Bentonite is a type of clay with unequalled swelling properties. When mixed with water, it can swell to 18 times its dry weight. Its swelling properties make it useful for filling holes and cracks in pond bottoms, and is often applied to pond bottoms when they are constructed. Sodium bentonite can also be used to help seal leaks and help prevent seepage in existing ponds. It is completely natural and cannot harm livestock or fish if applied correctly.
Drain the pond. Pump the water out until the pond is completely dry. This is the best and surest way to seal leaks, although it may not always be practical. Once the pond is empty, use shovels or a tractor to dig up roots and remove rocks and all debris from the bottom. Plough the bottom of the pond to loosen the soil and scrape up about 6 inches of soil off the top and place it on the side.
Sprinkle the bentonite evenly over the entire floor of the pond. A fertiliser spreader works well for this job. Mix the bentonite in powder or granule form well into the loosened soil with a tiller. The density should be about one part bentonite to five parts soil. However the density can vary with differing soil types, and it is essential that you ask the supplier for your local requirements.
Roll the bottom of the pond with a roller. Tamp the bentonite firmly in and compact the soil around it. Spread the top layer of soil back over the bentonite and roll it again to make it smooth. Take special care not to disturb the layer of bentonite. Spread a layer of gravel over the soil to prevent it being ripped up by animal hooves. Refill the pond using a low pressure flow so as not to disturb the bentonite.
Use an alternate method if it is not possible to drain the pond. Sprinkle a thick layer of bentonite granules on top of the water surface as evenly as you can get them. Make certain the pond is not disturbed to allow the bentonite to sink and settle on the bottom of the pond. This is not the best way to seal leaky ponds, but it may control water loss to some extent.