It's not for the wafers, it's for the equipment. Parts get coated up with various substances, and then have to get stripped back as part of regular maintenance. One of those substances is arsenic.

To sandblast the oxide layer off the GaS ingots.

Whoa there, cowboy. GaAs doesn't really make an oxide, and the last thing you want to do to an ingot is bead-blast it. You want to be in a room full of GaAs dust?