Note This is really a comment as I have no idea about the conditions in Bad Gastein, but I am making it an answer because there is too much information to put in a comment.
From How Does Radon Get Into Your Home? you can see that Radon is a gas that can permeate from the ground into buildings
Radon is a radioactive gas. It comes from the natural decay of uranium
that is found in nearly all soils. It typically moves up through the
ground to the air above and into your home through cracks and other
holes in the foundation. Your home traps radon inside, where it can
The issue is more about radon being trapped in enclosed spaces than in the open air.
But your comment about drinking water made me go looking for information and I found:
Radon and Drinking Water from Private Wells which states that
While most radon-related deaths are due to radon gas accumulated in
houses from seepage through cracks in the foundation, 30 to 1,800
deaths per year in the United States are attributed to radon from
So drinking water contamination is a potential issue.
I live in an area where radon is an issue and that doing a radon test is part of the inspection when buying a house. I know it can be mitigated with various pumping and sealing techniques, but as I said at the top, I know nothing about what you will encounter in Bad Gastein