If you know how they get inside the pump house, those entrances can possibly be plugged with wads of steel wool. The varmints won't or can't chew through it, and it's flexible enough to stuff into cracks and corners. Relatively inexpensive, I used it to keep some big city mice out of my apartment once upon a time, but it should stop a rugged farm rat just the same.
RE: Oh Rats, They Are At It Again!