How to Seal a Pond With Sodium Bentonite

Man-made ponds built on unsuitable soils are prone to seepage through fissures and gaps in the soil structure.

Sodium Bentonite as a Soil Amendment

Pond leaks can interfere with the use and appreciation of your pond.Pond leaks can interfere with the use and appreciation of your pond.
There are several options for sealing a leaking pond, but sodium bentonite is one of the most permanent and cost effective. Sodium bentonite swells to many times its normal size when wet, blocking gaps in the pond bottom where water might leak. It can be applied to the pond as either a soil amendment or a distinct layer.

Drain the pond if any water remains. Use a backhoe to dig a narrow trench on the downhill side of the pond or use a pump. Allow the pond bottom to dry enough that people and equipment can move around on it without sinking in. Replace the berm if you drained the pond by digging a trench.

Spread sodium bentonite in the pond at a rate of 3 lbs. per square foot. Sodium bentonite comes in 100-lb. bags and can be spread with a lime spreader or by hand.

Incorporate the sodium bentonite into the top few inches of the existing soil. A rototiller is sufficient for small ponds, but a tractor-mounted disc harrow is more efficient for large ponds.

Sodium Bentonite as a Distinct Layer

Drain the pond and let it dry. Scrape a 4-inch layer of soil from the pond bottom with a box blade or tractor bucket and pile it near the pond.

Compact the exposed soil with a sheepsfoot roller, which is a heavy, spiked cylinder that rolls across the ground behind a tractor or backhoe. Spread sodium bentonite over the bottom of the pond at a rate of 2 lbs. per square foot.

Spread the soil scraped from the pond bottom evenly over the sodium bentonite. The soil layer keeps livestock and wildlife from damaging the layer of sodium bentonite when they walk into the shallows to drink.

Compact the soil again with a sheepsfoot roller. Compaction reduces airspace in the soil and makes it more water resistant.

Things You Will Need

  • Backhoe or pump
  • Lime spreader (optional)
  • Rototiller or tractor and disc harrow
  • Box blade or tractor bucket
  • Sheepsfoot roller


  • Use fine-powdered bentonite to seal a pond, not the coarse or pelleted variety.
  • Old-timers turned hogs out around a leaky pond to seal it. The hogs wallowed at the edge of the pond, churning the bottom into a smooth mud that often sealed cracks and fissures.

About the Author

Based in central Missouri, Rachel Steffan has been writing since 2005. She has contributed to several online publications, specializing in sustainable agriculture, food, health and nutrition. Steffan holds a Bachelor of Science in agriculture from Truman State University.