We are not connected to the mains and only use water from our underground spring. The water has to run through a softener and a filter then it is fine and is tested annually so that we can drink it. We also use it fill the pool. As far as the pool is concerned, filling from a well should be not be a major problem, rain water falls out of the sky dircetly into the pool after all, but you will need to watch out for: 1. Dirt and stones 2. Algae! Especially if it is an open well, you might find yourself investing in a lot of Chlorine 3. Deposits around the water line depending on water softness 4. Ph can change dramatically every time you top up the pool You could end with a lot of work to save a relatively small amount on your water bill. Good luck