To my mind there are a lot of variables here. Dug well or drilled? Sandy soil or heavy clay? Sloped or flat ground? If it's a drilled well and there are no breaks in the cap you should be fine within 10 - 15'. If it's a dug well......then you need to think about all of the variables mentioned above.
I have a dug well that supplies all of our water. I have tried to be super careful with how the animal housing is situated as well as making sure all composting is done down hill from the well. I have heavy soil so it would be harder for contamination of the water to occur from the chickens free ranging but not impossible. For this reason, I buy one of the home test kits for water and check it at least twice a year.