TA slowly rising, have not added any TA increasers

[jrs_diesel]

Just like the title says, my TA has been slowly increasing since March and I can't figure out why. I have not added any baking soda, washing soda, or borax. According to my log book, in March, my TA was 80, and it has slowly crept up, and now it is 100 this week.

The only chemicals I've added since March has been CYA, muriatic acid, calcium chloride, and HASA liquid chlorine. I had to bring my CYA and CH up after we had several heavy rains, that caused the pool to fill all the way up, and me to have to pump it out to get the level back to normal. Liquid chlorine is added daily, and I use half a gallon of 12.5% on a sunny day (3 ppm). Acid is added as needed.

What could be causing the rise in TA?

 

[JoyfulNoise]

Can you post test results?

Can you also test your fill water to see what the TA is?


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[chem geek]

A small amount of TA rise will come from the excess lye in the chlorinating liquid, but that would only be around 2-3 ppm for 60 gallons of 12.5% chlorinating liquid from Hasa in your 20,000 gallon pool.

Once we get your test results as requested, including that of the fill water, we can see if evaporation and refill explains the rest of the difference. I assume you don't have a pool cover, correct? According to this report looking at the pan evaporation rate (which is net of rainfall) at "Thompson's 3 WSW" that is closest to you, the evaporation from March to June is 25.74". What is the average depth of your pool? If it's 4.5' (3' shallow end, 6' deep end) then you would get a 20 ppm TA increase if the fill water were (54/25.74)*20 = 42 ppm TA or higher which certainly seems plausible. The Water Quality Report for League City, TX shows a TA of 82 ppm so in fact your TA rise is even lower than expected from evaporation and refill. This is probably because of the Muriatic Acid you have been adding.

So we would have expected the TA to rise by say 3 ppm from the chlorinating liquid and (25.74/54)*82 = 39 ppm from the evaporation and refill so 42 ppm total. You only found a 20 ppm TA increase so this implies your adding 14 cups or somewhat less than a gallon of acid over that time. Does that sound about right? If you added more acid (let us know how much total acid you added since March) then there are yet other sources of TA increase. How old is your plaster?

You also mentioned that you had seasonal rains where you subsequently lowered the water level. That would also lower the TA some via water dilution so for balance you may have added less acid than I indicated above.

 

[jrs_diesel]

Current readings

CYA - 40
TA - 100
CH - 300
FC - 3 to 6
CC - 0

During the time (over several weeks) when we got the heavy rain storms my readings were:

CYA - 10 or less
TA - 80 to 90
CH - 250
FC - 2 to 5
CC - 0

I used calcium chloride (10 lbs) to bring up CH. And I added 2 gallons of liquid CYA to bring up that level.

I have not needed to add any make up water (thanks to the rain), so I have not tested my tap water. TA did not spike up after any of the rain storms, nor after adding any of the chemical. Seems to be more of a slow and steady rise.

 

[jrs_diesel]

Only recently (no rain) have we seen significant evaporation, about 2½ inches over the last two weeks. Our pool is a sport pool, deepest point is 5'3", sloping up to 3'10 at the ends (pool is symmetrical). And you are correct, we do not use a pool cover.

For acid, I only add 8-12 oz at a time, sometimes 20 oz if needed. But I have not gone through a lot of acid, maybe a gallon over this time period.

The plaster is old. The pool was built around 1992, and I believe the plaster is the original plaster. I cannot use a nylon/steel brush for brushing the walls, I get plaster dust if I do. I have to gently brush with a nylon brush, and even then I get a little bit of plaster dust. There are a few small spots where the plaster has been worn down to the concrete shell. And I see what I think is some of the plaster dust in the skimmer sock when I vacuum, and also in the Polaris debris bag. We plan on re-plastering next year.

Whenever the pool tops out from the rain, it's approx. 1500 gallons that I pump out to bring the water level back to normal. The pool topped out 4 times this year, and came close a 5th time. Three of those even happened during this time frame. I did not see a significant decrease in TA during this at all, only CYA and CH dropped.

 

[chem geek]

So you do not have an auto-fill? That is strange that you haven't seen the roughly 7" per month of evaporation normally expected for your area. Perhaps it has been raining more.

If you are seeing plaster dust, then you may be getting some calcium carbonate into the water. Your CSI from your latest numbers is balanced if your pH is 7.5 and your temperature is around 80ºF. So unless the pH is lower you probably aren't dissolving calcium carbonate that would cause pH and TA to rise, but the CH should rise as well.

So 1500 gallons out of 20,000 is a 7.5% dilution each time and you did this 4 times. So assuming the rain water was fairly low in TA (it normally would be) then perhaps your TA rose by 20 ppm overall you've measured plus about 20 ppm that the acid knocked down plus the roughly 20 ppm from the water dilution for a total TA rise source of 60 ppm. You said you did not see the CH rise during this time and instead it dropped by the expected amount from your water dilution from the rain, mix, removal.

If you saw the CH rise along with the TA, then that would explain TA rise perhaps from calcium carbonate dissolving. If it were from evaporation and refill, you should have seen the CH rise along with the TA since the water quality report shows 123 ppm TH (so probably around 85 ppm CH) with 82 ppm TA. I don't see any source for rising TA that wouldn't involve increased CH unless the Hasa bleach you were using had more excess lye in it than it should as that increases both pH and TA. The only other item you've added that would increase TA is the Cyanuric Acid where at a pH of 7.5 about one-third of the CYA ppm will show up as increased TA ppm. However, you were just restoring the CYA (and CH) that was lost from water dilution from rains so that should not be any net increase.

 

[jrs_diesel]

Yes, we would normally get a rain shower or two a week up until recently, which was nice since it kept up with whatever evaporation we had. Over the last week and a half, we've had no rain at all and a lot of sunlight. So my level is about 2 inches down from normal. And no, we do not have an auto-fill.

I keep the pH between 7.5-7.8, though recently I've kept it more towards 7.5 since the water has been so warm. Water temp is usually 88-92 lately. It's almost too warm to enjoy when it's in the 90's

On the CYA, I used Pro-tech liquid CYA. I did get a drop in pH when I added that, but I did not see a spike in the TA after I added it.

I've been using HASA 12.5% since we moved in last summer, and about 2 gallons of walmart bleach when I needed chlorine in a pinch. Is there any way to tell if there's more sodium hydroxide in the HASA chlorine than usual or should be in there?

 

[chem geek]

There's no easy way to tell about the amount of excess lye in the bleach except possibly to test it's pH with an accurate meter. If the pH is around 12.5, then the amount of excess lye is small, if it's 13.0 then it's almost 4 times higher.

The liquid CYA should have increased the TA level. CYA itself would not because the acidity lowers the TA by the amount that the CYA increases it. With liquid CYA that is sodium cyanurate, it should have raised the TA by about 1/3rd the amount of CYA added.

So while we haven't figured out the source, it's not a huge deal since lowering the pH and increasing aeration can lower it faster if you want.

 

[jrs_diesel]

I don't have a pH meter, so I won't be able to test my chlorine.

I will do the acid/aerate method if it rises much further. This will remain a minor mystery then as to the source.


Maybe the bigger mystery is where my CYA went? I've added 3 gallons of liquid CYA this year, which Pool Math says should have brought my CYA up to 55. I'm just barely at 40. Started the year at 30. With all of the rain and draining, that level should have only dropped to about 20. So I've had about 35 ppm of CYA just disappear since early spring. That's one I still haven't figured out.

 

[chem geek]

There is some slow chlorine oxidation of CYA, but usually in a pool that's only around 2-3 ppm CYA per month loss (in a hot spa it's around 5 ppm CYA per month loss). So that might explain a 10 ppm or so drop, but not 35 ppm.

 

[JoyfulNoise]

I never use liquid CYA as I prefer granular (99% CYA). Is it possible that the liquid CYA degraded and is not at the concentration that is stated on the label? Does the bottle have an expiration date on it or a date code for when it was manufactured?


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[jrs_diesel]

I didn't notice any dates on the bottles (which have long since been recycled). I seem to remember it had a 25% concentration on the active ingredients list.