What to do when a slow leak is possibly behind massive boulders

[kate gusky]

Hi - About 3 years ago, we used montmorillonite clay, the organic/edible powder form bought in a pouch, turned off any running water, and poured it very slowly into the area where there was a leak. We were careful with amounts because we don't know if the clay might damage fish gills/breathing, and we also have an in-water pump we don't want to clog with clay. There is a pepper tree and palm tree overhead and there is constant organic matter coming into the pool that makes cleaning and patching impossible. The fine dust gets water-logged, is drawn down into the leak out of sight, is very sticky, and looks like wet sculpture clay where it settles, is drawn down into the leak out of sight. We have old concrete rough koi ponds and sealing leaks was just driving us mad. Our whole lower mosquito fish pond, which hasn't a pump and is kept aerated by two large bubblers, is kept from leaking by having lined itself with the clay. You can't drain the area because the clay will crack and the leaks will start up again.

 

[Susan Barlow Holmes]

Hello! Thank you for reading, I’m new to Garden Pond Forum and curious if anyone has any ideas. Background...we have a two pond/3 waterfall feature in our backyard that was installed by previous owners. They also planted aspens around the ponds... so you can imagine how well that went. Last year during quarantine I was able to successfully patch/repair our bottom pond (the aspen roots had punctured the flexible liner), installed a Savio Skimmerfilter, and installed a PondMax pressure filter/UV clarifier. I was also able to move up to the second pond and make the edges more defined and repaired a few visible holes. It was a busy year and I have learned a lot.
This week I started back at it, and with great disappointment realized I now have a leak at the 2” mark of the base of the middle (second) pond. It could have been caused during my repair and repositioning of rocks last summer, or it could be caused by a new root puncture (aspens have been killed off via poison for 1.5 years now however there are a few conifers around this pond). It is a slow leak, I have tried the milk method on a non-windy day and the water just becomes cloudy over time with no movement or leak detected. There are some massive boulders at the back of the pond (see attached picture) that are too heavy to be moved.
First question... does anyone have ideas on how to determine where a slow leak is located?
Second question... what are my options now since we will not be able to reposition and check behind the massive boulders at the back? I’m assuming filling the pond with cement up to 3” wouldn’t work, but could it? Is there a product out there for this type of pond issue?
My plan now is to begin to pull apart my work from last year and inspect visually. I also think I could refill the pond (after removing small boulders along edge) and examine for leaks again.
Thank you so much for anyone reading and able to offer thoughts!

What about black spray foam between the rocks?

 

[GBBUDD]

Foam is not water proof it will only slow it down. But it also needs to be 100% dry in order for it to stick .

 

[goldfishgardennz]

I'm so sorry to hear of your leaking plight.
If you are putting a new liner in try to get some old furniture moving blankets to put underneath - the thick matted weave will prevent puncture of the liner by rocks or roots.
To move the rocks ... Do you have a car engine hoist on wheels or can you hire one? This is what we used to position and reposition our heavy pond rocks.
All the best with your endeavours!

 

[combatwombat]

Have you had any luck finding the leak?

Just another thought: If you didn't move the big boulders while reworking the pond, it seems unlikely that the leak would be there if wasn't there before you started working, Much more likely to be in an area that you disturbed during your renovation.