I'm a little over a year out from having a pond (currently a dirt pit) constructed on my property. Pond location had an existing "cattle pond" that was built into the beginning of a ravine. We had this cleared out and made into a much larger (close to 1acre) pond. After not holding water, I had the local NRCS agent come out to take a look. Soil borings show very a sandy loam with minimal clay and has given us a recommendation of 1-1.5 lb/ft^2 of bentonite clay. Comparing the cost of the bentonite, hauling in local clay, and liner, the bentonite is looking to be the most cost effective option. That being said, in reading many different comments on various sites, I feel there is risk. The sides of the pond are pretty steep and I dont have a way to fill on my own so I will be rain dependent. I am concerned about loosing the bentonite layer on the side walls and then will be chasing leaks for years. Looking for some guidance/advise from those experienced with bentonite.
Hard to tell visually, but it definitely looks like there is some clay in your pond materials. The cracked, red material in the pond bottom of Pic #2 certainly looks like clay.
You actually do NOT want 100% clay in a pond that will have portions be exposed to air during a drought. When it dries, it will crack. When the water level goes back up, those cracks usually will not heal.
Typically, material with about 20-30% clay content will make a good water seal in farm ponds. A sandy loam soil horizon should (by definition) have about 15-20% clay material. One cause of your leaks may be that they did not scarify (cut and mix) your soil horizons sufficiently. Any tiny horizontal layer of sand that is not broken up can leak a lot of water! If other layers had a higher clay content, then proper mixing might have brought you up to an average of 20-30% clay.
The other cause of leaks is poor compaction after the layers are mixed. The material must be moistened to the proper level, and then the material is compacted in lifts of 6" or less. (Did you observe them performing that step?)
You can roughly test for your clay content by yourself. Take a bucket of water and slowly moisten some of your material. I suspect you can roll the cracked, red material in the pond bottom into a small "snowball" in your hands. The clay material will make a nice ball and you might be able to roll it into some long pencils. Your sandy loam will probably make a poor snowball that wants to crack and fall apart. You won't even be able to moisten the true sandy or silica silty material.
If I am correct about your material and some flaws in your pond construction, then you probably came close to having a sealed pond. (That is a LOT of speculation on my part from a geologist that does not have eyes on your pond, so I may be wrong about several things!)
If you were close to a pond, then the bentonite is probably a very reasonable option for you. Bentonite works to seal a pond because it swells when it encounters water. Hopefully, some guys with bentonite experience can give you good guidance on the proper way to spread it and then till it into your soil material.
Is there any chance your pond site was grading down into more clay-rich material as the excavation went deeper? If so, digging a deep hole and creating a one-foot thick clay blanket on your pond bottom and sides (in compacted 6" lifts) would also seal your pond.
P.S. Can you finish your final grade around your pond and top it with good topsoil, and then get your groundcover established? You can see from the sides of your pond how running water will cut through bare material. When your pond is nicely finished, you do NOT want a big rain event to wash a foot of material out of your barren areas and lose the bottom depth of your pond.
