This is the correct answer. There're tons of microparticles of radioactive monzonite (and other radioactive minerals) in the dirt. And if you're near a granite

This is the correct answer. There're tons of microparticles of radioactive monzonite (and other radioactive minerals) in the dirt. And if you're near a granite body, you will show even higher levels of radiation. Downvoters need to pick up a geology book.