Naturally occurring arsenic in groundwater is the major source. Lots of parts of the US have pretty high concentrations. https://publicintegrity.org/environme

Naturally occurring arsenic in groundwater is the major source. Lots of parts of the US have pretty high concentrations. https://publicintegrity.org/environment/arsenic-levels-in-groundwater-across-the-u-s/ https://publicintegrity.org/environment/arsenic-levels-in-gr...