I have had similar problems as the OP losing CYA which left me scratching my head for the first couple of months of the season when I first moved to Florida... I was going through a considerable amount of CYA (granular). I try to maintain a level of about 70 ppm, but would find sometimes that after 3 or 4 weeks the CYA level would drop to a third of what it was originally. Eventually I started tracking conditions and came to the hypothesis that it was related to dilution from the heavy rains we get in Florida during the summer.
My pool has an overflow/filler and there is no stray water getting into the pools other than what is falling directly into it (Super-gutters catch all the roof runoff) But is a relatively shallow pool (12500 Gal pool with average of 48" depth, possibly less if I account for stairs, and shallow area) A rain accumulation of 2.5 inches works out to be a diluting the original water by roughly a 5%. Went through some days in July where I was getting 2 1/2" of rain several times a week. Checked some local stats and found we get in my area 50 to 55" of rain during the wet season. I actually believe in my location I may be getting more than that and plan to start tracking myself next year.
The other thing that can happen with automatic fillers with overflows (mine is sort of a toilet valve type mechanism) is that often they aren't installed properly and continually seep. I found mine was always seeping which adds to dilution problem. I noticed something was up when the grass around the filler overflow was greener than the rest of the lawn. In the summer I don't even need the auto-fill as the rain far exceeds any evaporation in this humidity.
In any case, I track it pretty closely now and can see the CYA levels go down immediately after heavy rains. I can't completely account for the total percentage loss of CYA, as the percentage of CYA I lose appears to be higher than the percentage of dilution, but my understanding is that CYA degrades with elevated temperatures.
I believe it is also possible that some of what I lose may be from filter cleanings and losing undissolved CYA, although in retrospect losing undissolved CYA shouldn't affect readings.
The other thing I'm not completely sure of is if dissolved CYA is homogeneous in the pool. I suspect it probably is, but if not and the concentrations were higher toward the surface (and subject to being displaced out the overflow) I would proportionally lose more CYA per inch of rainwater. Perhaps we have some chemists who understand that better.
tftestkits.net
