Bullcrap.
Uranium is mined and is a naturally occurring element. Also, Rock of Ages granite mine can certainly open up fissures for U-238. Nothing like MSM-based emotion-based logic.
Strontium 90 has a half-life of about 30 years, so if you can see it, it was made within the last 300 years (it takes about 10 half-lives for an isotope to become very, very hard to detect). if you detect Sr90 it is not natural, it had to have been made by us, somewhen. Same with Cs137, it’s not natural. Most likely source is Chernobyl.
Modern gear can detect a few atoms of a radioactive substance.