I recently bought a 6 1/2 acre parcel of land that has a pond on it. However, my new water hole has a leak at the base of the dam, which causes the water to seep out at the bottom on the back side of the barrier. I want to stock the pond, but can't do so because of the fluctuating water level. Is there anything I can do to seal this leak, short of draining the pond and reworking the whole dam with a bulldozer?
Leaky ponds are like most ailments, in that preventing the problem is a lot less expensive, and easier, than curing it. There are three types of remedies, however. The surest, but most expensive and time consuming, method is to drain the pond and repair the dam. This could involve complete reconstruction, remarking the case (a good dam should have a central core of impervious soil extending from the waterline to below the bottom of the pond), or simply compacting the existing structure with a sheepsfoot roller. Less costly, but still requiring pond drainage, is the application of dispersants or a clay blanket to the dry pond bottom. (Dispersants are chemicals that break down the bottom soil into tiny particles that will plug any seepage points when the pond is refilled. The most common one is plain old sodium chloride.) Finally, the simplest but least certain way to seal a leaky pond, and perhaps the method you're after, is to add bentonite clay to the water.
Bentonite is technically described as a “hydrous silicate of sodium composed chiefly of montmorillonite.” It's the montmorillonite that counts, as this type of clay has the ability to absorb several times its own weight in water, swelling to as much as 20 times its original volume! Consult an agricultural supply store for your closest source of the material. The clay isn't expensive, but you may need a lot of it, and freight costs can be prohibitive if it isn't available nearby.
The generally recommended dosage of bentonite is about one pound per square foot of pond surface, but as much as twice that amount should be used on very porous soil or in extremely deep ponds. The best way to apply the clay without draining the pond is to mix it with water to form a slurry, which is then spread over the pond surface from a boat. Because of the weight of the material and the labor involved, though, bentonite is commonly spread directly on the water in granular (not powdered) form. In any case, the grains will sink, and the force of the escaping water will draw the expanding material into the leaks, sealing them.
If you are certain that the only leak is near the dam, you can use a smaller amount of bentonite and do a spot application. (Note, however, that though you see water escaping only at the base of the dam, it's still possible that the entire bottom layer is leaking and flowing toward the dam.) Small, well-localized seepage points can also be sealed with cinders passed through a 1 inch screen.
There are two other possibilities that should be mentioned. Since you state that the problem area is “at the base of the dam,” I can't rule out the likelihood that the pond is actually leaking along the drainpipe, a common occurrence. In this instance, the only real solution is to drain the pond and reinstall the drainpipe, making sure to compact the soil well and put in an antiseep collar which should help prevent leaking. It may also be that, if your pond is relatively new, it simply hasn't had enough time to seal itself naturally. Heavy stocking and fertilization will hasten this process.
Obviously, this is very general advice that may need to be adjusted for local soil conditions. For more specific information, contact your local soil conservation service office.
— Bill McLarney, Ph. D. and founder of the New Academy Institute
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