The blanket method.
The blanket method of bentonite clay application is simply placing a uniform layer of granular bentonite between 2 confining layers of soil. In new pond construction, the pond bottom is over-excavated by 6 to 8 inches. The bentonite clay is then spread in a uniform layer before the over-excavated soil is then placed back over the bentonite clay blanket. The top layer of soil is compacted with a pad-foot roller to ensure proper sealing. When using the blanket method for bentonite the pond side slopes should less than 33 degrees (3:1) or the bentonite material will slump toward the pond bottom. This is an important detail because as the blanket of bentonite becomes saturated it takes on a gel like consistency and the bentonite can very easily slide down the pond slope.
The mixed method.
The mixed method for bentonite clay application provides some advantages over the blanket method. Using the mixed method, bentonite is spread at the desired application rate and then mixed with the existing pond soil using an agricultural disc harrow, rototiller, or other mixing method to be sure that the granular bentonite clay is completely mixed with the top 6 to 10 inches of pond bottom soil. Once complete, the soil mixture must be thoroughly compacted with a pad-foot roller. Very dry soil will need to have water applied to the soil/bentonite mixture before rolling to ensure complete compaction. The mixed method for applying bentonite clay does allow application on steep slopes up to 45 degrees. It is typically best to apply the bentonite at a higher rate near the top of slopes since it will tend to work down the slope when it is tilled into the soil.
Bentonite For Pond Sealing Application Rates
We get many questions about the proper application rate for a bentonite clay pond seal. The answer always is the same.... "It depends". The bentonite clay application rate depends on the soil type and local soil conditions as well as the level of compaction that can be achieved in the pond bottom. In some cases, when the pond bottom simply can not be dried due to springs and groundwater inflow, the application rate must be significantly higher to achieve pond sealing. Average bentonite application rates range between 2 pounds per square foot and 6 pounds per square foot. This means that the total amount of bentonite needed to seal most pond bottoms is measured per ton rather than in pounds. For loam soils with some clay content, 2 pounds per square foot may be adequate to seal the pond bottom. Sand and gravel pond bottoms will require 6 pounds per square foot or more of bentonite clay to seal the pond bottom.