POWDERED CARBON(?)

At work we have the archetypal water coolers, which I make use of fairly frequently. Sometimes I discover one to be empty, so I do the thing where I take the top off a new bottle and replace it.

Something I'm noticing, though, is that lots of times, these bottles have this weird dust on them that I wipe off before I put them on the cooler: an extremely fine, uniform black powder. What in the gibbon is that. Does the water company do that "activated charcoal filtration" or whatever, and is this somehow a byproduct thereof? That's my guess, anyway.