Arsenic occurs naturally in water. Certainly some activities may aggravate the problem...most notably mining...but it is a natural phenomenon.
the NaF "red herring" is generally NOT a natural occurrence. Absolutely not natural at the levels we inject.
So which is the atrocity?
Chlorination is another issue...because there hasn't been a disinfection method persistent enough to carry the water through distribution. That is definitely a problem of "at what cost"