I second the vote for Soil Flock. I also am curious about the spring behind the dam, was it always there, or did it show up wen the water started filling the pond?
If the bottom of the pond isn't sealed, then the 10 GPM will might not do you any good, like trying to fill a bathtub full of water with the drain plug removed.......
My personal pond is a watertable pond, dug in sandy soil, so the bottom is not sealed. Right now it's about 6.5' low, the lowest it's been since it was renovated in 2008/2009. My current well produces 25 gpm, and I tested it over a week's time a few years ago to see if it would be enough to keep the pond full, and it wasn't. As the pond level rose, so did the amount of water leaking out through the ground - I stopped the test when I was leaking 18 gpm, and the pond was still a few feet below full pool.
