I suspect you are on to something here. The average Massachusetts resident gets 475 MilliREM per year from radon. But the geology of Maine and New Hampshire has far more of the pegmatites that are the source of uranium and thorium minerals, such as uranophane (Can be collected at Ruggle's Mine, for example) and monazite. North of me by some distance, a deep (425 ft.) artesian well in pegmatite continually loaded water filters, and the homeowner brought it in for me to look at. It was plugged mostly with limonite, a hydrated iron oxide. But a thought occurred to me-(I am the Radiation Safety Officer for a Fortune 500)- I checked it with my counters, and it was emitting more than 8 mREM/ hour- Sufficiently high so the filter could not be legally mailed or shipped by UPS without a lot of special provisions and packaging.
What can you say to the homeowner...Never take a shower again?
Radon and Thoron (Rn220) are short-lived isotopes in the uranium and thorium decay series, respectively, and they have many "daughters" on the way down to lead 206. Before they are Radon, they are polonium and radium, to name a few, so these certainly are in the water also, as well as the U and Th. And some of these daughters are pretty hot alpha emitters.
I would not be at all surprised if your suspicions were correct. In fact I would be surprised if they were not.