What does rain do to pool water?

Rain causes several changes to pool water chemistry.  Two of them produce noticeable results in swimming comfort and the sanitary condition of the pool immediately. Other changes while having already happened may not be noticeable for a few weeks.  These other changes are due to dilution of stabilizing and water balancing chemicals.

In this post I want to focus on the changes in swimming comfort and the sanitary condition of the pool due to changes in the pH and TA.

Rain almost immediately causes the pH (Potential Hydrogen) in the pool water to rise while also reducing the TA (Total Alkalinity) slightly via dilution.  Higher pH will cause more of the chlorine in a pool to become inactive or “fall asleep” reducing it’s effectiveness. Lower TA via dilution also destabilizes the pH to some degree allowing it to move higher more quickly.

The power of chlorine to effectively do it’s germ-killing varies with pH level. As pH goes up, the ability of chlorine to kill germs goes down (see the scale below).

Another affect of high pH is a reduction in swimming comfort. A person’s body has a pH between 7.2 and 7.8, so if the pool water goes outside this range then swimmers will start to feel irritation of their eyes and skin. Ensuring the pH is kept between 7.2 – 7.8 will both ensure the chlorine’s germ-killing power is unlocked while also helping to reduce skin and eye irritation. See this Centers for Disease Control and Prevention (CDC) pH & Clorine article.

Chlorine Effectiveness Scale as Depending on pH (Potential Hydrogen):

Active Chlorine

pH

97%

6.0

91%

6.5

76%

7.0

66%

7.2

<<< 50% >>>

<<< 7.5 >>>

33%

7.8

17%

7.9

10%

8.0

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