Based on your description, it does sound like there is organic material in the dark clay.

Organic material in the core trench is a big no-no.

Perhaps there was a small pond there 30 years ago, they scraped the clay and organic debris out of that pond basin for the material in the dam core they built back then. (Reason doesn't matter.)

If there was a 1" diameter branch in the core trench material, and that has now decayed to create a water pathway, then that alone could create a leak that your garden hose cannot keep up with at full pool. Further, there could also be additional tiny leaks where small root networks were in the compacted fill, but have now decayed and created a perfect leak path ... shaped just like a tree root system.

You might need another core trench either to the pond side or the back side of the centerline of your dam. I don't know which side is optimal since you are moving beyond my expertise.

Perhaps you could get by just trenching and repairing the 35 yards that is consistently moist?

A core trench is also sometimes called a "keyway". That is because it locks the dam into place to resist the pressure of the water in a full pond trying to push the dam downslope.

IMO, your dam is probably "structurally sound" for that purpose. You just need to improve the "seal" in your dam. If you do try to repair only a portion of the dam, then any secondary clay trench must be tied to the existing clay in the core. Otherwise, the water will hit your barrier through the pre-existing leaks, and then move sideways until it can find a new path around your repairs.



P.S. There is also a member that is a leak expert (teehjaeh57). He sells a commercial product that can seal leaky ponds in some situations.

He also does free advice/consulting on pond leaks for members. You might contact him for more advice. I don't think his product would seal a 1" pipe (like my decayed tree branch speculation). However, his product would probably work on multiple tiny leaks (like my tree hair roots speculation).

tj@hudlandmgmt.com
402.730.4897