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Acid Rain - Wow!


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I live in MD. Have a 22K gallon plaster pool (10 years old). I use liquid bleach so the PH tends to drift up. Tested the water and PH is at 7.8. Go in the shed and I am out of liquid acid so I put it on the shopping list for tomorrow. We have a rainstorm that night and the next day I go to put the acid in and test the water and the PH is at 7.2. Wow, the rain dropped the PH that much! Don't remember rain affecting PH that much in the past. I wonder if our rain is getting more acidic? Just curious.

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That is strange. The aeration from rain drops sometimes has the pH rise from carbon dioxide outgassing, but even acid rain shouldn't lower the pH of the pool unless there was a LOT of rain. The reason is that the pool water is buffered while the rain is mostly not (except for some carbon dioxide from air). So usually the amount of acid in rain isn't enough to affect the pool pH. Nevertheless, this is what you saw so I'm baffled by this. As shown in this link, the pH of normal rain is acidic at around 5.6 while acid rain might be down to 4.2.

If I assume that there was 3" of rain and average pool depth is 4.5 feet (54") then I can start with pure water and add carbon dioxide to it to get to a pH of 5.4 and sulfuric acid to get the pH down to 4.2. If the pool TA is 80 ppm, then this results in the pH dropping from 7.5 to 7.36 so I suppose that with sufficiently acidic rain in large quantities that the pH could drop, but your drop was rather large.

Are you reaching down 1 foot or more below the water surface to take your sample? Maybe your earlier high reading was near the surface and after the rain the water got churned up so that you then were reading a more average value. The pH near the surface of still water will be higher due to the carbon dioxide outgassing.

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How close are you to an airport? I'm right in the flight path of DFW airport, sounds crazy, but we hardly notice the planes, except when they weren't flying post 9/11! anyhow. the jet fuel is dumped over us, and no, of course it doesn't rain down on us, but it does end up in our rain, and it affects our pH. Go outside and look at your gutters, if they are streaked (and it will be obvious) then you have a little jet fuel affecting your pH, no big deal, just keep an eye on it!

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  • 2 weeks later...

Well, must have been bad reading on my part. A couple more rainstorms and not a huge drop as I indicated. Thought it was weird. Thanks for the feedback.

Oh, and on the flight path, that is interesting. The planes to BWI airport do go over our house. I haven't noticed the streaks on the gutters but boy is it on the white fiberglass boats at the marina!

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