Speaking specifically to this picture
https://www.atomicarchive.com/history/trinity/afterwards.html
In: 685
The photo was taken in early September 1945, on a press “safari” to the Trinity site orchestrated by General Groves in order to try and disprove assertions that were being made about possible long-term contamination effects of radioactivity at Hiroshima and Nagasaki. So it was a deliberate propaganda photo of sorts.
The Trinity test did make the area immediately around the detonation intensely radioactive at first. Samples were taken from the test site in a lead-lined tank, and even that had limits to how close it could go.
But the drop-off in radioactivity from nuclear fallout is very steep. [Within 14 hours](https://remm.hhs.gov/RemmMockup_files/dose-rate-decay.png) of a nuclear explosion the radiation levels will have dropped to about 1% of what they were when it immediately went off.
Let us imagine that at 1 hour after the detonation, the test site radioactivity was a massive 10,000 R/hr. Exposure to 500 R is often a fatal dose for people. So that is radioactive enough to kill you in 3 minutes.
The simple rule for calculating radiation decay in fallout is called the Wigner-Way t^-1.2 law, and it basically is: R = D x t^-1.2 , where R is the “current” dose rate, D is the dose rate you are starting with, and t is the number of hours. So if D is 10,000, and t is 48 hours, then R = 96 R/hr, or 1% of the original. That’s still a lot of radiation! 100 R will give you radiation sickness, but probably not kill you.
OK, so what if we wait a week? Then it’s down to 21 R/hr, which is still not something you ought to be exposed to (it’s way higher than background), but it’s more in the “might raise your chances of cancer in the long run.”
The photos in question were taken about 60 days after the detonation. So that gets you down to about 2 R/hr. That is… still hot-enough that today, if you had a choice, you’d probably want to give that a pass. For comparison, the EPA doesn’t let people working in the nuclear industry get exposed to more than 5 R per year. The general public is not supposed to pick up more than 0.025 R per year beyond their normal, natural radiation dose. So 2 R/hr is what we would today consider to be pretty hot. If you are there for an hour, that is a low-enough dose that you wouldn’t expect any short-term health issues, and if a small number of people were visiting it only for a bit, you wouldn’t expect to be able to detect any meaningful increase in cancers. However, if you had people _living_ there, especially in large groups, it would be a bad idea.
All of the above assumes we know the starting radiation (10,000 R/hr) which is not a terrible assumption for the order of magnitude around the base of the tower. But we don’t really know that for sure. But there would potentially have been areas near ground zero (perhaps a bit downwind of it) with levels at that order of magnitude. So even though it is somewhat arbitrary, it’s not _totally_ arbitrary to go with that number for a weapon of this size, and an area like ground zero. But I would emphasize this would be the most intense part of the fallout — we are working from the worst case parts. Most of the fallout downwind of ground zero would be a lot less intense.
You’ll notice in the photos if you look carefully that they are wearing little booties. They are trying to avoid tracking the contaminated dirt back home with them — it’s one thing to visit it and then leave, it’s another to visit it, get contaminated stuff on your shoes, and then take it home where you (and your family, your kids, your pets) can inhale it.
The tricky thing about fallout is that it contains a lot of different elements, many of which are radioactive versions of elements your body wants and needs (or similar-enough to them that your body treats them like them), and so they can get inside you and sit there for a long time, radiating you over the long-term. So Strontium-90 is a nasty, medium-lived isotope in fallout that your body (and the ecosystem) treats like calcium, and so can get embedded inside your bones. So the raw exposure from visiting the site isn’t necessarily the total exposure at all.
Anyway the (shorter) answer is that because of the time passed, the radiation was a lot lower than what it was when the bomb went off, but it was still higher than most people who are telling you that it was safe probably appreciate.
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