I'm not sure how much I agree with the theory that Bruce presents. For starters,,, all earthen ponds leak; it is a matter of how much. Ideally when a pond is lined with clay, the purpose of the clay as a liner is to stop, impede, or dramatically slow the water flow out of the pond. In my experiences with well built, clay lined and sheepsfoot packed ponds, with good UNIFORM clay liners, you will get little if any seepage right away when the pond is full. The good well packed clay barrier acts as a seal to hold in water. It does the job.
Occassionally in my area, when a field drain tile is missed and not properly cut off it acts to draw water out of a pond. Porous patches of soil in the upper pond layers are also pathways for water seepage. Water can sometimes seep BETWEEN the sheets or layers of clay that have been packed with just a bulldozer. The well built dugout ponds in our area, even have clay liners that are 8 ft to 10 ft wide/thick (bulldozer blade wide) from the root zone up to the surface. Water has a hard time seepng out of thes ponds when built properly. Well built DUGOUT ponds in NW Ohio clay soils will lose only 10"-14" of water during a fairly dry summer and around 14"-20" in a drought summer. More than that is leakage in my opinion.
In some leakage ponds there can be a certain amount of suspended clay and or sediment that gets pulled and filtered into the areas of the bottom that are seeping out water. This action can cause the seepage to slow down and "somewhat seal" those areas. The success of this is real "iffy". This is what has probably happened in Bruce's example above and he got a little lucky. However, if the porosity of the seepage area allows suspended clay-sediment particulates to pass through, then obviously the seepage or leakage will not stop or slow down; at least not much. If you are losing 5 ft of water each summer I would conclude that you have a substantial leak. I think the contractor missed something in the building process IF there are adequate clay soils in your area AND IF this is a dug out pond. Ponds that are built with dams across a ravine are likely to leak due to inconsitancy of soils in the pond basin and sides. You dig a hole, and you build a pond.
