pH (potential hydrogen) measures the acidity or alkalinity of water on a scale from zero to 14, where 7.0 is neutral. Values below seven indicate acidity, and values above seven indicate alkalinity. For swimming pools, maintaining the pH between 7.4 and 7.6 is necessary for two primary reasons. This range ensures swimmer comfort by matching the pH of human eyes and mucous membranes. It also allows chlorine sanitizer to work at its maximum effectiveness, maintaining a clean and safe environment.
How Rainwater Changes Pool Chemistry
Rainwater is naturally slightly acidic because it absorbs atmospheric carbon dioxide and other gasses as it falls, forming carbonic acid. This typically gives rainwater a pH of 5.5 to 6.5, which is significantly lower than the ideal pool balance. Introducing a substantial amount of this lower-pH water directly contributes to lowering the overall pool pH level. However, the most significant chemical impact is usually the reduction of Total Alkalinity (TA).
Total Alkalinity (TA) measures alkaline substances dissolved in the water, functioning as a buffer to stabilize pH. Since rainwater has a TA level near zero, heavy rain dilutes the pool’s existing buffering capacity. As TA drops, the water loses its resistance to pH change, becoming susceptible to rapid and unpredictable swings. The acidic rainwater consumes this buffer first, which is why TA levels often drop more dramatically than pH immediately following a storm.
Contaminants and Dilution Effects
Beyond the chemical interaction of acidity and alkalinity, rainfall affects pool chemistry through dilution and the introduction of new material. Heavy precipitation adds a large volume of water, diluting the concentration of all existing chemicals. This includes the primary sanitizer, chlorine, and the chlorine stabilizer, cyanuric acid. When these chemicals are diluted, the sanitizer becomes less concentrated and less effective at controlling bacteria and algae.
Rain also washes physical contaminants from the air and surrounding areas into the pool water. These contaminants include dust, pollen, leaves, and runoff from the deck and landscaping. Runoff introduces nutrients like nitrates and phosphates, which promote the rapid growth of algae. This influx of organic matter creates a high chlorine demand, meaning the existing sanitizer is quickly consumed as it attempts to oxidize the new debris.
Post-Rain Testing and Adjustment Steps
The first action after a significant rain event is to physically remove debris from the pool surface using a skimmer, ensuring the water level is appropriate for skimming. Removing leaves and dirt immediately prevents organic material from dissolving and consuming chlorine. After this cleanup, the water chemistry must be tested, focusing on Total Alkalinity, pH, and Free Chlorine levels.
Total Alkalinity should be adjusted before pH, as correcting the buffer capacity is necessary to stabilize the reading. If TA is low, sodium bicarbonate (alkalinity increaser) is added to bring the level back into the ideal range. Once TA is stable, the pH can be adjusted; since rain usually lowers the pH, a pH increaser like sodium carbonate is often required to return the level to 7.4–7.6.
The final step is to restore the sanitizer level and eliminate contaminants that slipped past the filter. This is achieved by “shocking” the pool, which involves adding a large dose of chlorine or a non-chlorine oxidizer. Shocking rapidly raises the chlorine concentration to oxidize organic matter and destroy chloramines. Following chemical adjustments, the pool pump should run for at least 12 to 24 hours to ensure the new chemicals are fully circulated and the water is filtered.