It shouldn't be too much of a problem. Just make sure you age the water for more than 24 hours to reduce the chlorine. You can use the dechlorinating chemicals, but why waste money when you only need to age it.
Your problem could come from a sudden change in water hardness and pH going from well to city water. For instance, my well water were I first kept turtles was full of iron and slightly acidic. I took some baby Spotted turtles with me to my first year of College. The city water at the University was full of limestone and slightly basic. Spotted turtles prefer acidic water, and they simply didn't thrive...appetite loss and lethargy. I began to purchase reverse osmosis water from a very reputable tropical fish store and added some blackwater extract. They thrived once again. In short, keep an eye on your turtles. Testing both water sources and comparing might not be a bad idea. Well water can vary tremendously.
It's great that you're looking into this. Many people wouldn't even give it a thought. What kind of turtles? The snakes should be fine with aged water, unless they are some very delicate species.