How do you de-radiate water?

The question is a little vague... naturally radiated water from the ground i.e. radon, requires the water to be essentially filtered to remove the radon gas absorbed in it. If it is irradiated by a solid radioactive source, and unless the source material is in the water itself. Will have close to no if any effect. But to go to the extreme.... Just pickup a distiller...and that is it... you'll have as much water as you want based on it's size.