Note also that GCNP contains all kinds of radiation sources. I was there with my wife a year ago doing some backcountry hiking.
We went in via Horseshoe Mesa. There are signs near the campground on Horseshoe Mesa warning you to stay out of areas where there was mining activity. We shoved on from there because our permit for the night was for further down rather than any concern about radiation.
More titillating is the warning in the backcountry hiking guide for the Tonto Trail from the Bright Angel Trail to the Hermit Trail:
"There is water in the bed of Horn Creek about half the time, but unfortunately it is radioactive so don't drink it unless death by thirst is the only other option. The source of the radioactivity is a deposit of high quality uranium contained within a collapsed cave system geologists call a breccia pipe." [0]
We went in via Horseshoe Mesa. There are signs near the campground on Horseshoe Mesa warning you to stay out of areas where there was mining activity. We shoved on from there because our permit for the night was for further down rather than any concern about radiation.
More titillating is the warning in the backcountry hiking guide for the Tonto Trail from the Bright Angel Trail to the Hermit Trail:
"There is water in the bed of Horn Creek about half the time, but unfortunately it is radioactive so don't drink it unless death by thirst is the only other option. The source of the radioactivity is a deposit of high quality uranium contained within a collapsed cave system geologists call a breccia pipe." [0]
[0] https://www.nps.gov/grca/planyourvisit/campsite-information....