I would not attempt to fix a leak from the back side. To do it right it needs to be in front of the dam and done like is done in the center.

You say you do not want to lose water volume, you won't. Just dig until you reach a clay sub base ( My pond core trench was 4' deep by 10' wide for the entire length of the dam). And refill with good clay soil and make sure it is well packed.

Also, make sure there are no exposed rock vanes.

Wesley
Dallas, GA