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?