IMO your leak is in the upper 4 ft. That is the area that needs to be relined with clay and well compacted not the entire pond basin which may be cheaper than rebuilding the entire pond basin. However to get the job done correctly the pond may need to be drained.

For using the well IMO you first should know how fast the leak drains the pond. Any way to pump from a creek to fill it and measure rate of water loss?


aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management