Mixed blanket, pure blanket and sprinkling are the three main methods for sealing a pond with bentonite. Mixed Blanket Method Drain the area to be treated and remove all rocks and plant growth. Use 1 part granular or powdered bentonite mixed with 5 parts soil to fill large holes and crevices. Plow the area to be treated to a depth of 4 to 6 inches and allow soil to dry. When the entire area has been covered, mix the bentonite with the top 3 to 4 inches of soil, using a disc, spike tooth harrow or hand rake, and then roll or tamp to compact it.
Sandy soil works best for mixing with bentonite because it compacts better.
If clay soil is used it must be fine and without lumps. Pure Blanket Method Drain the area to be treated. Cover bentonite with 3 or 4 inches of soil, sand or fine gravel. This is the best method but requires the most care, because bentonite must cover the entire surface to prevent leaks. The pond does not need to be drained but the exact location of the leak must be known.