We've got a leak....but only when the pump is off?

[allif]

We've had our pond and bog running for about a week (pump running 24/7). The water level has remained steady in both bog and pond, and the amt going over the spillway has also remained constant.
Last night we lost power, I think for only about an hour. This morning when we came out the water level in the bog was fine but the pond was down about 2"
(over a surface area of about 40 ft sq). The bog was filled (the bog probably has roughly the same surface area).
We assume the leak is in the bog somewhere but when the pump turned back on the bog refilled first.
Question is, if there's a leak, why arent we always noticeably losing water? It's only obvious when the pump is off. What does flow have to do with it?

 

[BKHpondcritters]

Does the pond have an area where it overflows when too full? If so, perhaps the bog water drained down into the pond, causing it to be 'over full' and thus the extra water was lost. Once the pump was turned back on, the bog refilled & the pond area showed what was lost. (Am I making any sense? I don't have a bog, but this is what happens when my power is off & my biofall & upper pool area drain back into the main pond)

 

[poconojoe]

Does the pond have an area where it overflows when too full? If so, perhaps the bog water drained down into the pond, causing it to be 'over full' and thus the extra water was lost. Once the pump was turned back on, the bog refilled & the pond area showed what was lost. (Am I making any sense? I don't have a bog, but this is what happens when my power is off & my biofall & upper pool area drain back into the main pond)

This makes sense. I hope that is the answer.

I have noticed big fluctuations in pond water level in relation to bog water level.
It makes perfect sense too.

When I flush my bog pipes and or clean the straining holes on my submersible pump, there ends up being more flow to the bog. This increases the level of water in the bog and decreases the water level in the pond. So, the pond appears to have lost water.

Also, what could be happening, as @BKHpondcritters has stated... water from the bog, due to a vacuum or suction, is flowing back to the pond because your pump lost power. If your bog is higher than your pond, it will do this. I don't know if it's due to gravity or just that the suction wasn't broken and the water reversed flow, back to the pond. Your pond can now overflow and lose a bit of water.
Some people prevent this by adding a check valve or drill a small hole in the highest point of the piping as a vacuum breaker. The hole, obviously, has to be within the system as to not leak out.

I hope this is what you are seeing and it's not a leak.

 

[islandboy]

^^^ Agreed. My setup will lose ~100 gallons when the pump is off due to overflow. If I need to shut it down for maintenance I set aside100 gallons and wait for the pump to switch off. Still lose a little.

 

[combatwombat]

Second what @BKHpondcritters said. Would happen if your water line is very close to the lowest edge of your liner in the pond. When totally full like that, there is more water in the system than the pond can actually handle, but the "extra" is always in transit while the pump is on. When pump goes off, all that water in transit ends up back in the pond, overflowing the low edge. Turn pump back on, water is pumped from pond back to bog and now water is low in the pond.

 

[poconojoe]

Second what @BKHpondcritters said. Would happen if your water line is very close to the lowest edge of your liner in the pond. When totally full like that, there is more water in the system than the pond can actually handle, but the "extra" is always in transit while the pump is on. When pump goes off, all that water in transit ends up back in the pond, overflowing the low edge. Turn pump back on, water is pumped from pond back to bog and now water is low in the pond.

Exactly!
This makes total sense.
And again, I hope this is what is occurring and not an actual leak.

 

[allif]

Thank you all. I think I understand. And hope you're right.
The water level in the pond is indeed pretty close to the maximum possible, so that would all make sense.
It rained pretty heavily so it's hard to see where runoff/overflow may have been. Guess I could turn the pump off for a few hours and test it again...

 

[allif]

tested and confirmed....thank you all!

 

[TimL94]

Mini side note, depending on the layout of pond and bog, Probably is as others said but there is also the possibility that when the pump went off that a syphoning effect through the pipe caused a movement of water to overflow somewhere

 

[allif]

Thank you. Would a check valve somewhere in the line prevent that?

 

[Jhn]

Check valve would prevent it, but those eventually fail. Easiest way to prevent it is drill a hole in the pipe feeding the bog a little above the water line, this will break the siphon. Just be sure to drill in the pipe so as the pump is on the water leaks out inside the bog/pond area.

 

[allif]

Good idea, thanks!