Bentonite clay is a natural expanding clay sealant that when exposed to moisture can expand 15X or more creating a very effective pond sealant when applied correctly. Bentonite Clay Application Rates for pond/lake bottoms To properly apply bentonite to seal a pond, the pond must be drained and the bentonite clay then incorporated into the soil of the pond bottom and sides. The application rate for bentonite ranges from 2 pounds to more than 5 pounds per square foot depending on the soil characteristics. For clayey and silty soils, 2 pounds per square foot MAY be sufficient. For sandy soils 4 pounds MAY be sufficient and for soil containing gravel and rock, in excess of 5 pounds will be required to obtain a seal. Once the bentonite is applied, the soil should be disk harrowed to mix the soil and bentonite and then compacted. Alternatively, a continuous blanket of bentonite can be applied to the soil and then carefully covered with clean soil or sand to prevent the clay from mobilizing into the water column.