An impeller assembly includes a shaft, multiple scoops spaced circumferentially about the shaft, each scoop including an upper blade portion and a lower blade portion, spaced apart at the leading edges and joined at the inner edges, and a rib extending rearward from the inner edges, each scoop being coupled to the shaft by attachment at the rib. A system and method for mixing gas or liquid into liquid include a vessel for containing liquid, a drive shaft for extending into the vessel, and an impeller assembly adapted for rotating about the drive shaft, adapted for submerging below the liquid surface, and having multiple scoops, each scoop including an upper blade portion and a lower blade portion, spaced apart at the leading edges and joined at the inner edges, and a rib extending rearward from the inner edges, each scoop being coupled to the shaft by attachment at the rib.