The prepond's bottom is part lined from the exsesive amount of liner I had for the lined pond. The sides/wall of the prepond is not lined. It does however had a heavy about of blue clay on it. I will have to get in there with the big cans of spray foam. Not the stuff you get at the pond store....the bb-q tank size spray foam you get out of the GRAINGER catalog. That stuff sets up quick....and theres plenty of it. That should stop it.