I don't see a hole, maybe just my computer monitor but that appears to me to just be excess casting flash material on the surface. You could try to scrape it off with a chisle by hand or use a die grinder lightly with a small diameter, 120 grit cartridge roll. That's one of the things detailing an impeller addresses. If there is very much material there to remove the impeller will likely need to be re-balanced or at least checked. That's the high pressure area of the impeller, I'm surprised the apparent cavitation hasn't removed it already.
