total alkalinity constantly drifts lower...why?

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eye.surgeon

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Aug 23, 2015
8
California
I have a 21,000 gallon in-ground pebble-tec combination pool/spa in hot central California, open year-round uncovered, that is 4 yrs old and has been managed since day 1 with unstabilized liquid chlorine. I've never had a non-clear day and overall have had very good balanced water. I constantly struggle though with a battle between pH and total alkalinity, every 10-14 days the pH creeps up to 7.8 at which point I add half a gallon of acid and it goes down to 7.2 or so, and slowly creeps up. In addition, the total alkalinity seems to want to live at around 60...I add baking soda every 2-3 weeks and can get it up to my target of 80 but this makes the pH go up faster to the 7.8 level at which point I have to add acid...so it's a constant see-saw between adding acid and adding base. I've got to the point where I'm ready to stop worrying about total alkalinity being at 80 and just leaving it at 50-60, and just keeping the pH in the normal range with a little acid every couple weeks.

Any thoughts or suggestions on this? My other levels are within normal limits. This is the original water in this pool.
 
Welcome to TFP!

First, how are you testing? Are these your numbers or from a pool store?

Do you have any water features like waterfalls or sheer descents? Aeration causes pH to rise, so if you have a source of aeration, this may be why your pH tends to rise.
 
Wildcat said:
What's your CH? 60 TA can work with 7.5 PH. Check the CSI index in Pool Math
I have high CH, 460, and live with 60-70 TA to keep CSI low
Click to expand...
My CH is around 400.
tim5055 said:
Welcome to TFP!

First, how are you testing? Are these your numbers or from a pool store?

Do you have any water features like waterfalls or sheer descents? Aeration causes pH to rise, so if you have a source of aeration, this may be why your pH tends to rise.
Click to expand...
I'm testing carefully with a Taylor k-2006 test kit. I do have both a 2 foot spill-over waterfall from spa to pool (it's smooth water that I think doesn't aerate much but maybe I'm wrong) that runs ~6 hrs day and 3 jet fountains that shoot from the deck up and into the pool and run rarely, like 1 hr/week.
 
Great, that makes me feel a lot better. I was concerned that leaving my TA at 50-60 was somehow a bad thing, but it certainly does want to live at that level. And maybe it is the aeration from my spillover that causes the pH to slowly drift up..the pH is easily manageable though, it's the TA that's been stressing me.
 
eye.surgeon said:
Great, that makes me feel a lot better. I was concerned that leaving my TA at 50-60 was somehow a bad thing, but it certainly does want to live at that level. And maybe it is the aeration from my spillover that causes the pH to slowly drift up..the pH is easily manageable though, it's the TA that's been stressing me.
Click to expand...

As I mentioned, use the CSI index in pool math,it will give you comfort
 

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eye.surgeon said:
And maybe it is the aeration from my spillover that causes the pH to slowly drift up..the pH is easily manageable though, it's the TA that's been stressing me.
Click to expand...

Yes, increased aeration outgases carbon dioxide faster and that causes the pH to rise faster, but note that lower pH also does that so you don't want to lower your pH too much. Your acid addition is what lowered the TA over time so not needing to add as much acid will lessen the rate of TA dropping. So finding a lower TA level for more pH stability and targeting a higher pH level is fine. Because of your plaster surface, you can increase your CH if needed to have the CSI not be too negative.
 
I have the same problem with rising pH, I eventually found that the pool would hold at 7.8 for another week or so and then slowly rise. So now I wait until the pH rises to 8.0 before adding acid, and add enough via PoolMath to take it down to 7.6. It's now about three weeks between acid adds, down from every week previously.
 
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