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.