TA never changes

[C3Cl3N3O3]

So when I opened the pool back in May, my TA was low (50 or 60). I raised it with baking soda to 70. I've been doing straight BBB since then, and I've noticed the TA never changes from 70. I've probably used 5 lbs or more of dry acid, which should have lowered it by 12. In addition, I have a lot of aeration from a fountain and have backwashed quite a bit (all of which should have lowered it even more). Is this an expected behavior? How are these carbonates/bicarbonates magically getting replenished in my pool?

FC: 7
CYA: 50
TA: 70
pH: 7.4

 

[Schadenfreude]

Have you tested your fill water? That is a likely source.

 

[C3Cl3N3O3]

I'm on city water. I haven't tested it. That seems plausible though.

 

[C3Cl3N3O3]

OK I tested the fill water out of curiosity. The TA is 40. So it is picking up some alkalinity from the fill water. Also I wonder what the alkalinity of rainwater is? I'm not sure how the low alkalinity fill water can keep the pool right at 70. Hmm.

 

[camueller]

OK I tested the fill water out of curiosity. The TA is 40. So it is picking up some alkalinity from the fill water. Also I wonder what the alkalinity of rainwater is? I'm not sure how the low alkalinity fill water can keep the pool right at 70. Hmm.

Rainwater has no alkalinity unless it has run off of some other source into your pool.

I'm guessing the TA of 40 in your tap water is doing it. Consider yourself lucky. My tapwater has 250ppm TA. I try to avoid filling with my tap water because I definitely get a rise in my TA when I use it much. But I can only rely on rain so often.