After you put out the bentonite, can you back fill the rest of the ditch with clay, then compact? You don't need the ditch anyway. If the pond bottom is clay, and the ditch is sand....hmmmm...maybe push some clay....?
Here's another thought....25,000 gallons per 12 hours is just shy of two acre inches per day. That's significant flow rate. That's about a six month period to fill the lake, if evaporation is low, with no rainfall. That means you should be close to being able to maintain pond level all year, unless you still have a big leak.
