Sodium bentonite is a kind of clay that is available in granular form. It works to seal small garden ponds, as it can swell up to 18 times its size when in contact with water. Sodium bentonite is favoured over other products, as it is a natural substance, environmentally friendly and does not affect the water, fish or pond plants. You can add sodium bentonite to the clay when making the pond or sprinkle it on the surface of the water and allow it to sink.
Clear the pond of rocks, roots and other debris. Remove 4 to 6 inches of soil from the bottom of the pond.
Smooth out the bottom of the pond with a lawn roller.
Sprinkle an even layer of sodium bentonite over the pond floor. Replace the soil you removed earlier.
Roll the pond floor one more time with the lawn roller. Fill the pond with water.
Clear the bottom of the pond of roots, rocks and other debris. Moisten the soil slightly. Plough or till the pond bottom to a depth of between 4 and 12 inches. Roll the bottom flat. Sprinkle the pond with a thin layer of sodium bentonite. The layer should be thick enough to cover the soil.
Till the soil again to the same depth as before or use a hand rake to mix the soil and sodium bentonite.
Ensure that the sodium bentonite and soil are evenly mixed and distributed. Fill the pond slowly to avoid erosion of the sodium bentonite.
The sodium bentonite may take some time to swell and seal the pond. Allow a few days for the pond to become watertight. If you are not able to drain the pond or are not starting a new pond, sprinkle the sodium bentonite evenly over the pond surface. This process will be more effective if the pond bottom is free of debris and vegetation.