Nick,
You MIGHT still be able to seal the pond for less expense with proper compaction.
What is your max depth? A one foot clay blanket can readily seal to 10 feet of depth.
Procedure:
Drain pond and let dry.
Disc deeply. Use the heaviest disc that you can get into that small space. A 24" inch disk blade set at 25% depth would get you 6". You might try to set a little over 25% if that is workable for your conditions. (Your disc operations would have to be done such that you are NOT working clay high on the sides of the ponds down into the bottom!)
Get a water source and sprayer to the location. Spray lightly, then cease while water percolates downwards. Repeat as needed. The goal of this step is to have the clay dry enough at the surface so as not to be runny and slick, but wet enough that when you dig down 6" with a shovel and grab some clay, you can start to make a clay snowball.
Run in a sheepsfoot type compactor. Either pulled behind a tractor, or a self-propelled unit. Compact until the feet walk up out of the clay. You may have to lightly spray some more water during this process if the clay is drying out. (Dry clay cannot be effectively compacted.)
No guarantees on that procedure. However, it may be doable DIY if you own a tractor and can rent the necessary compaction equipment. Your builder could probably also do that work with his existing equipment plus what he may need to rent.
I totally agree with your builder that if a properly constructed and compacted pond heavily leaks, then a liner is a good solution. However, your pond was NOT properly compacted.
Maybe talk to your builder about my solution above and get his "chances of success" and a bid estimate? That would be much cheaper than a liner plus installation - but no guarantee.
Finally, a good, honest local pond builder >>>>> always greater than a random dude on the internet from far away!
