was named by the pioneers for the unusually good water in the stream.


The place (and branch) are both in Newton County, Missouri. The same reference linked above indicates that what was once called Sweetwater is now referred to as Boulder City (because of the boulders nearby).