I don't know of any radon measurements unless its just a standard water quality test. I don't know how expensive it can be but maybe a radon measuring device might be interesting to keep on hand.

On another note - I've heard that magnetic oddities can be a precursor - I have a note-board on my fridge - it is held there with magnets - its been up about a year - this morning it was lying on the floor - and it no longer sticks to the fridge... odd.

Canie