Sodium bentonite is a clay that swells to 18 times its dry size when wet. When applied to the bottom of a pond, it forms a waterproof layer that keeps the water in. Bentonite is totally natural and will not pollute the water or harm animals that drink from the pond, and if applied correctly, does not harm fish. Bentonite is often applied to the bottom of ponds during construction, but can be effective in sealing leaks if added to a pond full of water. Hand tools can be used for small ponds, but farm implements or construction equipment may be needed for larger bodies of water.
Drain the pond. If possible, drain the pond completely so you can seal the entire pond bottom, which will help seal existing leaks and prevent future leaks. Bentonite comes mixed with powder or in granule form. Either will work, but powder is better suited to this type of sealing procedure. Clean up and remove all debris from the bottom, including rocks and tree roots. Use shovels and rakes for a small pond, or a tractor for a larger pond.
Prepare the surface. Scrape up four to six inches of the soil and fill holes with a mixture of one part bentonite to five parts soil. Smooth with a roller or drag. Alternatively, plough the soil on the bottom of the pond to loosen it and ensure even application of the bentonite, and then roll or drag to make it smooth.
Apply the bentonite. Sprinkle the bentonite evenly to cover the entire surface of the pond. Check with the bentonite supplier for the density required for your type of soil. Even distribution and complete coverage of the area are essential. A fertiliser spreader works well for this application. If you have tilled the bottom of the pond, mix the bentonite into the soil with the tiller and roll or drag to make it smooth.
Cover with soil. Replace the top layer of soil with care. The bentonite layer must not be disturbed. Tamp the bottom to compact it. If the pond is to be used for livestock, spread a layer of gravel over the soil to help prevent their hooves from disturbing the bentonite layer. Refill the pond using a gentle flow. High water pressure will displace some of the bentonite.
Sprinkle the granules on top of the water. If it is impossible to drain the pond, sprinkle bentonite granules over the entire top surface of the water as evenly as possible. Allow the granules to sink and settle on the bottom, where they will fill any depressions and swell up to seal some leaks. This method is not ideal, but can help control leaking water to some extent.
Line a small garden pond with geotextile liners - rubber sheeting impregnated with bentonite. The bentonite fills any small holes that may develop.