Pond leaking Advice Please

Hi,

I have a system that consists of a pond connected to a concealed
reservoir that has a pump sending the water up to an upper reservoir
where the flow originates. It then goes on a downstream run of about
30 feet into the pond, and the cycle start all over again.

I started to lose a lot of water. To isolate the problem, I tapped
off of the 3 1/2 supply pipe in the upper reservoir, connecting it to
a flexible 2 inch line. That enabled me to isolate sections of the 30
foot run as well as the upper reservoir.

From what I can tell, the problem is in the upper reservoir. The
installer has the lining tightly wrapped around the vertical supply
pipe, and he also cemented the surrounding cavity. I'd like to pour
something into the cavity that would seal it once and for all. Any
idea's...

T.I.A.