I rebuilt my pond and tripled it's size. I went deep through blue shale and hit a seam of coal. I was afraid it would leak, but had no clay to seal it. It did leak showing up in a hay meadow 200-300 ft down the hill. I used 20 $5.00 bags of bentonite and sprinkled it out of a boat.
It didn't seal completely, but would say sealed 90%. Plan to add another 20 bags sometime, but pond is staying full.
Kent
