The trouble is that arsenic is a “semi-metal”. Above ground its primary use was as a wood preservative, often blended with an almost equally toxic copper solution. It was used as rat poison.
Plus, short of reverse osmosis, maximum contaminant level (MCL) of 10 μg/L is insanely hard to reach. I noted that Berkey filters promise to *reduce* arsenic levels, for what that is worth.
Before anything, I would check out what the contaminant levels are of your local water. As I said, the EPA is super strict about this, and it is a lot easier to clean if it isn’t contaminated in the first place.
Thx for info