I also have a small pond that is about 18 by 20 by 5 -6 feet deep. It was built on a slope and has a 3 foot high clay dam built up on one end. As it is a very moist spot the dam didn't get compacted properly when it was built.

I don't want to use a liner as there is a very small spring in the bottom of the pond and I rely on this to water a large garden.

I have had some success with a baseball bat. As the water level has gone down over the summer and the bank has dried a bit, I've followed up on this first compaction with a sledge hammer and the back of an axe with a heavy head.

It's a lot of work but it seems to be working. We are having a very hot dry summer here and all the ponds in the area are already down to the lowest I have ever seen them. My pond is at the same level it was in January during the wettest part of the year, before I began compacting it and inserting bentonite into the softest mushiest places in the dam. (There is a thread on how I did this titled "An Easy Way to Fix Some Leaks".)

My pond is about 16 inches down from being completely full, which is pretty good considering I have used at least 10 of these to water my garden. But I am still working on it as I think the small spring should keep it completely full if it wasn't leaking at all. So every day I am out there for my 15 minute work out, going whack whack whack, in the hopes I will get it so it stays full, next summer.