A surprising outcome

[chem geek]

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).

[AnnaK]

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?

[chem geek]

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.

[AnnaK]

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.