I agree with Bill.
Groundwater or a spring, it doesn't make a difference. Neither come with a one-way valve to only let water go one way. If a pond isn't sealed, both will add water to the pond because there is less pressure against the water in the pond area. Once the water in the pond (at the point of where it's leaking in) exceeds the pressure of the water in the ground, it will start to flow out of the pond. Roughly 1/2 psi for every foot depth of water.
re: clay. If you can take the clay and roll it between your palms and make a rope or a pencil out of it, then it's good for sealing the pond. If you can't, try adding or removing moisture. Even good clay can be too wet or too dry to compact properly.
Here's some reading for you if you haven't found it already:
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_030362.pdf
