I've been hunting down leaks for about 2 weeks. I just found this web site. This is what I've done, and what I've found. If anyone can add anything please do:
Lay of the land:
Basic rectangular IGVL, fiberglass steps, ladder, diving board, 2 strainers, bottom drain, no lights, DE filter, underground black poly pipe with worm drive clamps. Everything is about 20 years old except the liner, which is about 8 years old. This is not a flat landscape. Starting at my back fenceline and moving toward the house, I have level ground to the edge of the pool. Jump in, then cross the pool, and you walk out of the pool at the steps, then move 4 feet across some concrete toward the house, and you go down 3 steps to grade. At that grade, I have a paver patio, and a couple in ground drain boxes and a UG drain system that discharges to the curb line (which is another 7 foot drop in elevation, about 150 feet away). My pool equipment is off to the side, and the bottom of the filter housing is roughly at the elevation of the bottom of the skimmer inlets. So at normal operating water level (NOWL), the base of my pump and filter are actually below the water line.
Started losing water about 2-3 weeks ago. It will drop 2-3 inches from mid strainer down, in about 12 hours. That's 600 plus gallons.
We had a brutal winter. 3 feet of snow. Ice in the pool was a foot thick, right at the skimmer openings. I had drained it down past the jets when I closed it up, but I didn't get an opportunity to drain the excess water from the rains in late fall. The pool filled back up and froze.
Tests that I've done:
At first I thought it was leaking UG fittings. It was easy enough to dig up the fittings at the pump, so I did. No leaks. I then pressure tested the lines. Held pressure overnight. No leaks there. Went through a bottle and a half of blue dye, looking for leaks.
I then noticed that the water would drop to a certain point and stop. This corresponded to just under the nosing of the first step, and 1/2 inch higher than the bottom edge of the skimmer opening. I refiled to the NOWL, and spent an hour in the pool this morning looking at every square inch of the liner at the leak level, puffing dye here and there. I also noticed this year that the shrubs at my steps aren't "twins" anymore. One looks weak and a little droopy. I'm thinking it's getting the brunt of the water leak, and it's slowly drowning.
Things I've found:
There are 3 tiny quarter inch or smaller cracks in the 3rd and 4th steps. I guarantee you all 3 are weeping water out, but no way could I lose 2 inches from those 3 little cracks.
I actually found a decent hole in the liner, above the NOWL. Going to fix that tomorrow. But it can't be the source of my leak.
At first I thought the low end skimmer itself was cracked, but it's not. Then this morning, with my face up to the gasket plate, I see a bunch of crusty rust along the outside edge, on the pool water side. I think the ice jostled it and produced a leak. Why it didn't show up a month ago when I opened it, I'll never know. But when the water drains out, it stops draining when it reaches the exact height of the crustiest part of the rust. I think that's my leak. I'm going to try and get some photos today. Short term fix will be some marine epoxy. Long term, I'll fix it when I get the liner replaced in 2013 or 2014.
The low end jet seal plate has a crack in it, at the lower right screw hole. The crack goes clean through. During my pressure test of the line, I saw bubbles come right out of the crack, but I didn't lose pressure. I'm thinking in my effort to screw the plug in, I spread the crack, and allowed a leak to form. (That low end jet is a skant 5 feet from retaining wall that separates the upper level concrete pool deck and the paver patio below, and the droopy shrub, and is right next to the steps. The retaining wall itself has had some movement, and the concrete pool deck has settled on me about 3/4 inch in that area. I think it's washing out a little bit.)
5. Deep end skimmer
I accidentally overfilled the pool last week, and when I got up the next day, the pool was overflowing onto the lower patio. But it wasn't overflowing the top lip. I did some dye tests, and located a higher level leak in the deep end skimmer. Unmistakable leak through a crack in the deep end skimmer housing. And the water traveled 20 feet before it reached daylight. I could actually see where it was dripping out of the retaining wall (and bubbling up from patio at the base of the steps).
So I believe I have small leaks in the fiberglass steps, a decent leak in the low end skimmer, a small one at the low end jet, and a decent one at a high point inside the deep end skimmer. Plus a hole in the liner above the NOWL. But my underground lines hold pressure.
I think I have a handle on it, but if anyone can add anything to my findings, please do. Particularly about the rust stains around the low end skimmer (I'll post photos later).