Sorry, I had to not expand on my line of thought cause I had to go get ready for work. I see the silver lining in what looks like a cloud ( a problem) was n that if the water is pond safe, you now potentially have a way to top off the pond with out raising your water bill, or require chlorine neutralizers. The thought I had involves getting the water tested for the first step. Then, you would partially fill the hole with large round river rock, set up pipes so that the water will come up and out, possibly with a ball valve in there so you can prevent overflow or filling when you don’t want it. This would be a good area to t off the pipe so that the excess water can go to a French drain or away from the pond. After the rock and pipes are set up, you would then fill an appropriate amount with finer stuff like sand, the liner underlayment, the liner, fill the pond, etc. I’m no expert, so you might want to consult with one if that idea would even work, but it’s what I would try if I found that problem on land I own. Here my water table is 20+ ft deep, so as I don’t ever intend to have a pond that deep, it’s not an issue, but you now know you have a source of natural water which is free, and unless contaminated from something like an old land fill, or things like natural poison found with certain metals, radioactive materials, etc, you have lots of options on how to turn that into something nice. I don’t know where you live or how close the pond is to your house, but if it is close, I would recommend you do your research to find out about the water table in the area. If it’s well known to be high, chances are your house was built with that in mind. However, if it’s a natural spring, or high point of a natural aquifer, it could lead to other problems. Artisan wells are very popular out here, lovely cool sweet water with out ever being treated by a city. If you have enough water pressure, you could have one on land you own.