Pool Turns Green After Rain
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Why does pool water turn green after it rains? The short answer to this question is that there is not enough chlorine in the pool...but this would definitely be an oversimplification as there are many reasons why you could actually have enough chlorine but the water still is turning green. In this article you will learn which chemicals you need to be testing for in your pool water to get to the bottom of this green water problem once and for all.
First it is important to start at the beginning with pool water chemistry and explain that different pool owners have different notions as to what proper chemical maintenance and balancing looks like in a pool. Some pool owners simply "balance" the chemistry by how the water looks. Others add chlorine and nothing more to their water. In reality water testing and chemical balancing for pools is a science. You can not scientifically test water just by looking at it. Ignoring a dozen different important chemical maintenance values simply for a lack of understanding of their importance is also not how you properly balance pool water. Through luck pools cared for in this way might be clear some of the time, but mismanaged water will be the first pool to start to turn cloudy or green, especially after a heavy rain.
It is important to mention these water chemistry shortcomings that some pool owners have in order for us all to be starting on the same page here. If you think you can pick and choose what you need to maintain in your pool water, or you think that balancing your water chemistry values is unimportant, then you will certainly not be able to appreciate why your water turns green every time it rains, nor will you appreciate how to go about diagnosing and fixing the problem. As it turns out maintaining a permanently stagnant body of water to a sterilized and neutral state in an unprotected outdoor environment is actually kind of difficult. If you are ready to admit that you need to brush up on your water chemistry a little bit then take a look at the pool chemistry crash course.
Why does my pool water turn green after it rains? - The reason that your pool water turns green after is rains is most commonly due to the free chlorine level in the water dropping to zero (or effectively zero) levels. Rain itself is non-chlorinated which means that it directly dilutes your chlorinated pool water to a small degree, but it also contains contaminants which add to the bacterial and organic debris loads in your water and consume available free chlorine levels. Once the free chlorine levels have been depleted, algae growth begins immediately and can grow exponentially when given the right conditions.
In addition to diluting the water and depleting the free chlorine levels in your pool water the rain also exposes marginally poor water chemistry maintenance problems. For example, total alkalinity and pH values within the water will change from heavy rains. If these values were already out of range then this could contribute to having green water. Similarly other borderline deficiencies such as medium-high levels of CYA (cyanuric acid / stabilizer) or phosphate levels creeping up into the problematic range could be exacerbated by heavy rains resulting in the pool turning cloudy or green the next day.
How much does rain dilute your pool? A 20x40' pool is 500 gallons per inch of water. This means heavy rains that cause the pool water level to rise by two inches would be adding 1000 gallons of unchlorinated water to the pool. Using 5' average depth and 30,000 gallons as the pool total volume this means two inches of water would represent about 3.5% of the total volume of the pool. This is not an insignificant number. Also consider that heavy rains over many days would easily double to triple this amount of water at which point you have exceeded 10% dilution of the water in your pool with non chlorinated, zero alkalinity water. Also this rain water will contain environmental contaminants from pollution, smoke, phosphates and other organic elements which all contribute to reducing your free chlorine levels.
Total alkalinity levels - You ideally want your total alkalinity levels to be in the 80 to 120 ppm range. However the marginal range says that as low as 60 is acceptable for total alkalinity levels. In this way a pool that already operates with marginally good water balance parameters (60 ppm total alkalinity) has no ability to deviate further before it is squarely outside of the range it needs to be. A better managed pool with 100 ppm total alkinity has the ability to absorb some disruption from external factors (like a rain storm) and still remain with the ideal or acceptable ranges.
pH levels - Swimming pool water should remain within the pH range of 7.2 to 7.8 and more ideally within the smaller range of 7.4 to 7.6. If your pool water has an improper pH balance such as 8.0 or higher then this will cause reduced effectiveness of your chlorine. Above a pH of 8.4 chlorine in the water is well over 90% ineffective which means that you only have a fraction of the amount of chlorine that you think you do, effectively. While the daily sun exposure or bather turnover is not enough to cause the water to turn green, the reduced effectiveness of the chlorine is exposed after a heavy rain. With the amount of effective free chlorine at zero due to high pH preventing the chlorine from actually working, the algae growth would quickly outpace the ability for the chlorine to oxidize this organic debris.
Cyanuric acid levels - Cyanuric acid (CYA) also known as water conditioner or stabilizer is what prevents the degrading effects from the sun from burning off all of your chlorine every day. It requires between 30 to 50 ppm to accomplish this. Some pool owners use stabilized chlorine regularly in their pool, like chlorine pucks, and this causes the CYA levels to climb higher and higher over time. As the level approaches 100 ppm of CYA the chlorine is being limited slightly from functioning, and when you approach 150 ppm or more the chlorine is largely ineffective in the water. You can still measure a free chlorine level in the water, but the high CYA level prevents the chlorine from working. When it rains the pool grows algae readily as the chlorine levels are too low in effect to keep up with the rate of plant matter growth.
Phosphate levels too high - Phosphates are a technical subject but in simplistic terms phosphates are like a super food for algae. If you have a marginal problem with phosphates you might already be on the verge of having cloudy or green water. In this way rains could be the straw that broke the camels back. Further to this rain is often a source for phosphates to be introduced into your pool. It could be that your pool water turns green after it rains because your chlorine is being readily depleted by prolific algae growth, and this problem is being made worse every time it rains as the phosphate levels climb higher and higher. At 500 parts per billion phosphates become an issue in pool water. Above 2000 ppb the pool will struggle to maintain chlorine levels above zero. Phosphates do not make your water turn green directly like the other factors listed on this page, but they do contribute to green water problems in many pools.
The first thing to take away from this information is the importance of understanding and maintaining the different chemistry values in your swimming pool. The better you are at doing this the less likely you will be to have a problem with green water after it rains. The next most important thing to understand is how chlorine residual works and how having a very small chlorine residual like 0.5 ppm or 1 ppm might be enough under ideal circumstances however any heavy debris load like rain or a lot of swimmers might deplete all of the chlorine you have. This is why 2 ppm or more really is a better minimum free chlorine level to maintain.
In addition to this you might want to consider some preemptive action when you know heavy rains are coming. Many pool owners add extra chlorine to the pool when they know heavy or prolonged rains are in the forecast. This along with additional filtering time for your circulation system (if you normally would have yours off) will go a long way towards preventing green water after it rains. Every piece of organic debris you can remove from the pool with mechanical filtration is one less that your chlorine reserve will need to deal with. Add extra chlorine and filter your water more to prevent green pool water after it rains.
One more consideration for green water could be from the use of a solar blanket on your pool during the day. Ideally solar blankets for pools should be put on the pool in the evening to prevent evaporative losses and then removed again during the daytime. When left on the pool during the day the blanket heats the top few inches of water too warm which alters the water chemistry. Additionally water that gets trapped on the surface of the cover will not be reached by filtered or chlorinated water which means that it is fully stagnant and will grow bacteria aggressively. When you remove the cover from the pool surface this debris and puddled water is laden with bacteria, which is then deposited directly into your pool. Keep your pool cover clean, and remove the cover during the day time to reduce your chances of having green water.
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Swimming Pool Steve is an award winning, second generation swimming pool and spa specialist from Ontario Canada. With over 10,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the 2018 Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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