Measure the severity of the seepage. After a heavy rain place a stake at the water's edge. Check daily to determine how quickly the water level decreases. If water loss is rapid, you should drain the pond and skip to Step 4. If it is gradual, try a few simple fixes to save yourself from more intensive repairs. Severe water leakage will involve 6 inches to a foot of water in a week. A pond with minor leakage may be just an inch or 2 lower.
Purchase bentonite from your local extension office. Bentonite is a type of clay that expands greatly when wet. Sprinkle this substance on the surface of the water and allow it to sink to the bottom. As it expands, it often seals minor leaks in the pond bed. Several applications may be necessary, so monitor the water level after each application.
Drain the pond if leaking or seepage persists. Where possible, have several truckloads of heavy clay soil delivered to the pond, or use any available on your property. Spread this clay to a depth of 1 to 2 feet throughout the pond bed, then use a backhoe or bulldozer to tamp it down. Fill the pond slowly and measure for water retention.
Resort to a plastic pond liner only when the natural materials have failed. Plastic liners limit the ability of a pond to host a complete ecosystem, but you may prefer liners when no other option is available. Drain the pond and roll out the liner. Use a plastic seaming machine to seal the edges and make one impenetrable barrier between the water and the pond bed.