To a certain extent it depends on how deep the well is. I recently moved and when we had a well in our country house it was only 47 feet deep. I had it checked occasionally for bacteria levels and they were never high. I was told it was runoff water and therefor it was exposed to sun and air more than a well say, hundreds of feet deep, so the chance of bacteria is higher. It had an 8 inch casing and I always kept a cover on the top.
I've seen the quiet dead; and I've heard the living moan. This world's no place to live in, but it's home.