Please check and clarify the question.

Dear Susan

I hope I understand what you mean. 

There is no geographic reason. It is in fact the geological reasons which determine the concentration of fluorine in groundwater. Especially evaporite deposits or some volcanic rocks such as basalt contain high content of F. 

Please follow the following links and pdf attachment for the answer to your query.

http://www.fluorideindia.org/did-you-know/

https://books.google.co.in/books?id=MUja9Rt68I4C&pg=PA67&lpg=PA67&dq=what+are+geological+causes+which+determine+fluoride+in+ground+water&source=bl&ots=3sJagZaMIT&sig=zbGC6wXaaAGneYUWZizM6TvcQJk&hl=en&sa=X&ved=0ahUKEwi13rLD2O_LAhWIj44KHR8bDvoQ6AEISjAI#v=onepage&q=what%20are%20geological%20causes%20which%20determine%20fluoride%20in%20ground%20water&f=false

There is a meteorite crater in Manitoba, Canada (St. Martins' or Gypsumville) in which the fluorine content is relatively high in groundwater and has caused some medical problems.

The F has been attributed to the meteorite, but it can also be due to the evaporite deposits ( see Soroush Modabberi's answer above) or to the crater structure? 

The existence of fluoride minerals like

fluorspar