A discussion of the effect of rainwater on pools

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duraleigh

duraleigh

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Apr 1, 2007
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I see frequent posts wherein folks have attributed their pool water chemistry being out of whack because of rainfall.......I tend to disagree. I have been incorrect before and will be so again but here is my thinking:

Let's assume you get three inches of rain

Your pool averages 48" in depth

Let's also assume you have an AG pool or an IG pool that does not get surface water running back into it. If you're getting surface water, all bets are off and you need some landscaping ASAP.

Let's also assume that your pool before the rain was balanced perfectly...something like this:

FC - 4.0
CC - 0.5
TC - 4.5
pH - 7.4
Alk - 90
CH - 200
CYA - 50

OK, what does the rain change?

Chlorine? Not really. You got a 6% increase in your pool water so that means your TC is now 94% of it's former value....(3.76ppm)

CYA? Nope. Your CYA dropped from 50 to 47ppm....immeasurable.

CH - Exactly the same dilution....you now have 188ppm

"Okay, then, how about pH and Alk, Dave?", you're asking, "Don't you know squat about acid rain?"

Well, I can't do that logrhythmic math but it's been discussed here before and I can tell you your pH moves very little. Subsequently, your Alk will move very little as well.

What this adds up to in my mind is that many of us put w-a-a-a-y to much emphasis on rainfall's ability to dramatically alter pool chemistry.........What's your opinion?
 
This is interesting. I had a drop in PH from 7.5 where it was steady all summer to 7.2-7.3 ish after about 3-4 inches of rain over a week.... cover was on when it rained. I attributed it to dilution of the water, with low PH of the rain?

Aerating as we speak. My TA has also been steady at 110 all summer. Added nothing but bleach since May opening.
 
Hi, Jason,

Rain causes aeration, which can result in sudden increases in PH. I don't disagree, but how much? My pool has never budged in pH due to rainfall in 6 years. Also, Isn't that aeration effect minimized at pH 7.4 and above?

Rain also tends to bring along algae and other organic debris. Again, how much "extra"? I have never noticed an increase in chlorine demand in my pool.

Major storms can significantly lower the FC level. I assume you are referring to the above.

Again, I am not disagreeing but I do not observe this in my pool.....ever (so far). Do we have enough anecdotal evidence that your pH will increase consistently with every rainfall? If so, is there justification for mine never moving?
 
The rate of PH increase from rain/aeration is quite variable and depends on the current PH and TA levels. Your PH/TA may be at levels that don't cause much CO2 outgassing. Some people, notably people with very high TA levels see significant jumps in PH.

The "storm effect" is also highly variable, but the cause of the variation is much less obvious. One possible suspect is that people tend to skip chlorine additions when there is a nasty storm. One way or another, it is noticeably more common to get algae right after a storm than at other times.
 
I don't really have enough experience to comment on the technical things that happen because of rain, but I do blame my current problems on it.
We've only had a pool for a few months now and thankfully I found this forum early enough to be a great help in understanding how to keep a pool in good shape.
Early in our experience we got some algea and knocked it out quickly and enjoyed a crystal clear pool, until the rains came. I'm talking about a couple tropical storm rains, the second of which loaded the pool up with leaves, which we removed as soon as the wind quit blowing.

Since our first experience with algea I've been very diligent testing the FC every day and checking everything weekly. Our FC has been holding steady with a min. of 3ppm and CC usually zero to .5. The PH has been solid at 7.6. Ta has been 80 since getting it down. CH has been 225. The CYA has baffled me, it's been disappearing with great regularity. I have gotten it up to 80 ppm a week ago and today with cloudy water it comes in at about 55 or 60.

As I said the FC and CC has been good but the water clouded up and algea began to become obvious on the bottom of the pool after all the rain. I shocked the pool up to 21 ppm and it dropped to 15ppm overnight so I began putting the chlorine to it and it's getting better but it still dropped today from 21 to 17 ppm in about 6 hours on a cloudy day with more rain. Added more bleach tonight to bring it back to 21.

To me the rain and leaves have brought this algea into the pool where it took off.
Just don't understand where the CYA is going. It seems like I need to add 2lbs. every 3 weeks or so to keep it up to 80 ppm. The only time I've backwashed was to take a few inches of water out of the pool twice in the last few weeks because of the rain.
 
Regarding what happens there's theory and there's the real world. In theory, what Dave said is correct that the change in numbers should be negligible and even acid rain should not make the pH move down very much due to the pH buffering of the pool. The aeration effect and pH rise from it is real and seen by users in Hawaii with frequent and heavier rains (perhaps Florida as well) though this is dependent on the TA level.

What is harder to explain is the tendency of pools to get more algae growth from a rainstorm. The only thing I can figure is that stuff gets blown into the pool that uses up the chlorine so the chlorine level gets low and algae can grow. Whether algae itself gets blown in isn't as important as something getting blown in that consumes chlorine, including pollen or other organic matter, especially if it's small (since small substances have more surface area to volume and use up chlorine faster). I suspect that a wind storm even without rain could cause a similar effect.

Richard
 
chem geek said:
The aeration effect and pH rise from it is real and seen by users in Hawaii with frequent and heavier rains (perhaps Florida as well) though this is dependent on the TA level.
It is definitely seen in FL as well. Then again rain in FL is not quite the same as rain in other parts of the country. It rains hard and heavy! I think climate differences do need to be taken into account.

What is harder to explain is the tendency of pools to get more algae growth from a rainstorm. The only thing I can figure is that stuff gets blown into the pool that uses up the chlorine so the chlorine level gets low and algae can grow. Whether algae itself gets blown in isn't as important as something getting blown in that consumes chlorine, including pollen or other organic matter, especially if it's small (since small substances have more surface area to volume and use up chlorine faster). I suspect that a wind storm even without rain could cause a similar effect.

Richard
Click to expand...
We just has a heavy rainstorm and I found a LOT of garden soil and even several earthworms in my pool after. My FC had also dropped a bit so I believe it IS because of what gets into the pool. I think AGP owners would not see as much of an effect as IGP owners. BTW, my pool IS enclosed in a screeen room and it still had a lot of stuff in it after the rains.
 
We FINALLY had a really good downpour last night. I checked the pool this evening, after letting about an inch or so of rainwater out this morning, and the water was clear. A little bit of stuff had flown in - like two leaves a some dirt. But when I went to clean the skimmer, WOW! Lifted the lid and saw THOUSANDS of these little, TINY tan-colored bugs - just thousands of them. Also found at least a couple hundred drowned winged ants in the basket. When I reached in for the basket and pulled my arm back out, it was covered in these little bitty bugs (all dead, I think). I rubbed my finger over them and they just rolled off like soaked cardboard. After cleaning everything up, I added my usual 2.5 qts of bleach, ran the pump a little, turned it off and turned in for the night!

I got on here tonight to see if I could find a post that says we should always shock after a good rain! And if so, why? Our test results after adding bleach last night:

FC 6.0 (I usually bring it up to 7ppm, but shorted it this time!)
CC 0
pH 7.4
TA 90

Our results tonight before adding bleach:

FC 4.0
CC 0
pH 7.4
TA 90
 

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I think waterbear and chemgeek bring up a good thought....attributing much of the change to organics blown or washed into the pool. That makes sense to me.

Cherie's results are also worth noting. After a good, heavy rain, he/she apparently experienced nothing more than a slightly elevated chlorine loss.

For discussion purposes, I remind that I am not including extremes. If you have 12 inches of rain or you have landscaping issues and 6 inches of mud has washed into your pool, you are certainly gonna' have changes to your pool water.

As waterbear said, AG pool owners may be the best "test" cases because they won't experience nearly as much "wash-in" as the IG folks.

Any of you AG folks experience much chemistry change after a rainstorm?
 
I haven't seen much of a change in test results because of the rain in our agp.
That's the reason I got on this thread.
For several days I vacumed up the stuff on the bottom of the pool while the numbers were coming up good on the testing, every day the amount of algea on the bottom of the pool was increasing. I initially thought it was dirt or tree dust.
Only after bringing the FC up to shock levels could I see a drop in FC readings, CC readings always zero to .5.
 
chem geek said:
In theory, what Dave said is correct that the change in numbers should be negligible and even acid rain should not make the pH move down very much due to the pH buffering of the pool.
Click to expand...

So, RE: my pool :mrgreen:

Is my drop a "negligible" drop?

I'm confident in my PH results and that its not testing error. My tests showed a drop after the additional rainwater (about 3-4 inches) the PH dropped from 7.5 to 7.2. Its been steady prior to the rain and the last three results have been steady too (at the lower number) I'm trying to raise it by aerating, saw some improvement yesterday, about 7.3.
 
So, RE: my pool

Is my drop a "negligible" drop?
Click to expand...
I would say Yes and No!!

The very nature of the pH test lends itself to some innaccuracies. I'd speculate about plus or minus .1pH for the experienced user and .2pH for newbies.

Factor that in with a 4" rainfall (almost 10% of your water) and it could be considered "negligible".

But, like Richard said, theory and real world experience often collide. My original post is redneck theory at best but it does seem to hold true for my pool. It sounds like your pool may be a little different.

Back to the point of my original post.......I have seen several posts this season that go something like this.....

"My pool has been perfect all summer. The pool store told me all my numbers have been perfect, too. Then, a couple of days a go we had this torrential rainfall and now my pool looks like pea soup"

That's what I'm not buying into. A properly chlorinated, balanced pool can withstand a heavy rainfall and can suffer few, if any, ill effects. It is my thinking there has been too much emphasis on rainfall as the source for pool water problems. Rainfall may have been the trigger, but my bet is those pools were out of balance and woefully underchlorinated for an extended period of time.
 
frustratedpoolmom said:
So, RE: my pool :mrgreen:

Is my drop a "negligible" drop?
Click to expand...
3-4" out of 52" is around 7% of the total pool water volume. According to this link the most acidic rain is around a pH of 4.3 (normal rain has a pH of 5.6 from normal dissolved carbon dioxide that forms carbonic acid). 24' round, 52" high is 1960 cubic feet or 14,700 gallons. If I assume the pool has a TA of 110 ppm and starts at a pH of 7.5, then adding this acid rain to the pool (say, 1100 gallons) is equivalent to adding 10-4.3/10-(-1) * 1100 gallons = 0.0055 gallons which is 0.7 fluid ounces of Muriatic Acid (that has a pH of -1). That would result in a drop of pH to only 7.49 so negligible. If the water had no TA so was not pH buffered (e.g. distilled water), then the resulting pH starting from 7.0 would be 5.43. The carbonate pH buffering system makes a huge difference.

So I would call your drop "unexplained", not "negligible". It would take 15.6 fluid ounces of Muriatic Acid to drop the pH from 7.5 to 7.3.

[EDIT] WAIT! I realize that the above calculations work only for acid rain where the pH that is measured for such rain is due to a strong acid. However, that's not what happens. The pH is due at least in part due to a weak acid. Normal rain at a pH of 5.6 has, in 1100 gallons, 0.133 moles of substances with hydrogen and 0.0666 moles of carbonate substances which would still result in a negligible pH drop, but at a pH of 4.3 (solely from carbon dioxide, though that's not right since it's really from sulfuric and nitric acids) in 1100 gallons it's 42 moles hydrogen and 21 moles carbonate. So that needs to get added to your pool water which would result in a pH of around 6.94. To get a drop to a pH of 7.3, this is 10 moles of hydrogen with 5 moles of carbonate which is equivalent to rain with a pH of around 4.6 so quite a bit more acidic than normal rain. Note that I don't believe this calculation is the right way to do this.

So it is possible (with the above calculation) for a heavy rain of acidic rain to lower the pH, though the above calculation isn't right either since what I believe actually happens is that slightly buffered water from carbon dioxide in the air mixes with sulfuric acid and nitric acid (both strong acids) in the air and that wouldn't result in that much acidic capacity in acid rain at a pH of 4.3. It still seems that acid rain wouldn't have enough of an effect, but I'll look into this more.

The calculation is more complicated due to the weak acid (that is, the carbonate buffer system which exists in rain to some extent). This is similar to not being able to calculate the effect on pH from bleach that is 11.5 in pH or chlorinating liquid that is 12.5 in pH since the hypochlorite ion is a weak base so the bulk of the "capacity" of effect on pH comes from the hypochlorite ion that isn't measured in pH itself. [END-EDIT]

Richard
 
Dave... In the past I always tested after the rain and everything was fine on a regular rain.
On a large storm (3" rain) I would get about .2 drop in PH like frustratedpoolmom which I am do east in Ohio. I would add Borax to bring it up and the next day it would want acid to bring it back down. Now I hold off with the borax and the PH returns. I do run my PH low(7.2) for iron in my well. TA at 75. Most of our storms come from the Chicago area. I to believe geographical area makes a difference.
 
I live in west central Florida, and around here in the Summer the weather men program the fax machine with the same forecast: 40- 50% chance of afternoon t showers, and then go on vacation!. In any case I am pumping out 1 to 2" of water per week from my pool. I am not seeing any unexplained changes to my chemistry, just the normal dilution I would expect from adding that much water. In addition the "stable" chemicals: CYA, borates and salt, are all varying at about the same rate. I can't say much about pH as my pool has a rapidly changing pH (raising).
 
CHEM GEEK, I have a headache!! :poke: :-D



chem geek said:
frustratedpoolmom said:
So, RE: my pool :mrgreen:

Is my drop a "negligible" drop?
Click to expand...
3-4" out of 52" is around 7% of the total pool water volume. According to this link the most acidic rain is around a pH of 4.3 (normal rain has a pH of 5.6 from normal dissolved carbon dioxide that forms carbonic acid). 24' round, 52" high is 1960 cubic feet or 14,700 gallons. If I assume the pool has a TA of 110 ppm and starts at a pH of 7.5, then adding this acid rain to the pool (say, 1100 gallons) is equivalent to adding 10-4.3/10-(-1) * 1100 gallons = 0.0055 gallons which is 0.7 fluid ounces of Muriatic Acid (that has a pH of -1). That would result in a drop of pH to only 7.49 so negligible. If the water had no TA so was not pH buffered (e.g. distilled water), then the resulting pH starting from 7.0 would be 5.43. The carbonate pH buffering system makes a huge difference.

So I would call your drop "unexplained", not "negligible". It would take 15.6 fluid ounces of Muriatic Acid to drop the pH from 7.5 to 7.3.

[EDIT] WAIT! I realize that the above calculations work only for acid rain where the pH that is measured for such rain is due to a strong acid. However, that's not what happens. The pH is due at least in part due to a weak acid. Normal rain at a pH of 5.6 has, in 1100 gallons, 0.133 moles of substances with hydrogen and 0.0666 moles of carbonate substances which would still result in a negligible pH drop, but at a pH of 4.3 (solely from carbon dioxide, though that's not right since it's really from sulfuric and nitric acids) in 1100 gallons it's 42 moles hydrogen and 21 moles carbonate. So that needs to get added to your pool water which would result in a pH of around 6.94. To get a drop to a pH of 7.3, this is 10 moles of hydrogen with 5 moles of carbonate which is equivalent to rain with a pH of around 4.6 so quite a bit more acidic than normal rain. Note that I don't believe this calculation is the right way to do this.

So it is possible (with the above calculation) for a heavy rain of acidic rain to lower the pH, though the above calculation isn't right either since what I believe actually happens is that slightly buffered water from carbon dioxide in the air mixes with sulfuric acid and nitric acid (both strong acids) in the air and that wouldn't result in that much acidic capacity in acid rain at a pH of 4.3. It still seems that acid rain wouldn't have enough of an effect, but I'll look into this more.

The calculation is more complicated due to the weak acid (that is, the carbonate buffer system which exists in rain to some extent). This is similar to not being able to calculate the effect on pH from bleach that is 11.5 in pH or chlorinating liquid that is 12.5 in pH since the hypochlorite ion is a weak base so the bulk of the "capacity" of effect on pH comes from the hypochlorite ion that isn't measured in pH itself. [END-EDIT]

Richard
Click to expand...
 
I second that, Poolmom! Believe it or not, when the kids are not around and I can just sit here and concentrate really, really hard on what he's saying, I can actually comprehend some of the stuff! I would be lost totally, without this site and the patient moderators/responders :salut:
 

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