The first week I came to live in Guatemala from the USA 3 brothers from a nearby village dies of asphyxiation at the bottom of a hand dug well which had a generator running too close up above.
I have a large well for my water (machine bored - wells here have terrible recovery), when the pump went out a few years back I looked down the hole and decided no way will I climb down there. I bought a pump and used hoses and extension cords to lower it into the well from above, then a week latter hired someone to come out and do the proper install. They had the proper safety gear so they couldn't fall down the hole. More importantly they knew how to use the - I know where to buy the gear, but that is one case where improper usage will kill you before you find out you were wrong.
I'm going to declare a new law, a la Bettredge's Law: "When a small probability is claimed, without evidence, to be less than that of death by lightening strikes, it is safe to assume the small probability is in fact higher than the risk of death by lightening strikes".
Find me the evidence and I'll admit that my new law is invalid. :)
I don't need evidence to know that the odds of casualties falling into a modern well (that is a few inches in diameter) such as the one being drilled here are less than being hit by lightning (assuming lightning is possible).
I won't rule out the possibility of an ankle sprain.
The biggest risk of construction comes from handling the concrete rings. I haven't heard of a suffocation accident. These are much more common when people maintain their sanitation tanks.
I think it will solve itself as people move to steel pipes which are generally less labor intensive to drill. They also eliminate the danger of falling into the well.
Don't risk it. Confined spaces are dangerous.