Alkalinity change

[Kimmmg2003]

Noticed some eye burning today. Have had a whole week of rain. pH is 7.2 which is where it has stayed pretty much since opening. My alkalinity today is 40. Last I checked it was 90 (3 wks ago) Have not had to add acid this season. Would rain change the alkalinity this much?
Should I add baking soda? Then see where the pH is or add both baking soda and borax?

 

[JoyfulNoise]

Did you actually let your pump run for a while to mix up the pool water before testing? My guess is your low alkalinity is a result of testing mostly rain water. It takes a lot of mixing to homogenize fresh water from rain and brackish pool water.

 

[Kimmmg2003]

Today was the first day without rain, pump ran 7hrs

 

[JoyfulNoise]

I suggest you brush and give it another day or so. Your pool water has high TDS from being a salt water pool and rain water has almost no TDS. So the rain water will essentially float on top of the pool water and remain unmixed for quite some time. In order for your alkalinity to drop from 90 to 40, you’d have to exchange half your pool volume with rain water. I doubt that much rain happened. Give it time.