In the book i'm currently working on, the main protagonist has a special (yet unknown himself) mutation because his ancestors have been involved in nuclear disasters while their mother was pregnant. more precisely: his grandmother was conceived a few months before Chernobyl, in Pripyat. After the Soviet Union dissolved, she managed to escape to Fukushima, and was pregnant with the protagonist's mother during the incident there. The mother then moved then moved to Belgium, and was in Antwerp during a terrorist attack on Doel around 2034, when she was pregnant with the protagonist.
He ends up on a kinda futuristic version of Earth, with Belgium being the global powerhouse. Most of this was caused due to the inhabitants of Belgium having had similar radiation back around 100 BC or something like that, basically when they were being invaded by the Roman Empire. The effect of the radiation has faded for about 500 years now, and the radiation itself hasn't been around for over 2000 years. However, the muttaion, combined with a certain chemical in the atmosphere, gives above-average attributes to the person infected with it, explaining why Belgium managed to conquer most of the world.
the problem is that I need to find a way to expose a 30,000 km² area to a constant yet low amount of radiation, that only lasts about 50-70 years (enough for 3 generations to be born), in a way that appears coincidental to casual observers, almost a freak of nature. The idea is that they won't have a method to even know that there was radiation for a while, long enough to make most signs of the radiation disappear, even in the affected people.
The real reason for the radiation is that it's been placed there by a precursor race that has their own goals for humanity, so a solution that can feasibly be done by such a race is preferable.
Basically, what I want to do is expose Roman-Era Belgium to a small amount of radiation for a short timespan, with no way for the Belgae to notice this for at least 1500 years. The method should be doable by an outside force thousands of years in advance.