The simplest, most economical, and most environmentally friendly way to seal a pond is by applying sodium bentonite. Bentonite pond sealer, a natural clay product, chemically attracts to itself and swells up to 20 times its dry size when moistened. Expanded bentonite forms an impermeable gel seal which makes an excellent pond sealant. Sodium Bentonite is environmentally safe and does not affect the water, livestock, or wildlife. When properly applied, it will not harm fish. For these qualities, sodium bentonite also makes an excellent liner for new pond construction. Safe, natural, relatively inexpensive, the watertight membrane that may be your best answer to sealing your pond is sodium bentonite clay!
When sealing a leaking pond to prevent water loss through seepage, sodium bentonite can be applied directly to the soil in the pond bottom or it can be sprinkled on the surface of the water and allowed to settle to the bottom. There are three basic methods: the blanket method, the mixed blanket method and the sprinkle method. Sodium bentonite is easily applied. In the case of small ponds, it can be applied with simple hand tools and for larger ponds, with ordinary farm equipment.
Mixed Blanket Method
Begin by preparing the surface to be sealed. Remove all rocks, trash, and vegetation, including trees and roots. Roots left in the ground will decay over time, which leaves the potential for future leakage problems. If the soil is dry, lightly moisten it, then plow, disc or till the bottom of the pond 4" - 12" deep, then roll or drag the surface until it is smooth. This initial plowing and blending of the soil will assure consistency of the soil throughout the bottom of the pond before the bentonite is mixed into it. When this is complete, apply the prescribed amount of sodium bentonite. We recommend Pond Sealer, a mixture of powder and granular sodium bentonite for this application.
The bentonite can be spread by several different methods, however, we recommend a fertilizer or seed spreader to ensure even spreading. Mix the bentonite with the top 4" - 12" of soil by discing, tilling, or by hand raking. It is extremely important for the bentonite to be mixed uniformly with the soil. To ensure that no bare or thin spots will be left, we recommend multiple passes over the area. If livestock will have access to the pond, the cover coat must be thicker than 4 inches. In this case, a top
layer of gravel would be best to protect against puncture by cattle hooves.
Finally, compact the pond bottom by rolling or tamping. Use care when filling the pond. A gentle water flow is recommended to prevent erosion of the soil/bentonite layer during this final phase. It may take several days for the bentonite to reach the point of full saturation and fill the voids in the soil.
The Sprinkle Method The sprinkle method is recommended only when it is not feasible to drain the existing pond or in cases of isolated leaks when a spot treatment may be a possible alternative. This method is less effective than the other two methods for several reasons: Uniform and accurate placement of the bentonite is difficult; the pond bottom may include debris such as trash, vegetation, logs, rocks, etc., which may prevent sufficient coverage. If the bottom of the pond can be cleared, this method has greater chances of providing a good seal.
The granular grade of bentonite, “Granular Seal," is best suited for this method of sealing leaking ponds. The granular bentonite should be used when using the sprinkle method because the particles are large enough to sink to the bottom, while finer particles cloud the water and stay in suspension. Begin by simply sprinkling the prescribed amount of granular bentonite (Granular Seal) on the water surface and allow it to fall into place. The granular bentonite particles will sink to the bottom of the pond or be drawn into porous areas where they will swell and reduce the seepage rate.