Radon in water mitigation
Do you have well water or do you get your water from a public water supply? If it's the latter, then I would certainly contact the water company. If it is the former, there are two main methods of treating for radon in water: granular activated carbon (GAC) and aeration.

I recommend this link for more information...

https://archive.epa.gov/water/archive/web/html/basicinformation-2.html