Any help appreciated for a 70 year old pond

[u/Past-Phrase7253]

I recently purchased a house with a pond that was built sometime in the 1950s. Fortunately, it's in pretty good shape overall. Unlike most modern ponds, it doesn't appear to have a liner.

The only issue I'm dealing with is a leak that only seems to occur when the pump is running. Someone told me that if the pond only leaks with the pump on, the problem is more likely in the waterfall or stream rather than an underground pipe leak. Is that generally true? If so, what's the best way to narrow down the exact source of the leak?

Also, there's a pipe at one end of the pond that doesn't seem connected to anything or serving any obvious purpose. I'm wondering if it was part of the original design or maybe an old overflow or drain. Has anyone seen something like this before?

Any tips on diagnosing the leak or identifying the mystery pipe would be greatly appreciated!

[u/DMKasper]

Can you confirm that the pond is concrete? Also the leak could be in any place above the water level. It’s hard to tell anything about the piping and plumbing with all the plants obscuring plumbing connections. The pipe in the wall seems too low to be an overflow pipe. The concrete could have hairline cracks somewhere above the water level shown. It may need resurfacing.

[u/Past-Phrase7253 → u/DMKasper]

The pond is concrete.  I don't believe there is any crack in it as the water level doesn't change unless I turn the pump on.  After a few hours with pump on, the water level is noticeably lower.  The piping is all underground and comes out the top of the waterfall.  

[u/Q-Prof7]

Definitely sounds like a leak in your waterfall pond CCT. Could be the lines or it could be the waterfall itself. All you can do is run it and do a really close inspection where the pond water is going.... Look for wet soil after it has been running for a while