Rain forms from evaporated water, and before it falls as raindrops, it’s some of the purest water on the planet. As raindrops fall, they gather dust, pollen, spores, and other contaminants. Heavy rains will often raise water levels in swimming pools, filling them with these pollutants and causing skimmers to cease functioning as debris gather on the water’s surface. This accumulation of rain in pool water can also change a pool’s chemistry.
How Rain in Pool Water Affects Its Chemistry
To understand rain’s effects on pool water, think about a pool full of water as a carefully balanced chemical solution. Anything added to the water will affect this delicate balance, including rain. In pool water, contaminants brought by falling rain can negatively affect pH balance, alkalinity and calcium levels. Enough rainwater can dilute chlorine content, making it unhealthy for swimming or even accelerating algae growth.
While light rains likely won’t affect pool water, heavy rains almost certainly will. Large amounts of rain can additionally lead to runoff that can end up in the pool. As water drains from a neighboring deck, lawn, or landscaping, it brings with it dirt, leaves, and other debris.
Though initially, rain has a neutral pH of 7.0, raindrops rapidly absorb contaminants as they fall. These pollutants will normally be low in pH, resulting in what’s essentially “acid rain” that contains varying amounts of carbonic and sulfuric acids. These acids are corrosive, affecting both swimmers and the pool itself. Throughout much of the United States, rain in pool water is commonly acidic, which can cause deterioration to swimming pools.
Low pH (acidic) rain in pool water can cause:
- Corrosion of metal fittings in heaters, filters, and pumps
- Decreased chlorine levels that lead to bacteria and algae building up
- Deterioration of metal surfaces and accessories like railings, light fixtures, and ladders
- Dry hair and skin
- Erosion of concrete, grout, plaster, stone, and tiling
- Itchy skin
- Stinging of the eyes and nasal passages
- Vinyl surfaces become brittle, making cracks and tears in linings more likely
Though rain tends not to be overly acidic, with pH levels usually between 5.5-6.5, this is low enough to lower pH in the pool’s water and cause the above issues over time.
As rain in pool water increases acidity, it also lowers alkalinity. Heavy rains can lower alkalinity levels by 5-10 parts per million (ppm) per day. Low alkalinity too can cause corrosion, damaging underwater metal surfaces on ladders, pool lights, and rails. It also damages filters, heaters, pumps, and other pool equipment through which water flows. Additionally, it can cause pool surfaces to become etched or stained and vinyl liners to become wrinkled and corroded from metal parts leading to discoloration or stains. Low alkalinity can also turn pool water green, unbalance pH and burn the eyes of swimmers.
Essentially rainwater is soft water, which means it’s free of calcium salts. During heavy rain, pool water becomes diluted and loses its calcium hardness. If the pool’s water has a high calcium hardness count – over 400 ppm – rain actually benefits, as it helps resolve issues like scaling on equipment and pool walls, along with cloudy water. However, sufficient rain in pool water will lower calcium hardness, with levels less than 200 ppm leading to other issues.
Low calcium hardness damages vinyl, plaster, liners, grout, and concrete decking. This happens because water is essentially an aggressive solution that seeks to dissolve certain minerals like calcium into it. When calcium levels are too low, water will draw calcium from plaster, causing it to pit and ultimately disintegrate. It also contributes to vinyl pool liners losing elasticity. Along with these issues, low calcium hardness causes metals to corrode slowly. When combined with low pH, these corrosive effects from the rain in pool water will only increase.
Rain dilutes all the chemicals that balance pool water, including chlorine. For this reason, it’s a good idea to add chlorine shock before a storm, especially when there’s a high chance of heavy rain. In pool water, chlorine is key to preventing the growth of algae or bacteria, and keeping it safe for swimming. As rain in pool water lowers pH, chlorine becomes hyperactive as a result, creating a situation where unbalanced chemistry makes pool water unsafe, as well as damaging the pool’s finish and pool equipment.
While rain in pool water doesn’t cause algae to grow, it provides the conditions for its rapid spread. Algae likes to grow in damp places and in standing water, which makes a swimming pool after a storm an ideal environment. Not only do debris like dirt and leaves from runoff pollute pool water, but all this detritus also helps introduce algae spores into the pool. Rain additionally brings organic contaminants like nitrates and phosphates that aid plant growth, enabling algae to flourish.
To help prevent algae growth, it’s a good idea to keep the pump running when rain is forecasted, and the pump should continue to run constantly for about a day after the rain ends. Another good idea is adding and maintaining high levels of algaecide, which won’t degrade in sunlight or when exposed to chlorine. These levels should be added every week to help prevent algae from forming colonies.
Rebalancing Act: Negating the Effects of Rain in Pool Water
It’s important to keep ahead of any changes in a pool’s chemistry caused by rain. In pool water, testing is the key to preventing unsafe swimming conditions. When a test shows a chemical imbalance, this must be addressed immediately. As a reminder, pH should fall within 7.2-7.6, alkalinity should fall between 80-120 ppm, calcium hardness should be between 100-300 ppm, and chlorine levels should fall between 1-3 ppm.
Here are a few easy-to-follow tips for how to address issues of rain in pool water:
- Brush the sides and vacuum the pool floor soon after the storm to remove contaminants.
- For pool owners who use cartridge filters in rainy regions, consider installing an outlet valve past the pump to release water until it reaches an appropriate level for the skimmer.
- Those with a sand filter or diatomaceous earth filter should backwash to quickly lower pool water levels, though care must be taken to ensure backwashing doesn’t lower the water level enough to damage the pump.
- If excessive soil or mulch gets into the pool water, test for phosphates and use a phosphate remover.
- Remove leaves, twigs, and other debris from the pump baskets and skimmer baskets to keep them from getting caught up in the pump and restricting circulation.
- Secure yard furniture or other loose objects around the pool before stormy weather to prevent them from blowing into and contaminating the water.
- Use pool shock immediately after a heavy rainstorm to remove nitrogen or other nutrients that can turn water cloudy or green.
- Utilize a pool skimmer or pool rake to remove larger debris in pool water after it rains.
Rain in pool water isn’t a good thing for your swimming pool, so testing pool water after a rainstorm is always a good idea. Remember too that it’s very important to deal with any chemical imbalance as soon as possible after it rains. And it’s also not a bad idea to consult a reputable pool supply company like Halogen Supply with questions regarding rebalancing your pool water after it rains.