Congress itself would not decide what a safe level of arsenic is, EPA experts would.
But as you said the EPA only would have the authority to regulate "clean drinking water", and since arsenic isn't drinking water, then the EPA would have no authority to regulate it unless Congress specifically says so.
But as you said the EPA only would have the authority to regulate "clean drinking water", and since arsenic isn't drinking water, then the EPA would have no authority to regulate it unless Congress specifically says so.