I refurbished an old farm pond and expanded it a couple years ago. This old pond had a breech in the dam and during a drought was almost dry. It had a seep on the backside of the dam ever since I can remember as a kid because there was a place we did not drive the tractor because of the potential to get stuck.
When I dug all the muck out with the dozer had a lot of extra dirt. Also expanded the pond from about a half acre to an acre.
The point, which I am actually going to get to, is as I was doing all this I remembered the seep on the back side of the dam. It was still a soft spot. What I did, right or wrong, was I dug down with the dozer (almost got stuck and couldn't back out) as deep as I could in this seep on the back side of the dam. Then I filled it and packed best I could with the dozer. Then we proceeded to put fill on the back side of the dam, mostly clay from the new excavated part, hauling with a rubber tired 12 yard scraper packing it in as we went. Made the dam about twice as thick as the original and made it sloped enough so we could mow (original dam was too steep to mow).
Did I fix the leak? Maybe. At least it is not wet where it used to be. There is a small seasonal creek right on the back side of this dam so potentially I might have only pushed the seep lower where it is not noticeable. But I think if I did not actually "fix" the leak, I must have pretty well slowed it to almost nothing. At least this pond looses water to evaporation no faster than my other ponds or ponds in the area. So there must not be a leak of any significance.
So my point is, though it is generally recognized you can't fix a leaky dam by working on the back side of it, if you do have enough dirt to almost build a second dam and if you do pack it in well, you can slow or stop a slow seep. Or at least push it lower out of sight.
My results. Yours may vary. But if you end up having a lot of extra dirt from your excavations, might be worth a try. No self respecting contractor would recommend it. But that description does not fit me so I did it.
