Like Dave alluded to, springs are nothing more than a permeable layer of dirt in the ground that lets water out (or through it) because there is more pressure (most likely uphill) pushing the water out. If the pond is built over the spring, and water pressure in the pond exceeds the water pressure in the spring, it will act as a drain in a bathtub, letting the water out until the pressure equalizes between the ground and the pond.
Before I made plans for a pond, I'd make sure that there was enough good clay in the area of the pond to build the dam and to properly seal the bottom of the pond.
If clay isn't found, then if you want a pond, it would have to be trucked in. At that point, (trucking in clay) start comparing prices of clay vs. a liner.
