holy crap budman - only go with your contractors advice if he guarantees it in WRITING and will come back to fix leaks. I would withold some or most final payment as leverage Because if he does it his way, he will be back.

With that type of subsoil you are going to be damn lucky if you get a pond that will hold water well without an artificial liner.

Clay Blankets. The USDA: Ponds- Planning, Design and Construction, recommend that clay blankets be a minimum 20% clay and should cover the entire area over which water is to be impounded as well as the upstream slope of embankment. Thichness depends on water depth to be impounded. Minimum thickness is 12" for all depths up to 10ft. Increase the thickness 2" for every foot of water greater than 10ft.

Spread clay in 6" to 8" layers and compact each layer with at least 4 to 6 passes of sheeps-foot roller under OPTIMUM moisture conditions. Clay balanket at high and low water levels should be protected from freezing and drying with 12" to 18" of stone or riprap.

With a small pond like yours I would invest in some betonite to mix with the clay blanket before compaction. It is cheap insurance.

Where would you put your confidence and bet your money on for the better seal??? with a 12" - 18" clay blanket or a 3" to 4" blanket?

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