Last weekend I opened up my in ground pool. Since then the water level dropped significantly. Here is the sequence of events.
Last Saturday I put water pump on the turpentine cover to remove water from it. The water level was about one feet from the ground level. Pump run for about 2 hours then it started to rain, so I shut off the pump and went inside. It rained all night. However, in the morning I was surprised to see that all water is gone from the cover and the level dropped to about two feet from the ground level. At first I thought, eh... no big deal, probably "communicating vessels", as it looked like the other end of the hose was lower then the pump on the turp. Then I removed the cover, started water pump, dumped 18 gallons of chlorine in the pool, connected leaf bagger and started collecting leafs. After I was done with the leafs, I left water hose running to full up the pool.
On Monday, I noticed water level dropped because skimmer was making noise by sucking the air. I refilled the pool with water and had water pump off over night.
Here is a picture I took Tuesday morning.
This is how it looks today.
Also note wrinkles, it didn't look anything like that when I pulled off the cover.
I was suspecting plumbing, but as the water level dropped below the return jets doesn't seem to be the case. I can't see any obvious damage to the lining.
TF-100 kit says chemistry looks as follows:
pH 7.3 - 7.5