If a pond’s water level drops much more than 6 to 12 inches in one month, it may have a serious leak. Summer drought conditions should be taken into consideration when determining whether or not a pond leaks. Fortunately, pond leaks can often be sealed by adding clay material to pond soil or using a plastic liner.
Bentonite clay is a product commonly used to seal leaky ponds that is commercially available at farm and drilling equipment stores. It is sold as a dry powder and it expands more than other clays when wet. Bentonite should be applied to dry pond soils at a rate of one to three pounds per square foot of pond basin. Bentonite is most effective when incorporated into the pond’s soil using a rototiller or disk and then compacted using a sheep's foot roller, dozer blade, or soil compactor. If it is impractical to drain a pond, a slurry may be added uniformly across the surface of the pond. However, this method is often less effective because of uneven settling onto the pond bottom.
Geomembranes and other plastic liners can also be used to seal leaky ponds, although they are much more expensive solutions than bentonite. Geomembranes are thick plastic sheets drawn across the pond basin and covered with a thick layer of sand and soil to hold them in place and protect them from punctures.