Wikipedia (https://en.wikipedia.org/wiki/Zone_Rouge) states that "as arsenic can constitute up to 17% of some soil samples" and references "Bausinger, Bonnaire, and Preuß, 2007".

I found this study (https://hal-insu.archives-ouvertes.fr/insu-01267232/document) but I am afraid I don't know how to read the results.

I assume that 17% is the absolute maximum proportion of arsenic ever found in a single small sample.

I don't really know how to read this either, but the number 17 as such does not appear anywhere in that paper. This quote may be relevant: "Concentrations of As vary between 72,820 and 1,937 mg/kg" (page 13).

These commas appear to be thousands separators, so that's a maximum of 73 grams per kilogram or 7% per mass of a soil sample. I'm not a doctor, but I would advise against eating significant amounts of these soil samples.

I'm not a doctor either, but I would advise against eating soil altogether.

Eh, it's good for building up the immune system.