Last Friday I lifted the plastic and looked at my koi pond--to my shock, it was about half empty. The filtering system had shut down, but at least the air stone was going. It was really cold here, and I had not looked under the plastic for a couple of days.
I did not see any signs of tearing of the liner at the water level, but it had settled close to the bottom of my mid-water drain. I put that drain in about a year and a half ago; it is a 4" pipe, but runs about 18"-2 feet deep. I know it wasn't below the frost line, but I knew I covered the pond and thought it would always have 40 degree water running through it, so I reasoned it should be o.k. to not go deeper. I think I might have been wrong!
When we had a thaw this past week after a brutal cold spell here in mid-Illinois, my theory is that the pipe froze, then when it thawed out it began to leak.
So Friday, I put a plug in the mid-water line right where it enters the pond, filled up the pond Friday, and thought I'd be o.k. until I redo that pond this spring. Wrong again...
By Saturday morning, the water level had dropped 4" in about 12 hours since I had topped off the pond. The pond is about 11 x 17, so 4" is a lot of water. (No sign of it on the ground anywhere.)
By Sunday morning, it was now down about 7.5", and by Monday morning, it was down 11"! At this point, it was at the base of the opening for the skimmer...so I thought I would see if dropped any more, proving it was the skimmer line that was leaking instead of the new, shallow, mid-level drain line (after all, it was plugged so couldn't leak any pond water, right???)
Well, tonight (Monday night), the water had fallen another 2" (not quite as fast this time, but now we are 13" down. The mystifying part to me was that the water level inside the skimmer box was also down about 2" below the bottom of the skimmer mouth opening...matching the lower level in the pond itself! How could water fall in both isolated containers--the pond and the skimmer box--at the same rate when they were no longer connected by the skimmer box opening?
Then it hit me--there are three pipes that are all connected by the RDF box! Even when the filter is shut down, water can still flow from the skimmer box, into the RDF, then backward out into the mid-level drain pipe, and out a break into the ground--even if the other end of the mid-level pipe is plugged.
So I shut all the valves, eliminating the RDF box, and I will watch and see what level continues to fall, if any, tomorrow. If the skimmer box water level falls, then the skimmer pipe broke. If the levels all remain steady, it could be a break in the shallow mid-water drain line (where I suspect it is). And if the leak happens to be in the BD line, then the pond level will continue to fall, but the level in the skimmer box will not.
I will post a simple drawing in a moment to illustrate...