Pond Boss Forums POND CONSTRUCTION Renovating a Pond or Lake? Black Plastic Sheeting

As far as your question about degradation, I believe you are correct., in the absence of UV and air exposure, the plastic sheeting should last longer than you. However, there are a few problems...

Easy to puncture with a few stones, sticks, or hooves. One hole is all it takes to render it useless. For the same reason, non welded seams just wont work. The plastic to soil boundary will be teaming with organisms and plenty of water pathways. Water will simply need to travel further with the plastic, that's it.
It doesn't stretch much before it rips. Think settling. That is why most ponds use rubber materials rather than plastic.
Any back pressure from a water table will cause it to lift from the bottom like a large balloon if it isn't buried.

The thing that bothers me is the chance for plastic to float off of the bottom and become an annoyance or eyesore. I hate the stuff in gardens, it is forever pulling up and getting tangled in things.
I have seen advertisements for one huge contiguous sheet of plastic or rubber that can be used for lining waste water lakes, but something that size would have to be placed in new construction, and it is much heavier material than hardware store stuff.

I would guess you have a leak down deep, where the pond and surrounding water table like to neutralize pressures. It is also possible this is two directional flow. When things are wet, water comes in, when dry, it goes out. This makes plugging via ess materials or pouring in clay not work. You would have to compress or pack clay very tightly to stop that.

Have you thought about lowering your expectations of what is considered full pool? You could contour the exposed banks and bring in topsoil, and deal with what you have. No more leak in a sense, just a shallower pond than you hoped for. sometimes a lot less frustrating and expensive to reset the goal.

What others have suggested is likely true if you want the pond fixed, but it means draining and starting over. It doesn't sound like that is something you are willing to do. However, if it works, you can be done with the frustration once and for all, pending a large earthquake.