FHA, as well as all other lenders, have underwriting requirements. You'd be amazed as to how thick the volumes are as to the requirements to lend money to people to buy homes. (But that's another story).
FHA requirements on well water is as follows:
"A domestic well must be a minimum of 50 feet from a septic tank, 100 feet from the septic tank's drain field and a minimum of 10 feet from any property line."
Now the requirement that the well be a certain distance from a septic tank and a leach bed makes sense. As for the distance requirements from a property line, I don't know the rationale. But it's there. If the home you want to purchase does not meet this requirement, then there are two options: Get the seller to dig a new well that conforms to FHA requirements, or else find another house to buy.
Hope this helps.