Calcium rise…or lack thereof

[Lake Placid]

Hi all, it’s been a while since I’ve posted, been slammed with work. I have a question that’s perplexing me and haven’t been able to find sufficient data from other sources to support my theory so I thought I’d post it up here and let those smarter than me chew the fat and hopefully provide an explanation….or just call me crazy!

-25,000 gallon Pebbletec pool - pebble tech was installed 10 years ago and pebble is in very, very good shape for its age
-Historically chlorinated with trichlor - I just converted it to SWG after some coping and tile work necessitated a fresh drain and refill. The homeowner described his annual issues briefly touched on in the next point, and called me in to balance his water which lead to deeper discussions and ultimately the conversion.
-Homeowner historically “took care” of the water himself with the help of Leslie’s. Always fought mustard algae late in the season due to CYA overdose, and was sold loads of phos free to mask the real issue…CYA levels that were stupid high even by their ”reports”.
-Homeowner never balanced calcium hardness, but did address PH and Alk as best he could with pool store advice.

Prior to conversion I tested the water and made minor adjustments to PH and Alk to bring them to spec. Hardness tested out at 128 ppm. By the math 40 pounds of calcium chloride should raise to 300 ppm target. 4-5 days after the initial dosing when I was testing the water at conversion time the hardness came back at only 178 ppm.
I’ve not seen this before excepting 2 other pebble pools where hardness historically had never been addressed by the homeowner. In each of those cases it took multiple doses over subsequent periods of time and finally calcium reached the desired level, but with a significantly higher volume of calcium chloride than what the math would indicate should be necessary.

So my postulation is that over years of neglect of hardness levels, the water had been leeching calcium from the pool’s surface. Subsequently, when I dosed to achieve appropriate calcium levels in the water, the pool‘s surface that had been having its calcium leeched began leeching the calcium back out of the water. When the surface ultimately reached its desired calcium saturation after leeching it out of the water from multiple additions of calcium, the testing reflected it and no further additions were necessary as I achieved the desired target.

Is this what is actually happening or is there some other issue at play chemistry wise? I can find plenty of data on over saturated water depositing nodules on the surface, but not finding much on the converse where the surface may pull it from the water to satisfy some demand. Thoughts are appreciated.

 

[Newdude]

Is this what is actually happening or is there some other issue at play chemistry wise?

I woulda gone with a bad batch of calcium, but you do you.

Seriously though between the ones that are 75% salt and all the ice melt brands out there, it wouldn't take much for the factory to make/send the wrong batch.

Leeching to me takes time and the water should reflect the addition for quite some time.

 

[Lake Placid]

I woulda gone with a bad batch of calcium, but you do you.

Seriously though between the ones that are 75% salt and all the ice melt brands out there, it wouldn't take much for the factory to make/send the wrong batch.

Leeching to me takes time and the water should reflect the addition for quite some time.

Thought provoking but I used calcium from 2 different suppliers when the phenomenon happened. 1 is a known concrete supplier that I use and is high volume and the other is a distribution house that supports high volume for commercial pools. Keep spitballing though my brain seeks knowledge lol….

 

[proavia]

Have you considered doing a drop based test to verify the PoolLab results?
If you add a specified amount of product to a known pool quantity and don't get the expected results, it's either a product issue or a testing issue.

 

[mgtfp]

I don't think it would be possible for calcium to controllably leech back into the plaster. Either the CSI is very high and you get scaling of the ugly kind, or the CSI is not and the calcium stays in solution. I don't think that the plaster would actually attract calcium carbonate to dock back into the appropriate positions.

That might be a question for @onBalance.

 

[Lake Placid]

Have you considered doing a drop based test to verify the PoolLab results?
If you add a specified amount of product to a known pool quantity and don't get the expected results, it's either a product issue or a testing issue.

Absolutely. I keep a TF100 Salt kit with me as a verification tool if I get a weird result from the PoolLab. In the previous 2 cases I verified the result. In this case I didn’t, (the day at the account ran way to long and honestly I was fried at that point.) I’m back at the account next week and will verify it, but I don’t necessarily have reason to suspect the test result based on other test results at other accounts. That’s not to say I didn’t necessarily get a bad reagent, but the likelihood seems low as it’s liquid based reagent that I’ve used at other accounts without issue.

 

[SoDel]

I agree with @mgtfp . If the water is not yet saturated, it will take calcium from the plaster and water CH will rise. If the water is not excessively overdosed with calcium, I don’t see how the plaster or pool could possibly take it back (in the form of scale or precipitate) — the water will seek saturation and that’s that. You obviously know what you are doing so even without seeing your other numbers, my sense is the water is not being overdosed with calcium. There must be some other cause of your observations. Is it possible there’s some “magic potion” in the water from prior years that is binding with the calcium and causing it to “disappear”?

 

[onBalance]

Nope, the plaster finish did not and does not absorb calcium.
There a few other possibilities for this issue. If adding calcium chloride while the pool water has a very high pH and CSI, then the added calcium chloride became converted (precipitated) into calcium carbonate and then got filtered out.
Another possibility could be that the pool water contained a lot of phosphate or sulfate and caused a precipitation to occur and then was filtered out. But that situation is very unlikely.
Another possibility is a mistake on the assumed water volume of the pool. Here is a way to accurately determine pool volume.

A Chemical Way of Calculating Pool Volume

There is a way to use pool water chemistry to calculate pool water volume without using the traditional geometric calculations. The geometric formulas are fine for pools that are traditionally shaped. However, when pools do not fall into these convenient shapes, a chemical method can be used...

www.troublefreepool.com

 

[Lake Placid]

I agree with @mgtfp . If the water is not yet saturated, it will take calcium from the plaster and water CH will rise. If the water is not excessively overdosed with calcium, I don’t see how the plaster or pool could possibly take it back (in the form of scale or precipitate) — the water will seek saturation and that’s that. You obviously know what you are doing so even without seeing your other numbers, my sense is the water is not being overdosed with calcium. There must be some other cause of your observations. Is it possible there’s some “magic potion” in the water from prior years that is binding with the calcium and causing it to “disappear”?

The water is a 2 week old fresh fill after an acid wash and coping/tile install.

 

[Lake Placid]

Nope, the plaster finish did not and does not absorb calcium.
There a few other possibilities for this issue. If adding calcium chloride while the pool water has a very high pH and CSI, then the added calcium chloride became converted (precipitated) into calcium carbonate and then got filtered out.
Another possibility could be that the pool water contained a lot of phosphate or sulfate and caused a precipitation to occur and then was filtered out. But that situation is very unlikely.
Another possibility is a mistake on the assumed water volume of the pool. Here is a way to accurately determine pool volume.

A Chemical Way of Calculating Pool Volume

There is a way to use pool water chemistry to calculate pool water volume without using the traditional geometric calculations. The geometric formulas are fine for pools that are traditionally shaped. However, when pools do not fall into these convenient shapes, a chemical method can be used...

www.troublefreepool.com

PH was 7.6 prior to ajusting to 7.4. Alk was 66 prior to adjusting to 70. CSI before and adjustments was calculated at -.41 and if targets would have been reached should have settled at -.24.

The volume calculation may be off slightly but the PH and Alk adjustments performed as expected with the given volume so volume can’t be too far off, especially not far enough off that the calcium addition should have deviated that significantly.