
A little rain never hurt anybody...but a lot can kill your pool chemistry. Most pros have seen a storm turn perfectly clear water cloudy or green, or knock its chemical balance sideways. So, what exactly happens to the pool when it rains?
The answer, of course, is complicated β but understanding some of the basics can make aftermath maintenance easier.
COLLECTING CONTAMINANTS
Know this: Rainwater is not completely pure. βWater vapor is incapable of condensing into droplets without something for it to grab onto,β explains HASA chemistry expert Terry Arko. βThis could be anything in the atmosphere at the time, including dust, smoke, industrial pollutants, nitrogen dioxide, sulfur dioxide and airborne organic material.β
Rainfall might introduce any number of these contaminants into the pool, where theyβll steal the attention of sanitizers. The more contaminants in the water, the higher the chlorine demand β and at lowered levels, chlorine isnβt as effective as it should be.
Debris closer to the ground compounds the problem. βHard rain washes a lot of things into a pool that will eat up your sanitizer residual,β says Alicia Stephens, education and training manager at Biolab. βYouβll get ground runoff, stuff thatβs coming out of the trees, leaf litter, and if you have any chemicals in your lawn, theyβre draining into the pool, as well.β
SEEING GREEN
If a storm leaves the pool looking swampy, itβs because rainwater can foster the right conditions for algae growth.
βHeavy rain can dilute your sanitizer level quickly, just because youβre adding so much fresh water. When you donβt have enough sanitizer, bacteria and algae can grow,β says Stephens. βBoth of those will lead to cloudy water pretty quickly once they get started.β
Not to mention that algae can feed on backyard runoff; specifically, nitrates and phosphates.
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βDepending on whatβs getting washed into the pool, you get a lot of phosphates from the environment, so youβd expect to see the phosphate level rise after a storm,β says Stephens.
βNitrate levels can increase considerably after a rainstorm,β adds Arko, βwhich could lead to an algae bloom from diluted sanitizer and an increased nutrient for the algae.β
ACID AND ALKALINE
Depending on the amount of rain that falls into the pool, storms can often mess with pH levels. And pH is the cornerstone of pool chemistry, affecting chlorine efficacy and the tendency toward corrosion or scale.
The pH of your rainwater often depends on geography. βIn large cities with lots of cars, the rainwater is acidic,β says Arko, citing nitrogen and sulfur dioxide buildup in the atmosphere. βMost rain that falls in these areas has a low pH, so when it enters the pool, it can cause a reaction.
The primary effect could be a slight lowering of both the total alkalinity and pH.β
At low pH (acidic) levels, water becomes more corrosive to all metals in the pool system, from ladders, rails and light fixtures to heater elements.
But in some places, rain can also be alkaline. βThatβs due to some EPA regulations that changed years ago that require factories to put scrubbers on their smokestacks, and that drives up the pH of their emissions,β explains Stephens. βSo if you live close to a lot of industry or a lot of factories that are putting high pH emissions into the air, the rain picks that up, and then when rain falls, it drives the pH up.β
At high pH (alkaline) levels, water can cause calcium scale to form on plumbing, equipment and tiles.
βAnother consideration of rain in relation to pH is that the disruption of the pool surface from a good downpour could actually create some aeration, which could cause the pH to rise,β adds Arko. βProbably not too drastically.β
CHECK THE FORECAST
Stephens and Arko agree that some extra pre-emptive effort will ease the damage control later. Anticipating the effects of a storm can help a pool pro quickly put pool chemistry back in range after a storm has altered it.
βDonβt just let the pool get bad and then clean it up,β Stephens advises. βIf you have the right chemicals in there, it never gets bad in the first place, because itβs just cleaning everything up as it comes in.β
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βAlways, always, always shock the pool before a storm if youβre able to,β she continues. βThat will build your chemistry up to where itβs a little more forgiving of whateverβs going to get washed into the pool during the storm. But you donβt want to add it too early. If you see itβs supposed to storm on Tuesday, and you shock your pool on Sunday, thatβs not very helpful.β
Stephens also suggests adding an enzyme and clarifier blend. βPutting that into the water before a storm will help break down any contamination that comes in,β she says.
Arko recommends buffering up both the total alkalinity and pH, by at least 10ppm and between 7.5 to 7.6, respectively. βIf there hasnβt been any rain for a long time, then thereβs a good chance that there is a lot of pollutant in the atmosphere, so that first rain will be loaded with it,β he says. βTop the chlorine up, and even add a polyquat algaecide to prevent a bloom from the high nitrates that could enter the pool.β
βIf a very heavy and lengthy storm is expected, it may be prudent to protect the pool and the equipment from flooding with strategic placement of sandbags,β Arko adds. βMake sure all equipment and chemical storage rooms are protected from excessive moisture.β