The Blanket Method is usually the most effective method for sealing leaking ponds with bentonite. However, more care is required during it's application. Pond Sealer (MSDS), a mixture of powder and granular sodium bentonite, is best suited for this method unless working in an extremely windy area, in which case, we would recommend Granular Seal (MSDS).
The first step is to prepare the surface to be sealed. Remove all rocks, trash, and vegetation, including trees and roots. Roots left in the ground will decay over time, leaving the potential for future leakage problems. Remove the top 4" - 6 " of the soil from the pond bottom. Fill any deep holes or crevices with a uniform mixture of one part bentonite to five parts soil. Then smooth the area by roller or drag.
Cover the area with the prescribed amount of sodium bentonite. It is very important that the bentonite layer is even and that there are no bare or thin spots. This will ensure complete coverage and a water tight seal. The bentonite can be spread over the area to be covered with a fertilizer or seed spreader, or with a front-end loader. After the bentonite has been spread as evenly as possible, carefully replace the 4 to 6 inches of soil that was removed earlier. While spreading the soil, be certain not to disturb the bentonite layer, as this could leave bare spots which could allow water loss to continue. 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 and bentonite layers 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 underlying soil. If a considerable amount of time elapses between applying the bentonite and filling the pond, it may be necessary to protect the treated area against drying and cracking. A mulch of straw or hay pinned to the surface by the final passes of the sheep foot roller gives this protection. Use rock riprap or other suitable material to protect areas where water inflow into the treated area is concentrated.