I noticed a decline in the pool water level and did some research. Checked the obvious. No leaks in the above ground plumbing, pump skimmer gasket, etc. There is no air in the pump skimmer.
I marked the pool level and did a 24 hr pump on/pump off test with all the returns and skimmers open. I had a bucket on the steps for evaporation comparison. The pool lost 5/8" pump on and 3/8" pump off. The evaporation rate was comparable to the pump off test.
This did not really confirm anything, except it leaks more with the pump on.
I plugged up all the skimmers and returns. I marked the pool and bucket and turned the pump off. The water level dropped 3/8" in both the bucket and the pool.
I think this confirms the integrity of the pool. Water drop in pool equals evaporation.
This leave the plumbing! I believe there is a return side leak.
I notice a small amount of bubbles in the returns. AND there is a sunken bit of concrete over one particular return.
The bubbles, coupled with the lack of air in the pump skimmer and the evaporation rate being equal (with the return and skimmers blocked) leads to the return side leak conclusion.
Am I thinking correctly?
Do I have any logic flaws?