I think you're on the right track with your plan to compact the entire basin. With the geology you and I are faced with I don't think tracking down the leak you have now will help for long, the water would just find another path. I know it's generally recommended that people build cores in their dams but I think the best tactic in karst areas is to basically seal or 'line' the entire basin with a compacted blanket of material, whether that means trucking in clay or just compacting what's there. Think of a small pond with a synthetic liner, there would be no need for a core with a rubber liner, because the water would never get to the core in the first place. Same thing when you pack the basin. If the water can't seep into the ground in the first place, it can't find it's way to bedrock, or pipe between the rock and subsoil. I still want to come down and see your pond one weekend. I haven't called yet because it's good dozin weather! Don't give up on it, you'll get there.
