The water level and resulting radiation attenuation has nothing at all to do with the steel being "low-background steel" (1) it's just that's where you typical

The water level and resulting radiation attenuation has nothing at all to do with the steel being "low-background steel" (1) it's just that's where you typically find untouched pre-1945 steel objects. The contamination occurs during the manufacture of the steel. 1) > Low-background steel is any steel produced prior to the detonation of the first nuclear bombs in the 1940s and 1950s https://en.wikipedia.org/wiki/Low-background_steel https://en.wikipedia.org/wiki/Low-background_steel