My pond leaks. What can I use to seal it?

First, make sure that the pond is actually leaking. 

In Kansas, evaporation can be expected to range from about 4 feet per year in the eastern part of the state to about 6 feet per year in the west. Almost all ponds will leak to some degree, especially new ponds.  The pond owner can determine his pond?s leakage rate by measuring the water level drop with a marked stick during a period of cold or very humid, calm weather.  Techniques are available to seal the leaky and potentially leaky areas. Most sealing techniques are expensive and require considerable work.  Materials and methods for sealing a leaking pond can be found here.