Creeping pH

[Pool in Carolina Blue]

I have a 23,500 sf in ground gunite pool with attached spa. Yesterday afternoon I backwashed my DE filter and recharged with DE. Due to water loss, I added water that is sourced from my well. This well water has a low pH of roughly 6.2. After adding the well water yesterday afternoon, I tested the pool and the pH was 7.5. Test results were:

FC 5
TC 5
pH 7.5
TA 120
CH 270
CYA 65

We had a very light rain last night and I tested again first thing this AM. All of the results were the same except for pH which has creeped up to 7.9. I have had a continuing problem with the pH rising and I am not sure the cause of it. Also, I have not added CH in a week yet my readings have held steady at 5. Swimmer load has been fairly light with 3 or 4 kids twice this week along with my Portuguese Water Dog. Any ideas on the creeping pH and non reduction of CH?

Your thoughts are greatly appreciated.

[chem geek]

There are several possible sources of rising pH and I need more information to narrow down which source is the dominant one in your case. I assume since you talked about adding chlorine (you said "I have not added CH in a week", but I assume you mean chlorine) that you do not have an SWCG system and are manually dosing with some form of chlorine (chlorinating liquid or bleach).

You should test your well water for CH and TA as well (you already tested it for pH). It is possible that your well water is high in CH so even replacing large amounts of your pool water may not reduce the CH.

Since you say you did not have to add chlorine for a week and that your FC has remained steady at 5, do you use a pool cover (either an electric opaque cover or a manual solar bubble cover)? Is your pool exposed to direct sunlight? It would be extremely unusual to hold the chlorine level over a week unless your pool was covered or not exposed to sunlight at all, and even then it should still drop at least some. How are you measuring your chlorine level -- what kind of test kit are you using? Is it OTO test where you look for the intensity of the yellow color for chlorine, or is it the DPD test where you look for the intensity of the purplish color for chlorine, or is it the FAS-DPD test where you count the number of drops to get the chlorine level? Perhaps the "5" reading is the maximum in your OTO or DPD test and you really have more than 5 in your pool.

You say you have a gunite pool. How old is it or when was it last replastered? If it is less than a year old, then the rise in pH could be due to the curing of plaster which will increase both Calcium Hardness (CH) and pH.

My guess is that you are outgassing carbon dioxide from your pool/spa since the spa probably has significant aeration when in use. You can lower your TA level to reduce this problem by using Ben's Lowering Your Alkalinity procedure. If your pool has been outgassing carbon dioxide, then you should see a slow decline in TA over time (after adding acid to lower you pH back to normal levels).

Richard

[Pool in Carolina Blue]

Richard:

Yes, I am manually adding bleach as the source of chlorine. I have an in-line feeder but I stopped using it when I found this site! I have some pucks in the feeder but it is turned off. I did test my well water for CH and there was no trace at all. I did not test for TA though.

My pool is five years old and it does not have a cover. I am in Charlotte, NC and the pool gets direct sun in the morning up through 1:00 PM. I agree that it is unusual to hold the 5 level for this period of time, but your point is well taken in that maybe my level has been above 5 and I just did not know it. I have a Taylor kit (DPD) that only reads up to 5 so maybe it has been at 10 or higher and I have not differentiated the color at these darker levels. Would the FAS-DPD be a better choice?

I moved into this home a year ago and to the best of my knowledge, it has never been replastered. I will follow your advice and try Ben's Lowering Your Alkalinity procedure. Another question, since my spa "feeds into" my pool via a waterfall and the chemistry for a pool is different for a spa, is there a way for me to attain different levels of chemicals for each vessel?

Thanks in advance for your help,

Mike

[chem geek]

Would the FAS-DPD be a better choice?

Mike,

Yes, FAS-DPD would be a better choice, especially since you will be keeping chlorine levels high (for your spa). You will be able to measure high chlorine levels and it will be more accurate as well. You can either get Ben's kit or an appropriate Taylor kit (the K-2006 would be the one to get unless you already have another Taylor or similar kit with other components in which case you could get the K-1515-A for just the chlorine FAS-DPD part).

Richard

[Pool in Carolina Blue]

Thanks for the help Richard. I will order Ben's kit.


Mike

[waterbear]

Richard:

Another question, since my spa "feeds into" my pool via a waterfall and the chemistry for a pool is different for a spa, is there a way for me to attain different levels of chemicals for each vessel?

Thanks in advance for your help,

Mike

Since the spillover is always in operaton (except when you have the spa on, I assume) then treat them as one body of water. I assume your spa is heated only when in spa only mode when you are using it. Your FC of 5 is fine for the spa. If you want you can bump it up anothe 1-2 ppm before use with bleach. Once you shut off the spa I assume it returns to spillover mode. This effectively changes your spa water so you can keep the chemistry the same as for your pool. Actually, your numbers look good for either pool or spa (as long as you keep the pH down to around 7.5-7.6)

[Pool in Carolina Blue]

Waterbear:

Yes, your assumptions were all correct about spillover, spa mode, etc. Thanks for the advice and I will "treat it as one." I wish I had found this site a year ago!

Thanks,


Mike