CYA dropped without replacing water?

[momspool1]

Hi there,

The CYA in my pool dropped from 90 on August 24th to <30 yesterday/today. I took over care of the family pool recently but was away from Aug 26th-Sep 11th.

I had just started slamming the pool before I left and asked my mom to continue for me, but she did not have time to vacuum the pool daily (which it needed- there was a bunch of sludge on the bottom), so I told my mom to just keep the chlorine at the normal recommendation with the poolmath app instead of at SLAM levels.

I got back yesterday and the CYA level was <30. I repeated the test this morning and got the same— I can see the black dot even when the column is entirely full. I put in enough chlorine to reach SLAM level last night and this morning my FC was =<1.0 ppm, so this fits with the lack of CYA.

My mom did not excessively back wash while I was away and she did not do any partial drains.

How is it possible that the CYA dropped?

 

[mknauss]

The only way that happens without water exchange or testing error is that a bacteria entered the pool system when the FC was zero and that ate the CYA. It results in ammonia in the water that takes large amounts of chlorine to combat. The symptom is rapid loss of chlorine (minutes), low pH, and high CC.

Ammonia - Further Reading

www.troublefreepool.com

 

[Gary Davis]

I agree with the OP that something isn't adding up correctly.

For one example, while I'm told CYA disappears very slowly over time, the CYA is mainly going to so precipitously drop from CYA 90ppm to CYA 30ppm with a large amount dilution (or reverse osmosis, which isn't happening, or some other major degradation process).

Given the previous caretakers would have had to replace a substantial portion of your pool water to drop from CYA 90ppm to 30ppm, my first suggestion would be to strongly suspect the initial CYA 90 reading.

My second suggestion would be to keep watch over the CYA to see if these large drops occur again, as you need more data to back up such extraordinary readings.

On the chlorine levels, if you set the FC the night before to SLAM levels and it dropped to FC <1ppm in the morning, sans sunlight - that is most likely NOT due to CYA levels for two seemingly obvious reasons:
1. CYA of 30ppm is just fine to protect FC levels (even in sunlight)
2. Sunlight doesn't happen overnight to degrade FC levels

Offhand, my first observation is that, if you were thinking of SLAM'ing, then I'm assuming you have large amounts of algae, which could affect the FC levels overnight.

Do you have large amounts of algae?
And, what is the nature of this "sludge" anyway?

Why isn't the filter & cleaner not picking up the sludge?

 

[momspool1]

The only way that happens without water exchange or testing error is that a bacteria entered the pool system when the FC was zero and that ate the CYA. It results in ammonia in the water that takes large amounts of chlorine to combat. The symptom is rapid loss of chlorine (minutes), low pH, and high CC.

Ammonia - Further Reading

www.troublefreepool.com

I followed your link and read about ammonia. My pH is 7.6, so this piece of the puzzle doesn’t fit— but the rest of the description sounds right.

I will do a 30 min FC loss test this evening after work to look into this further.

 

[wireform]

As Marty ^^^^ said othere then the water exchange which wasn't the case it would be ammonia as long as your story holds that it was 90. How old are the testing reagents. Are they kept in a cool place. Perhaps some testing error.