I am going to give you something to do until someone smarter than me can help,buy a can of penetrating oil,PB blaster or whatever is good in your parts,and everytime you go near the thing spray the shaft at the impeller,after a few days of that you may be able to wiggle it off.i think that is a splined shaft,after all the soaking try a screw driver and rubber mallet and lightly tap the center of the impeler where it touches the shaft,a little patiences will pay off.i hope that helps