Thinkly said:
I also of course can't measure for combined chlorine because the water never turns pink after adding the 871 powder. It just stays clear.
You don't have to have the sample turn pink for you to test CC. If you don't even see a flash of pink with the powder or you add extra powder to make sure (where clear means zero FC), then just add the 5 drops of R-0003 reagent at that point and if the sample turns pink/red, then you've got CC. I suspect that is what you will see.
As for the TA test, perhaps monochloramine causes the effect you are seeing.
As for pH, perhaps it is what happens when ammonium chloride reacts with chlorine:
NH
4+ + HOCl ---> NH
2Cl + H
2O +
H+
Ammonium Ion + Hypochlorous Acid ---> Monochloramine + Water +
Hydrogen Ion
though in practice with CYA in the water the following is probably what primarily occurs:
NH
4+ + HClCY
- ---> NH
2Cl + H
2CY
- +
H+
Ammonium Ion + Chlorine bound to CYA (ion) ---> Monochloramine + Cyanurate Ion +
Hydrogen Ion
I wonder if the yellowing of the walls came from the pH rising from the storm (perhaps aerating the water with rain/wind) and was really metal staining. Shocking would not have helped in that case and could make it worse due to increasing the pH even further. The algaecide itself probably didn't do anything directly, but the lowering of the pH redissolved the metal stains. That's my best guess at this point.
Anyway, try the CC test again as I described (i.e. don't wait for the pink if you don't see it and just proceed to adding R-0003). If that confirms you've got CC, then the solution is to add lots of chlorine to get rid of it.