Only a few areas have naturally occurring fluoride that will benefit dental health. In other places it is added to the water. In San Benito County we do not add fluoride to the water supply nor does our well water have enough naturally occurring fluoride to benefit our teeth.
Community water fluoridation is effective , safe, and inexpensive way to prevent tooth decay. Fluoridation benefits Americans of all ages and socioeconomic status. Good scientific evidence supports the use of community water fluoridation and the use of fluoride dental products for preventing tooth decay for both children and adults.
Fluoridation of community water has been credited with reducing tooth decay by 50%-60% in the United States since World War II.
Fluoride in the water reduces tooth decay by 25% in children, and helps adults fight tooth decay when gums recede.
Check to see if you water is fluoridated
https://nccd.cdc.gov/doh_mwf/default/default.aspx