Tim,
Sorry I did no make myself clear.. If it is the kind of pump union I think it is, it will have two O-Rings.. One is between the pump body and the union and the other is between the two union halves. From your pic, it appears to be O-Ring between the pump body and the union is leaking.. The only way to get to that O-Ring is to remove the lower half of the Union..
I just don't see a problem here.. The unions allow you to just disconnect the pump and move it out of the way, which will allow you to remove the lower part of the union and replace the O-Ring or just inspect it, lube it up and reinstall it. While I had it apart I would also look at the intake union.
In "theory" when you rely on the threaded connection to prevent leaks (Air or water) they tend to leak after a few years.. The whole point of a pump union is that you don't rely on the threaded connection, you rely on the O-Ring between the pump body and the union. You can see the O-Ring on the threaded end of the union, if you look at the pics in the link I posted.
Thanks,
Jim R.