A surprising outcome

About a month ago I posted that my pump had died two days before we were leaving for a 3 week camping vacation. There was no time to buy and install a new pump. I added enough LC to bring the FC to 20 ppm. CYA at the time was 40-50. I also adjusted TA to 80 ppm (it normally 'wants' to be 60) to help steady the pH during our absence. Then I put on the solar cover and we were on our way.

Great vacation! We camped along all five of the Great Lakes in excellent fall weather, dry, sunny, warm. In contrast, it rained here in Pennsylvania nearly every day during that time.

I had expected a slightly green pool upon our return. Amazingly, the water was absolutely clear with a fair amount of leaves on the bottom. The new pump arrived and we installed it yesterday, a Pentair Dynamo 1 HP 2 speed. Runs great and is very quiet.

The test results just now returned: FC 3, pH 7.2, TA 50-60, CYA < 30, temp 60F. I had expected the pH and TA drops but am surprised that the FC is as high as it is, relatively speaking. The solar cover kept the UV off the water very nicely!

Given that it's near closing time I'm not going to adjust CYA but I will raise TA to 80 and then adjust pH to 7.5. We'll do the full closing toward the end of the month. Until then, the dogs still swim until their lips turn blue.

BBB rocks!

An FC drop from 20 ppm to 3 ppm over 3 weeks is a loss of 8.6% per day. Between the solar cover and the cloudy/rainy days, there was little UV from sunlight getting through. So the main chlorine loss was from chlorine reactions which are dependent on water temperature. Actual loss depends on the organics in the water and what the chlorine can react with (cover, filter, etc.) and there is the slow chlorine reaction with CYA itself. At 88ºF in my own pool, there can be 15% loss per day with no sun, but this drops to 2.5% per day at 50ºF. I'm guessing your water temp is below 73ºF which is where I'd expect the loss you are seeing if you had no UV at all (so the water temp might be lower than that and the rest of the loss is from the little UV getting through).

The water temp was probably 68 F when we left and is 60 F now.

I'm curious why the TA and pH dropped, can you help me understand that? The solar cover would have been a shield against the rain which, I'm told, was very heavy at times. In the past I've not noticed appreciable changes in water chemistry after heavy rains, i.e., Irene and Lee. Would just the sheer volume of rain be enough to decrease the alkalinity?

If you had enough rain to overflow and therefore dilute the pool water then it could lower the TA and pH (assuming acidic rain, which is typical, though it would take a lot to move the pH). From historical weather data, it looks like you had about 0.16" of rain every day so over 3 weeks that would be 3.4" so with a 4.5' avg. pool depth that would only be a dilution of 6.3% so I wouldn't expect much drop in TA or pH. It's possible the water didn't mix well (since your pump wasn't working) so unless you were taking a sample from depths, you might be measuring closer to rainwater measurements.

I take the test sample from approximately 12" down.

I had expected the decrease in FC, of course. That CYA, TA, and pH also dropped was a bit of a surprise, but so was that I had any free chlorine left at all. I had fully anticipated coming back to a slightly green pool.

The new pump is up and running and doing a fine job.