Tap water is fine. It helps to put in some water conditioner like Reptisafe to neutralize the chlorine, which is a mild eye irritant for the turtles. If you use distilled water then you don't need to use a conditioner.
Yes you should be treated your tap water with at least dechlorinater. The better water conditioners remove nitrites, nitrates, choloromine, and chlorine. If your water comes from a well then there's no need to condition it.
Almost all tap water is going to have chlorine or chloromine. That's how the public water supply is treated to make it safe. You can call your local water department to make sure but almost sure to have been treated this way.
It's safe to say a water conditioner (which removes chlorine et al) would be a good idea. Reptisafe is a good kind.
If your tap water comes from a city supply that more than likely as SpotsMama says, it's treated with chemicals which need to be removed for your turtle. If you get your water from a private well, then more than likely it's not treated and it should be fine for your turtles.