If applied properly it's a forever thing. I researched this a year or so ago. Determine what type of soil then go to a chart that shows lbs/sf of bentonite to be applied. Disk or till into the soil. Compact the soil. For optimum results, then apply layer of bentonite across pond bottom then cover with dirt. As G said, if the bentonite is not covered, over time it will lose its effectivness. It was going to be an expensive proposition for us to expand our mudhole to 1/4 acre. We have creek stone not far below the surface. When the knee deep water warms up a bit, I'm going to wade out with a probe to see how far down the stone is.Is bentonite a compound that will stay in forever or does it dissipate over time?