wivell,
If your pond is surrounded by higher ground (even at quite a distance), there's a fair chance the spring won't run backwards.
If you start packing the seep, and it's real hard to stop the water, that's good news. It means the spring has pretty good pressure and may never run backward. If it's easy to pack and stop the leak, that's good news too.
If you had a neighbor with a well, maybe you could buy some water and fill it, and see what happens.
