Wivell...
Being concerned about the spring going in the opposite direction is a well founded concern because it is likely. The water will flow in the direction of least pressure. If there is more pressure coming from the pond, it will act like a drain. As far as sealing it goes, I will leave that to more experienced people. I am curious to hear their response to your pier question as well. I would think packing the holes with a foot or more of clay would be sufficient, but I have no experience with that.
