For homeowners that have them, ponds are a vital part of the natural landscape. They serve every purpose, from fishing and recreation to livestock and wildlife management. In addition, they provide a natural water reservoir that can sustain a water supply during drought. A pond that leaks, though, may not provide any of these benefits and can actually become a haven for pests (such as mosquitoes). Repairing a leaking pond dam, then, is an important -- albeit it sometimes difficult -- step in maintaining a properly functioning pond.
Verify that you do indeed have a leak in the dam, rather than a water supply problem. A pond that is not built in the lowest part of a property may not get the runoff necessary to counteract the natural process of evaporation -- particularly during unseasonably hot or arid times. If it is indeed a leak, you'll notice that the pond rapidly loses water to a certain point, then holds fairly well from then on.
Determine your treatment method based on pond size and leak location. If the dam is particularly steep, you won't be able to use spot applications of bentonite or chemical sealant because these treatments will sink below the location of the leak. A small pond can be efficiently fixed with a pond liner, but this is an unrealistic solution for large ponds.
Start with a bentonite or chemical sealant application if the dam is not extremely steep. You'll be able to avoid draining the pond, renting equipment, and harming pond life this way. These additives, which can be bought at a local agricultural supplier, are simply spread on the surface of the pond throughout the area in question. They sink down to the pond floor and often expand greatly as they become saturated with water, thus filling any gaps in the pond bed.
Drain the pond below the determined level of the leak if Step 3 does not adequately address the problem. For small ponds, or large ponds where the leak location is specifically known, rent an industrial tamper to compact the pond bed in the area. Follow with a heavy application of clay or bentonite, and then tamp again. Allow the pond to refill and monitor water levels for 2 to 3 weeks.