How Heavy Rain Affects Pool Water Chemistry - and How Ozone, UV, and AOP Systems Can Help
- Brian Richardson
- Jun 2
- 3 min read
Being in the swimming pool industry for decades, I’ve seen firsthand how a heavy rainstorm can throw your pool chemistry completely out of balance. You spend all week keeping your water clean, clear, and perfectly balanced—then a big downpour rolls through, and suddenly your pool looks cloudy, your sanitizer demand spikes, and your pH is all over the place.
So, what exactly happens to your pool water during heavy rain—and more importantly, how can advanced sanitation systems like ozone, UV, or AOP help protect your water quality?
What Rain Does to Your Pool
Dilution of Chemicals Rainwater adds a large volume of untreated water to your pool. This dilutes the existing sanitizer (like chlorine or bromine), which means your pool may not have enough free chlorine to keep bacteria and algae at bay.
Introduction of Contaminants Rain washes in organic debris, fertilizers, oils, and other pollutants from nearby landscaping or decks. These contaminants increase the oxidant demand, forcing your chlorine to work harder—and often not fast enough.
pH Swings Rainwater is naturally acidic (with a pH around 5.0–5.5). A heavy storm can lower your pool’s pH, especially in smaller or lightly buffered systems. Low pH can corrode equipment and irritate swimmers’ eyes and skin.
Alkalinity and Hardness Drops The soft, acidic nature of rainwater can reduce both alkalinity and calcium hardness, leading to instability in your water chemistry and possible etching of pool surfaces.
Algae Blooms Combine warm water, diluted chlorine, and added phosphates (from soil or plant runoff), and you’ve got the perfect storm for algae growth.
How Ozone, UV, and AOP Can Help
While you can always correct water balance with chemicals, proactive pool owners are increasingly turning to supplemental sanitizers like ozone, UV sterilizers, and AOP (Advanced Oxidation Process) systems. These technologies can significantly reduce the workload on chlorine and stabilize water quality—even during stormy weather.
Ozone (O₃): The Powerful Oxidizer
Ozone is a natural molecule that destroys organic contaminants on contact. It oxidizes oils, sweat, ammonia, and algae spores far more efficiently than chlorine, but without leaving harmful residues.
How it helps during rain: Ozone breaks down the organic load that washes in with stormwater, preventing cloudy water and reducing chlorine demand.
Bonus: It also helps degrease filters, making post-storm cleanups easier.
UV Sterilizers: Killing Microorganisms with Light
UV systems use ultraviolet light to inactivate bacteria, viruses, and protozoa by damaging their DNA. Installed in-line, UV sterilizers treat water as it circulates, acting as a disinfection checkpoint.
How it helps during rain: After a rainstorm introduces pathogens, UV light ensures that any live microorganisms passing through the system are neutralized—keeping your swimmers safe and your chlorine working more efficiently.
AOP (Advanced Oxidation Process): The Best of Both Worlds
AOP systems combine ozone and UV to generate hydroxyl radicals (•OH), the most powerful oxidizer available for pool water. These radicals break down virtually all organic contaminants instantly.
How it helps during rain: AOP offers the fastest, most complete oxidation of organic and microbial contaminants, handling everything from phosphates to algae spores—great for pools exposed to unpredictable weather patterns.
Final Thoughts
Heavy rain is one of the most common disruptors of pool water chemistry. While regular testing and adjustment are essential, adding an ozone generator, UV sterilizer, or AOP system provides an extra layer of protection. These systems don’t replace chlorine—but they dramatically reduce your dependency on it, help stabilize your water, and make post-storm maintenance much easier.
If your pool is constantly battling the effects of weather, it might be time to upgrade your sanitation system. Over the years, I’ve seen clients go from frustration to peace of mind simply by adding one of these advanced systems, forever making their pool maintenance easier.
Need help choosing the right system for your pool? Reach out and let’s talk through your options, because the only thing your pool water should be soaking up is sunshine, not stormwater.
Contact us today at 877-281-7603 or visit www.UltraPureLLC.com
