I did a full pool water chemistry set of tests today and noticed something very unusual. I have a mostly opaque electric safety cover, but I have a pool cover pump where I dump rain water from the cover into the pool which then overflows to a drain so that I can get water dilution over the winter. I do that to prevent a buildup of salt and to generally refresh the water each season. This year had less rain than others, but even so I got an expected drop in Total Alkalinity (TA) from around 90 to 55, a drop in Cyanuric Acid (CYA) from 25 (at end of season -- was higher earlier) to around 10 (guess based on turbidity, but could still see black dot at 20 mark), and salt level drop from around 1700 to 1300. Based on the rainfall, I'd expect somewhere in the neighborhood of a 25-35% dilution and for the most part that's been true, BUT my Calcium Hardness (CH) only dropped from around 400 to 380 ppm. There were some problems with one of the test reagent bottles, but I had 3 kits and got consistent results with 2 of them (the 3rd was too high).
I noticed this past year that I was using more acid than usual, needing to add it every week or two instead of once a month (remember I have a cover on most of the time) even though the TA was not unusually high. I also noticed that the plaster, which is now 9 years old, seemed to have tiny holes in some areas, though it does not feel rough, and these areas aren't only where chemicals are added over a return. The saturation index was near zero during the summer yet having the pH rise requiring more acid and now seeing that CH must have gone up has it seem as if there was dissolving of calcium carbonate. However, that would have resulted in the TA rising even after acid addition. The TA did rise a little, but not very much. If calcium hydroxide were to dissolve, then the CH would rise but the TA would not (after adjusting pH back down using acid).
I suppose it is possible that the lower saturation index during the winter rain dilution dissolved some calcium carbonate (though that does not explain the increased acid usage during the summer) but the temperature was fairly low around 50ºF (until I turned on the solar recently) and even today when I measured everything the saturation index is only at -0.2 so not very low (and during the winter with the cold water temps the pH was kept at around 7.8). Also, this problem didn't seem to happen in previous years. Does slightly older (9 years isn't very old though, is it?) start to have issues? Anyone have any thoughts about this?
I noticed this past year that I was using more acid than usual, needing to add it every week or two instead of once a month (remember I have a cover on most of the time) even though the TA was not unusually high. I also noticed that the plaster, which is now 9 years old, seemed to have tiny holes in some areas, though it does not feel rough, and these areas aren't only where chemicals are added over a return. The saturation index was near zero during the summer yet having the pH rise requiring more acid and now seeing that CH must have gone up has it seem as if there was dissolving of calcium carbonate. However, that would have resulted in the TA rising even after acid addition. The TA did rise a little, but not very much. If calcium hydroxide were to dissolve, then the CH would rise but the TA would not (after adjusting pH back down using acid).
I suppose it is possible that the lower saturation index during the winter rain dilution dissolved some calcium carbonate (though that does not explain the increased acid usage during the summer) but the temperature was fairly low around 50ºF (until I turned on the solar recently) and even today when I measured everything the saturation index is only at -0.2 so not very low (and during the winter with the cold water temps the pH was kept at around 7.8). Also, this problem didn't seem to happen in previous years. Does slightly older (9 years isn't very old though, is it?) start to have issues? Anyone have any thoughts about this?