Yes, but what’s your point? It can’t be arsenic because it would have to be nearly pure arsenic, the chemical that is odorless, colorless, and tasteless, and so wouldn’t be detected unless a chemical analysis is done of the water, and only 1/8 of a teaspoon is lethal?
Why couldn’t it be arsenic? It could be in the ground and leached into the aquifer, as happens sometimes with wells, and it isn’t easily detected. It’s a toxin that only requires a small amount to kill, so why can’t it be what poisoned the well?