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Thread: Pool Alkalinity Drifts Down

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    Pool Alkalinity Drifts Down

    We have a 40,000 gallon plaster pool with a sitting pool that waterfalls into the main. We use liquid chlorine and muriatic acid. The PH and Chlorine is controlled by a CAT 2000 controller. The PH is typically 7.4 and the chlorine runs between 2-3. Pool temperature is 82-84F. Alkalinity was at 40 when first measured. Started adding baking soda at rate of 8 lbs every couple days. Got alkalinity up to 90 by adding about 40 lbs over a couple weeks. Now in the last week or so it has drifted back down to 50 and thick white scale lines are forming on the side of the sitting pool where it waterfalls into the main pool. I am worried scale may also be forming in the heat exchanger as the heater doesn't seem as efficient as it used to be (runs all the time and return water line feels cool). Can anyone help explain what is happening and give me advise as to what to do?

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    Re: Pool Alkalinity Drifts Down

    Downward TA would not contribute to scale....just the opposite.

    What is the CH of your pool?
    Dave S.
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    JasonLion's Avatar
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    Re: Pool Alkalinity Drifts Down

    The acid feed is going to consistently drive the TA down to it's equilibrium level, which is going to be fairly low because of the waterfall.

    Is the white scale forming on surfaces that get wet but mostly sit there letting water evaporate off of them? Or is it forming in places with consistent water flow? Areas that experience consistent evaporation will always form some scale because the minerals in the water get left behind when the water evaporates.
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    Re: Pool Alkalinity Drifts Down

    OK. I tested the CH. It measured 100. I never tested it before so I wasn't really sure how blue the sample should get. It started pink then went to purple and was a blue purple color when I stopped. The scale on the sitting pool is in an area where water runs continuously, but most the water cascades out while some may dribble down along the tile. I included a picture where you can see it behind the water on the right side about two tiles in from the edge. It actually stands out much more than seen on the picture. I scraped it off last weekend and this weekend it was back. I use a scraper and it actually flakes off.
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    Re: Pool Alkalinity Drifts Down

    One more thing to note. The sitting pool has it's own two HP pump for of a waterfall effect (kind of a dribble without it running). It runs continuously. There are air adjust dials around the sitting pool that suck air in such that the jets shoot water and air bubbles. Usually they are wide open.

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    JasonLion's Avatar
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    Re: Pool Alkalinity Drifts Down

    In that kind of situation, the wall behind the waterfall will constantly get wet and constantly be evaporating, so there will always be some scaling accumulating on the surface. However, getting to visible levels in only a week is quite unusual, so something additional might be going on. Unfortunately, nothing comes to mind.
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    Re: Pool Alkalinity Drifts Down

    I'm going to be brutally honest with you. I sympathize with you and would feel exactly the same about seeing this, but there is nothing you can do to prevent it. You have an intermittently wet/dry area there as Jason said, and in those places, water will leave behind whatever dissolved solid is in it. I wish I had a better answer for you, but it is the reality. You will have to keep removing it or live with it. As an aside, it sounds like you need to push your CH test another couple of drops, maybe a few. Faulty Chemistry isn't your problem for this though. Sorry to be the bearer of bad news...
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    Re: Pool Alkalinity Drifts Down

    That white area seems quite localized and I suspect it is efflorescence. That's when dissolved minerals migrate from behind the tile out to the surface where they precipitate and form solids.

    There is no practical cure but cleaning weekly with muriatic should work. IT most always stops with time but that can be a LONG time
    Dave S.
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    Re: Pool Alkalinity Drifts Down

    Thanks for the great answers. The next question is should we be constantly fighting this low alkalinity by adding baking soda regularly or should we let it go to 40 where it seems to like to be? Should we raise our PH target on the controller from 7.4 to 7.6 or higher? Would it help to turn off the waterfall overnight? Just seems like the aeration throws us in a pattern of high chemical use. For pools with aeration would it be better to use solid chlorine tablets that tend to lower the PH? Sorry for all the questions, but it really great to talk people who really know about pools. I've heard so many different theories from people who think they know all about pools because they use to swim in one.

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    JasonLion's Avatar
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    Re: Pool Alkalinity Drifts Down

    I don't recommend letting TA get down below 50, unless you also have borates in the water. Very low TA leaves the PH with too little buffering, which can allow sudden PH swings. Borates provide a separate PH buffer, which allows you to lower TA a bit lower than you otherwise would.

    Raising the PH target on the controller will significantly reduce the rate at which TA goes down, and might even eliminate the drift (thought that is unlikely). Depending on your setup, keep in mind that raising PH will have other effects that need to be compensated for. For example ORP is very sensitive to PH and will need to be recalibrated to actual FC levels if you change the PH.

    Chlorine tablets have a number of effects, and it is fairly unusual that you would want all of them. In this situation the main issue is that they constantly add CYA, and eventually your CYA level will get too high, which will cause worse problems then the ones you are having now.
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