There can be multiple layers of water table. When they dug it out they likely hit a sandstone, shale, gravel or limestone rock layer and failed to cover and compact it with clay.
If you can get a good layer of clay comacted over the rocky area you should be fine.
In a true water table pond it would be extremely hard to seal because the pressure outside the layer of clay seal at times would be greater than the pond water trying to leave causing the water to try to push away the seal into the pond. But where yours is in the bottom, as soon as you have enough water filled in the pond to equal the pressure of the water coming in, the clay layer should stay intact. If that makes sense.
In our area we have solid clay so we rarely have to worry about sealing. The exceptions are if we hit a sandstone layer or an ancient creek bed with gravel. Then we have to make sure that area is sealed with compacted clay or the gravel or porus rock can cause a leak.
My main pond dam butts up against a seasonal creek. When digging it we ran into what was likely the old stream bed thousands of years ago. We quit digging in that area and sealed it over with clay best we could (water was slowly seeping in as you describe). Had we not done so it would have been an avenue for water to follow the gravel bed to the existing current creek. I would not swear we do not have some seepage to this creek because the creek area behind the pond never goes dry while other portions of it do. But the seep must be minimal because the pond level in relation to the overflow stays essentially the same as my other ponds and area ponds. So no leak of any significance.
Last edited by snrub; 12/11/17 07:57 AM.
