Are you sure it's a spring and not just the water table? How much pressure does the water have? When you dig a hole in the ground anywhere where I live, you will have water "spring" into the hole, this includes my pond. There were veins everywhere where water came in, from the water table I'm sure.
Fill the pond up, the pressure cause leakage
Over the past two years though, this seems to have greatly slowed down to about nothing.
