Unless wanting a "water table pond", the entire pool area of a pond has to be lined with a minimum of 1 foot of well compacted, quality sealing clay on the sides, and 2 feet of compacted clay on the bottom.

Springs need to be sealed off. As esshup said, springs can give water, but they can also act as drains if the pond level is higher than the water level of the spring source. (water always flows to it's lowest level)