Fluoride
Fluoride is added to the water to protect teeth, as required by state law passed in 1969.
http://www.cdc.gov/fluoridation/
According to the American Dental Association, persons who drink fluoridated water have a 20% to 40% reduction in the number of cavities that would have occurred without fluoride; and the use of fluoridation in drinking water has been recognized by the CDC as being one of the 10 greatest public health achievements in the 20th century.