Once the leak lowers the level to a hole and then the rate of loss
drops off, you will have established the aproximate depth of the
leaks. Add enough water to raise the pond level back up an inch or so.
Move around the pond with a dropper and use milk to track where the
flow passes through the wall. You can patch with any number of cements
or expoxies which work when wet.
Ultimately you may be forced to empty the pond and line it with EPDM
rubber or some other resealing technique.

--
Crashj

Believe it or not, I had a leak that I couldn't locate with all the tried
and true methods that actually defied this very sensible logic. The leak
was on the very bottom of the pond, but the level only dropped eight or nine
inches and then would level off. We can only guess that it had to do with
the ground being saturated already. We patched it yesterday (without
draining the pond, would you believe?!) and once the resulting bubble under
the liner was pressed down by the newly added water, enough ground water was
forced up, that the driveway looked like we left the hose running all night.
I hope the leak in the concrete pond isn't like that because he won't be
able to grab the bubbled-up liner and dry it off for patching. Instead, he
will actually have to drain the pond.
The weird thing was that the bubble flattened out gradually, and the water
level actually fell again as the displacement went away, leading me to
believe that we had yet another leak that so far wasn't detected.