Clearly, the bacteria aren't going to magically destroy the arsenic. But eat the arsenic, die and float to the surface of the well where they can be skimmed off

Clearly, the bacteria aren't going to magically destroy the arsenic. But eat the arsenic, die and float to the surface of the well where they can be skimmed off? I'd say that's a simple goal for some genetic engineers to start gunning for. It's also plausible they could metabolize it into some non-toxic form. (We've got a variety of biomolecules we've never seen before, some of which could be non-toxic.)