Many pool owners question the effect of rain on their pool water, with the large amounts of rain falling over Metro Atlanta. We are going to explore how your pool reacts to rainwater. 

Rain which is formed from evaporated water, leaves behind minerals, carbonates and dissolved solids on earth, as it evaporates into the air. And just before it condenses into rain droplets it is the purest water on the planet. As rain falls through the air, it picks up small amounts of pollutants, spores, dust and other contaminants. Sounds much like phosphates in your pool water, right?

When heavy rains move over your home, the rainwater can raise the water level in the pool. If the water gets high enough, the skimmer can stop functioning, meaning it no longer circulates and skims the surface of your pool. This allows leaves, bugs, flower buds, and other debris to accumulate on the water surface and eventually settle to the bottom of your pool.  It is very important as rainwater falls into the pool, that you backwash your pool down or use a pump to pump the water down if it gets to high. 

What About Your pH?

Rain is initially neutral in pH, a perfect 7.0 – but very quickly absorbs low pH contaminants as it falls through the sky. The effect of low pH rain on your pool pH level is usually small, except for heavy rains over a period of days, which can require a need to raise your pH. Acid rain is rainwater that has absorbed carbonic acid and sulfuric acid from atmospheric gases. Such rain is not usually extremely acidic, but may fall in the 5.5-6.5 pH range, low enough to affect your pool pH levels, again causing a need to raise your pH. 

Will Rain Change My Alkalinity?

It is a fact that rain will decrease your Total Alkalinity level in the pool over time, requiring a pool owner to raise the alkalinity. Rainwater has a Total Alkalinity near zero. Heavy rains will dilute your pool alkalinity level, causing it to fall by 5-10 ppm a day.

Low Alkalinity and low pH levels in pools can create a corrosive water environment. Corrosive pool water can result in the damage of underwater metal surfaces, such as pool lights, ladders and rails. As well, corrosive water can damage pool equipment that water flows through, such as heaters, pumps and filters. As well, etching of masonry pool surfaces, wrinkles can form in vinyl liners, and the corrosion of metal parts can release into the water that may lead to staining and/or discoloration. Corrosive conditions can be controlled by the adjustment of the pH, total alkalinity and calcium hardness.  

What Will Heavy Rains Do To My Pool Calcium Reading?

Rainwater is soft water, having near zero hardness, as measured by the amount of calcium salts in the water. If your pool water is soft to begin with (less than 200 ppm), the addition of hundreds of gallons of 0.0 ppm rainwater can decrease your pool water calcium hardness, requiring the addition of Calcium Hardness to the pool. If your pool water is hard to begin with (over 400 ppm CH), rainwater could be beneficial to lower hardness levels, and being the only known method, the solution is dilution.

Heavy Rains and Chlorine- Should I add more Chlorine?

You may see a trend developing here. Rainfall dilutes pool chemistry levels and lowers the readings for pH, alkalinity, hardness, stabilizer, and chlorine.

Rainfall does not contain chlorine. As well, rainfall brings with it small amounts of contaminants that are washed into the pool. Leaves, dust and debris also creates a higher chlorine demand and uses up your chlorine. It is smart, as a pool owner, to raise your chlorine levels if you know a storm or heavy rain is forecasted for your area. 

What About Salt?

Salt pools may be diluted in the event of heavy rains, requiring additional salt to the pool water. Storms can also add environmental debris, requiring additional chlorination. 

Can Heavy Rains Cause Algae?

Rainwater itself does not cause algae, but it can provide the right environment for algae. Rain will bring phosphates, nitrates and other organic contaminants into the pool. As we discussed earlier, rain also reduces chlorine levels. If large amounts of rain are forecasted for your area, make sure you are running your pool around the clock for twenty-four hours while the rain is present and after it ends. 

It is very important that you add a maintenance dose of algaecide weekly to your pool. This product will help prevent algae development. If your pool turns green from algae, use a copper containing algaecide to rid the pool of the algae. 

It is important to maintain your pool water chemistry on a weekly routine. If we have daily rainfall, you need to be checking your chemicals after the rain. Remember, routine maintenance is your best prevention plan for problems with your water chemistry.