Rainfall dilutes pool chemistry levels and lowers the readings for pH, alkalinity, hardness, stabilizer, and chlorine.
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 reality, an inch of rainfall can only increase an 11,000-gallon pool by around 1.5% in size, which is an unlikely culprit for any major dilution issues. This is even less of a concern in Texas, which only sees on average 25 inches of rainfall per year.
Rain almost immediately causes the pH (Potential Hydrogen) in the pool water to rise while also reducing the TA (Total Alkalinity) slightly via dilution. Higher pH will cause more of the chlorine in a pool to become inactive or “fall asleep” reducing it's effectiveness.
Heavy rains will deplete many of the chemistry levels in your pool. Generally your alkalinity will drop significantly. It is a good idea to have Alkalinity, Muriatic Acid, Chlorine (or Salt), and Shock on hand to be able to test your water and treat your pool immediately after the rain stops.
Baking soda can help kill algae and make your water clear again after a heavy rain. Rainwater will lower your pool's pH, making it more acidic.
Losing more than ½” of pool water per day indicates you likely have a leak in your pool's structure or your pool pump system. You should call your pool service for a thorough leak inspection. You might not be able to keep up with refilling your pool at this point.
After heavy rains, your pool water level will almost always be beyond what it should be around the halfway level of the skimmer's opening. You need to get the water level to mid-skimmer level. For that, you'll need to hook up your pool pump and set it to 'waste' or 'backwash'.
If you need to quickly lower chlorine levels in your hot tub, use a chlorine neutralizer, dilute the water with fresh water, or leave the cover off and run the jets to let the chlorine dissipate. These methods can help reduce chlorine levels to the recommended range of 1-3 ppm.
The consistent inability to maintain chlorine levels is also called “chlorine demand” and is most often caused by a high level of inorganic and organic contaminants in the water. Those contaminants force the chlorine to work extremely hard to oxidize them, leaving little chlorine to protect swimmers.
The maximum safe chlorine level is 3 parts per million (ppm). Anything above 5ppm is considered a hazard and should be addressed immediately. If you find numbers as high as 7-10ppm, the pool should not be used until chlorine levels return to safer levels.
Two different chemical agents that can reduce chlorine levels are Sodium Thiosulfate and Pool-Grade Hydrogen Peroxide (specific to pool treatment).
It contaminates the pool water, making it more alkaline or acidic than usual. It can lead to corrosion of metallic parts or even the walls of the pool. The chemical imbalance in the pool can also increase the algae growth inside the pool, which will cost you extra effort to clean it.
Sodium hypochlorite, the active ingredient in most pool chemicals, can irritate eyes and skin. Do not allow chlorine tablets, pellets or flakes to get moist or wet, as they will start to emit highly corrosive and toxic chlorine gas!
After a heavy rainstorm, your pool's chlorine levels will lower and by shocking your pool it will replenish the chlorine lost from the rain. Be sure while shocking your pool that you keep the pump running. Quick Tip: You can shock your pool in the rain, YES, it's safe!
Backwashing is a great way to clean out clogged filters but there are times you should avoid the process. If your pool has or had an algae issue is it recommended to by-pass your filter entirely and vacuum directly to waste.
Rainwater is known to have acidic properties that can negatively affect your pool water chemistry balance. This can directly affect anything from the pH balance to alkalinity levels and more. Read on to learn more about the effects of rain on your pool and learn how you can prevent these changes from happening.
Pool Losing Water After Heavy Rain
Due to a heavy rainstorm, water loss is more common with vinyl liner pools that can be damaged, come loose, or float to the top. With other types of inground pools, it is vital to get the water level down. If not drained, the water balance will change, causing cloudy water and more.
As for normal chlorine demand, it varies by pool, but is usually somewhere between 1.5 and 3 ppm FC per day with most having around 2-2.5 ppm FC per day. This assumes that the FC starts out at around 10% of the CYA level and that the CYA level is high enough to protect the pool in sunlight.
Many new pool owners are surprised to find out how often they need to add more water to their pools. Evaporation alone will cause you to lose over 25,000 gallons of water each year! Typically you will need to add two to four inches of water each week, although this can vary a bit based upon weather and usage.
Contrary to what some might think, chlorine raises the pH level of the water. When you mix chlorine with water, the pH level typically increases and becomes more alkaline/basic. Therefore, you must monitor and adjust the pH level accordingly when using chlorine as a disinfectant.
If you put too much baking soda in your pool, your alkalinity will rise. That means you'll need to take steps to lower your alkalinity.