Questions in preparation for pool drain to correct high CYA

[Cetanorak]

Here in Texas we haven't had substantial rains in many weeks/months. I may attempt a drain and refill in the future when the temperatures subside a bit and we are no longer in a Stage 3 drought. I'd prefer to do a full 60-70% drain prior to refilling in order to most efficiently/economically reduce CYA. Regarding the pool popping due to high water table, my pool is an in-ground above-ground hybrid of sorts (see photo below). The pool is sitting up on a rocky cliff that drops a couple hundred feet to a dry riverbed.

Is it safe to assume that there is not a water-table concern (in regards to draining the pool) at this site?

 

[Texas Splash]

I would doubt that any ground water would swell-up that high. Were you in that house 2-3 years ago when Wimberley received that massive flooding? I suspect you could always check with your country office to confirm potential water levels in your area.

 

[Cetanorak]

I would doubt that any ground water would swell-up that high. Were you in that house 2-3 years ago when Wimberley received that massive flooding? I suspect you could always check with your country office to confirm potential water levels in your area.

No, I wasn't here during those historic and devastating floods. We have just purchased this home. It sits high enough up that it was not at risk during those floods, thankfully.

 

[Cetanorak]

I tested my tap water from an outdoor spigot and have CH=175 and TA=290. If I were to drain and refill with this tap water, would the results be better CYA levels but exccessively high TA and CH? My latest pool water results are below:

My last pool water results were (I've since added recommended dose of Muriatic Acid and left to aerate, haven't re-tested)
FC=15.0
CC=0
pH=8.2
TA=140
CH=325
CYA=140

 

[Texas Splash]

It's good you are trying to maintain a higher FC level to compensate for that CYA, but I would still try to lower it when you can. Your CH and TA are not bad at all. If the CH remains anywhere in that wide range of 300 - 600 you'll be fine. The TA can always be lowered later. Managing the pH is most important of the three to prevent scale. But remember the CYA is forcing you to keep a higher FC level and that is impacting the pH reading. Just another reason to eventually bring the CYA down as soon as possible. Maybe one day we'll actually get some rain (hopefully not all at once) and be able to exchange some of that water for free.

 

[Cetanorak]

Regarding high FC and it's consequent effect on pH testing accuracy...is the inaccuracy of pH measurements completely random with higher FC levels or does it tend to error towards being specifically high or low?

In consideration of the inaccurate pH readings, I wasn't sure if I should be adding muriatic acid or not while I deal with high CYA and maintain high FC levels.

 

[Texas Splash]

Regarding high FC and it's consequent effect on pH testing accuracy...is the inaccuracy of pH measurements completely random with higher FC levels or does it tend to error towards being specifically high or low?

Specifically when the FC is at 10 ppm or higher.

In consideration of the inaccurate pH readings, I wasn't sure if I should be adding muriatic acid or not while I deal with high CYA and maintain high FC levels.

Therein lies one of the problems of maintaining an elevated FC level.. It leaves some ambiguity. If your CYA was say 50 or 70, you would have an FC of around 6-9 ppm and your pH test would be reliable. You can just about bet you would be controlling the pH each week, especially with a TA of about 140. Currently the elevated FC is skewing the results, but you still need to control the pH. So if you ride-out the elevated FC for a while, you may elect to let it fall to at least 10 ppm just long enough to get a good pH reading, adjust the pH, then bump the FC back up.

 

[Cetanorak]

So I've done two no drain water exchanges and, as of yesterday ,CYA is down to 100 from 140. Yesterday pH tested at 8.2, so I added the recommended 80oz of 31.45% muriatic acid to target 7.2 (to lower TA). Today the pH tested at 7.8 (FC=8.5)...I know that high FC levels affect pH reading, is it normal to have added 80oz of MA yesterday and the pH is still at 7.8 today.

I'm also not exactly sure about my pool volume, the 20K gallon is my best guess. If the pool volume is not accurate, that would also affect the impact of my chemical additions.