RAH's situation is a good example of the natural variability of the sediment deposition that creates the sub-soils for our ponds. Every leak is going to be somewhat unique.
However, in that type of scenario, if you knew exactly where the water was exiting the pond, you could excavate a single narrow trench with just a backhoe or small excavator outside of the pond boundaries. If you successfully exposed the edges of the sand body, you could excavate an extra foot, and then backfill your trench with lifts of compacted clay. You would create the equivalent of a "core trench" just to block the pathway that is causing the leak.
In the real world, it is difficult to determine "exactly" where the leak is going, but it is possible.
