Is it unreasonable to ask the builder to fix the leaks by creating and compacting a core on the backside of the dam on his dime?
The worst he can do is say "no", the only way to find out would be to ask. I would be diplomatic about it, even if he ultimately says "no" to doing the repair on his dime he might be willing to do something for you at a very reduced rate or even split his actual cost for doing the repair work.
Before you ask, did you have any type of contract that spelled out fixes if the pond leaked once it was built?
