# Thread: The effect of rainwater on swimming pools

1. ## The effect of rainwater on swimming pools

I'm gonna' suggest a possibly controversial idea and speculate that we here on the forum get too caught up in how much rainwater influences our pool chemistry.

My basic premise is this......given that most of our pools average 4 or so feet in depth, 1" of rain (that's quite a bit) has only changed the makeup of the water by 1/48th. The other 47/48ths are still original chemistry and, I believe, will keep the pool very near it's original parameters.

Now, just like you, I can think of many, many exceptions to that idea (2 feet of rain, for example) but my point is those exceptions are not very likely and can be taken into consideration when they occur.

Acid rain is a good example......and I'm gonna' need help with this math. From what little I've picked up on the net. Acid rain averages about 5.0 (again, there are certainly exceptions....let's not "dig" for the exceptions)

So, if your pool is a nice 7.5 and 1/48th of it goes in as 5.0, what is the resultant pH? (I know pH is logarithmic, so I don't think you can simply average the two volumes) My guess is it will be about 7.3 - 7.4 Math help anyone?

Will it affect Alk? Probably some, but I question how much.

FC - Very, very little (unless you get runoff into the pool)

CH - I would think it will hardly move. I don't think rainwater contains calcium. Anyone?

CYA - straight math there....a 1" rain dilutes your CYA by 1/48th...you can't measure it....if it was 40, now it's about 38.4

Now, if your decking on your IG is poorly designed and you get runoff into your pool, that's a significant change and may well imbalance your water enough that it needs attention. At the very least, it will surely get your pool really dirty. For this discussion, we all know run-off could easily "drown" good balance so I'd treat that seperately.

After some posts, I would hope we could get a consensus as to the effect of rainwater on a pool.

2. I agree. In almost all cases the only significant impact of rain is that it brings contaminates, caused more by the wind than the rain, which can lower the FC level temporarily. At the same time, getting two feet of rain over a couple of weeks, while rare does happen in some places, and will have an impact.

3. My own observation here in eastern PA is that even after an extended period of rain the only significant change is a measurable decrease in alkalinity IF the pool was left uncovered. No difference when the solar cover is on.

Our rain does carry a lot of dirt and fertilizer dust which precipitates to the bottom and requires vacuuming.

There's an interesting article on acid rain at Acid Rain - Chemistry Of Precipitation. A partial quote of the chemicals in precipitation:

"Water in precipitation contains a mixture of positively charged ions (or cations) and negatively charged ions (or anions). The most abundant cations are usually hydrogen (H+), ammonium (NH4+), calcium (Ca2+), magnesium (Mg2+), and sodium (Na+), while the major anions are sulfate (SO42-), chloride (Cl-), and nitrate (NO3-)."

Interestingly, rain in agricultural areas is "not acidic (average pH 6.0) because of the influence of calcium-rich, acid-neutralizing dusts blown into the atmosphere from agricultural fields."

4. One of the biggest effects rain can have is aeration of the water if the rain is severe which can actually cause a pH rise (but this will probably be influenced by the TA in the pool to begin with). In some parts of the country 'acid rain' can have a pH below 4.0 and might have some impact. Bottom line, after a rainstorm it's good practice to shock the pool and then rebalance if needed since it cah have an impact on all aspects of the water chemistry if there is a lot of rain.

5. ## Re: The effect of rainwater on swimming pools

Originally Posted by duraleigh
So, if your pool is a nice 7.5 and 1/48th of it goes in as 5.0, what is the resultant pH? (I know pH is logarithmic, so I don't think you can simply average the two volumes) My guess is it will be about 7.3 - 7.4 Math help anyone?
1. Convert the 7.5 and 5.0 to base-10 (on my calculator, I entered 7.5, then hit the "log" button).
2. Calculate a weighted average of the two resulting numbers: (47/48)*log(7.5) + (1/48)*log(5)
3. Convert the result back to a log (10 to the power of the result from step 2)

the whole thing would be: 10**((47/48)*log(7.5) + (1/48)*log(5))
(where ** denotes exponentiation)

rounded to the hundredths place, the result is 7.44.

6. Bottom line, after a rainstorm it's good practice to shock the pool and then rebalance if needed since it can have an impact on all aspects of the water chemistry if there is a lot of rain.
Hmmm. I have never shocked after rain. I realize it's only anecdotal, but my pool remains virtually unchanged after a rain.

Wait a minute, you weren't talking about the forty days and forty nights, were you?

The potential increase in alk because of the aeration makes perfect sense but I have not observed a measurable change.

Jules, thanks for the math. That is about as I suspected but I am simply not smart enough to calc it. Thanks!!

7. Originally Posted by duraleigh
Bottom line, after a rainstorm it's good practice to shock the pool and then rebalance if needed since it can have an impact on all aspects of the water chemistry if there is a lot of rain.
Hmmm. I have never shocked after rain. I realize it's only anecdotal, but my pool remains virtually unchanged after a rain.

Wait a minute, you weren't talking about the forty days and forty nights, were you?
Don't know what the rain is like up in your area but when it rains here in Florida it REALLY rains! Sometimes it seems that an Ark might be a good idea!
The potential increase in alk because of the aeration makes perfect sense but I have not observed a measurable change.
Like I said, it really depends on where your TA and pH are sitting whether you have any measurable effect. I don't see any changes in my pool but several of my customers with high TA do seem to get a slight pH rise.

8. I think the recommendation after an extremely heavy rain (South Texas is very similar to Florida), would be to test, not necessarily to shock. Sure it won't hurt, but if you are able to get out the debris, it may not be necessary.

As far as a couple of weeks of heavy rain, I don't really think that's an issue as you should be testing more often than that anyway - even if it rains every day. I think Dave is questioning the effect of a heavy downpour in a day - the 1-2" example.

9. Thanks for the math refresher Jules! Brings back memories of sleepless nights in college!

10. My experience with rain is that I have to vacuum the pool more often. Other than that the water balance remains, for the most part,unchanged.

11. Like mentioned above, rain is a great aerator. I often use it as a tool when planning lowering my TA. That and 13 year olds...

12. Rangeball,

I'm still trying to ferret out someone that can definitively say that rain has increased their pH. It sounds like that's been your experience. How much rain over what time? How much pH rise did you get?

13. Originally Posted by duraleigh
Rangeball,

I'm still trying to ferret out someone that can definitively say that rain has increased their pH. It sounds like that's been your experience. How much rain over what time? How much pH rise did you get?
It absolutely has, but only in the face of elevated TA. When I get my TA below 90ish, it's much more stable, even after a rainfall.

At 100+ and especially the 150+, it isn't uncommon to see a .2-.6 rise, depending on the amount, duration and force of the rain (lighter rains produce less increase than heavier rains). I attribute it entirely to the aeration process from the rain drops hitting the pool, nothing specific from the rain water chemistry itself.

14. That's an interesting post and makes sense. The more and more I learn about T/A, a range of 80-100 seems a little more practical than the 80-120 we normally recommend.

15. We have high TA fill water which makes it a bit difficult to keep TA low the entire season. I strive to get TA to 80, as I know it's going to go up as I add water due to splash out or evaporation, and give myself the most room possible.

It's another reason I'm highly considering adding borates to 50 ppm. In fact the only thing holding me back is I need to drop my TA from it's current 170.

For a vinyl lined pool, I think 80 is a good target TA.

16. Well, say what you will, but I've had rain storms that have lowered the ph by a good .3. Myself and my neightbors all keep a container of ph up handy.

And the vacuuming that has to be done afterwards.

17. Not disputing that at all, and I should probably clarify that I'm in the central midwest. Your rain chemistry is most likely completely different than ours.

Have you ever collected some rain and tested it? That'd be interesting, and isn't the NE more likely to encounter acid rain?

18. Hotrod,

I have certainly heard of your situation before. The math is what makes me struggle with that. Refer back up to guilietta's post on my hypothetical at the top of this thread.

For even a 2" rain to have much effect on pH, the rain would have to be IMPOSSIBLY acid.

Yet, more folks than you report the same thing (pH changes) so what's going on? I haven't a clue.

PS - Buy 20 Mule Team Borax to raise your pH and save some money over pool store "pH up"

19. ## Re: The effect of rainwater on swimming pools

Originally Posted by giulietta1

1. Convert the 7.5 and 5.0 to base-10 (on my calculator, I entered 7.5, then hit the "log" button).
2. Calculate a weighted average of the two resulting numbers: (47/48)*log(7.5) + (1/48)*log(5)
3. Convert the result back to a log (10 to the power of the result from step 2)

the whole thing would be: 10**((47/48)*log(7.5) + (1/48)*log(5))
(where ** denotes exponentiation)

rounded to the hundredths place, the result is 7.44.
That only works when there are no buffers in the water. It is useful for a worst case but the TA in the water will limit the change in PH. Chemgeek has a spreadsheet somewhere which takes the buffers into acount and will give you a more accurate answer.

20. Let's say that one has an extraordinary 3" of rain and that the average pool depth is 4.5 feet. That's 1/18th. Let's also assume some particularly nasty acid rain with a pH of 4.0. The way to use my spreadsheet for such a calculation is to calculate the equivalent amount of Muriatic Acid which has a pH of -1.0. So one reduces the volume by a factor of 10 to the 4 - -1 = 5 so 1/(18*10^5). Multiplying that factor by 10,000 gallons gives 0.71 fluid ounces. This would have the pH drop (at a TA of 100) from 7.5 to 7.48 which is very small because of the pH buffering.

Were it not for the pH buffering, then the exponential/log formula that was used would be appropriate and would result in the following (which my spreadsheet also calculates if you zero out TA, CYA and other parameters):

-log10( (17/18)*10^-7.5 + (1/18)*10^-4 ) = 5.3

Note that the formula posted by giulietta1 is not correct (it's backwards) since pH is the negative logarithm of the hydrogen ion concentration so one must exponentiate the (negative) pH numbers first, then add the resultant hydrogen ion concentrations, and then take the (negative) log after. Even this isn't exactly correct since it ignores the hydroxyl ions and their relationship to hydrogen ions near neutral pH, but that's not a big factor. [EDIT] Actually, it is a big factor in this case. [END-EDIT] The giulietta1 example would really calculate as follows:

[EDIT]
The following is what I had initially, but isn't really accurate: -log10( (47/48)*10^-7.5 + (1/48)*10^-5 ) = 6.6

The added hydrogen ion combines with the excess hydroxyl ion (at a pH of 7.5 there is an excess of hydroxyl ion) to make water so that must be taken into account in this case since the resulting pH will be too close to neutral. The easiest way to calculate it is to consider the NET hydrogen ion vs. hydroxyl ion via [H+]-[OH-] which is 10-7.5-107.5-14=-2.85x10-7 which is negative since there is an excess of hydroxyl ion so first figure that this amount of hydroxyl ion wipes out the equivalent amount of hydrogen ion to produce water. Then, the rest of the added hydrogen ion is excess. So,

-(47/48)*2.85x10-7 + (1/48)*10-5 = -7.0x10-8

which is negative meaning that there is an excess of hydroxyl ion to hydrogen ion so the net pH is actually above 7.0. The hydrogen ion concentration can be calculated from the following:

-7.0x10-8 = [H+] - [OH-] = [H+] - 10-14/[H+]

[H+]2 + 7.0x10-8*[H+] - 10-14

[H+] = ( -7.0x10-8 +/- sqrt((7.0x10-8)2 - 4*1*(-10-14)) ) / (2*1)

[H+] = 7.1x10-8

pH = -log10([H+]) = 7.15
[END-EDIT]

So the effect some people see of a measurable pH drop after a rain comes from something we have not yet explained.

Richard

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