pH dropping

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poolguy575

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Jun 21, 2009
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I'm trying to help a friend out with his pool.

The pool is in a heavily wooded area in southern Oregon. Over the winter and during the spring he really let it go with leaves and needles from the trees settling on the bottom of the pool. And then when the weather got warm it turned really green and nasty.

Anyway, when we started in early June the readings on this 19000 gal vinyl liner pool were pH 6.8-, FC 0, Alk 40, cya <20, Calcium 120, TDS 800.

I started by getting the FC up to 15 ppm with bleach at the same time as using tri-chlor to raise the stabilizer level a bit. It took 4 high doses of bleach before we started getting a FC reading. We added 6 lbs of sodium carbonate to raise the pH.

We also added about 4 lbs of sodium bicarbonate to help with the alkalinity. We brushed and vacuumed and scooped and cleaned the filter and now the pool looks great, crystal clear.

What I'm curious about is the pH. Last week it was 7.3 with an alk of 100 and FC of 3 ppm. I expected it to rise a bit with the continued addition of bleach. This week it is 6.8 with an alk of 90 and FC of 10.5 ppm. The cya is just a little more than 20 ppm (melamine turbitity test). He was adding about 96 oz of 6% bleach every day last week. We aren't using the tri-chlor anymore.

Can you think of anything that would be driving the pH down?
 
I keep my pool covered most of the time, but my pH is very stable and only rises very slowly over a month. I only use 12.5% chlorinating liquid.

So I can understand why the covered pool isn't rising in pH, but it shouldn't be falling unless there is some source of acid. If Trichlor isn't being used, then perhaps he's using non-chlorine shock, potassium monopersulfate (MPS) which is also acidic. Even Dichlor is net acidic.
 
chem geek said:
I keep my pool covered most of the time, but my pH is very stable and only rises very slowly over a month. I only use 12.5% chlorinating liquid.

So I can understand why the covered pool isn't rising in pH, but it shouldn't be falling unless there is some source of acid. If Trichlor isn't being used, then perhaps he's using non-chlorine shock, potassium monopersulfate (MPS) which is also acidic. Even Dichlor is net acidic.
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Nope, the pool is being kept covered and not being allowed to outgas at all. Seen it happen many times.
poolguy575 said:
We also added about 4 lbs of sodium bicarbonate to help with the alkalinity. We brushed and vacuumed and scooped and cleaned the filter and now the pool looks great, crystal clear.

What I'm curious about is the pH. Last week it was 7.3 with an alk of 100 and FC of 3 ppm. I expected it to rise a bit with the continued addition of bleach. This week it is 6.8 with an alk of 90 and FC of 10.5 ppm.
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Do the math! :wink: :)
 
Evan, if the pH and TA are dropping, that indicates some sort of acid being added from somewhere. What could it be? If you've seen it before, was there anything in common in those covered pools?

poolguy575, the continued use of bleach is mostly pH neutral because the consumption/usage of chlorine is acidic and compensates for the initial rise in pH upon addition. So at the same FC level, the pH doesn't change (i.e. it goes up when you add bleach and goes back down as the FC drops back down to where it started).
 
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