Since the buckets were in a curatorial facility, presumably they were geological samples, probably from the Orphan Mine, situated on the South Rim not far away, and now no longer active. It was a critically important mine in the 1950s.

I dealt extensively with curatorial issues during my career in the NPS and I can guarantee that simply putting samples in a bucket is not the proper way to handle any museum specimen, even if not radioactive. It is feasible to properly store and care for radioactive items, but care is required - special packaging for sure.
Geezer in Chief