Pool chemistry changes can indicate leaks

By Robert W. Lowry

I got a call I got from a pool service tech who said, “do you know why the salt level has gone down from 3,400 ppm to 1,900 ppm, calcium hardness from 350 ppm to 230 ppm and my CYAto be down by 30 ppm?” I said, “how long did it take for the readings to go down?” “Well, I checked it a couple of months ago, but they usually don’t change so I don’t test them very often.”

Lost Water Replaced by Automatic Refill

A pool may have a leak or many leaks that are small but cumulatively, the pool can lose a considerable amount of water. Recognizing that a pool is losing water is difficult with automatic refill devices and regions with high evaporation. The lost water is replaced automatically, and no one knows how much water is being replaced. (It would be cool to put a water meter on a pool refill line sometime.) It is not just the cost of water that is important.

Water Chemistry is Diluted

Water loss means that the water chemistry is being diluted and more chlorine may be needed, more CYA to protect the chlorine, salt may be lost, and salt generators may have to run longer to make chlorine. So, more electricity is needed, salt cells may develop scale quicker, and more frequent cleaning may be needed. The pH may be increasing because the salt generator is running longer. The pump may need to run longer because the salt generator needs to run longer. Water balance may be off requiring more time and chemicals to balance. If not attended to, corrosive water and staining may result. The pH buffering capacity is changed due to lower total alkalinity, lower CYA and lower borate levels (if used).

Complete Water Testing Not Usually Performed

Most service techs do not test for all 7 or 8 water conditions every time they visit a pool. Testing for everything is expensive, time-consuming, and 4-5 of the tests never change much. The only water conditions that change frequently are free chlorine, pH and total alkalinity.

So, weekly testing of calcium hardness, CYA, borate, salt and TDS are deemed not necessary. As a result, many service techs only test for these water conditions once a month or even every couple of months.

Lost Water Costs More than just the Cost of the Water

A pool that is losing water costs money, time and effort. Pool chemicals are lost to a leaking pool, which results in additional problems. For example, if borate concentrations are low, fewer algae are prevented, more chlorine is needed, and the pH can rise quicker. If CYA is low, the UV protection is lowered requiring more chlorine. Low concentration of both lower the pH buffering against pH decrease. Lowering the calcium level can create corrosive water conditions, a negative LSI, or even plaster and grout problems. Low salt can cause premature salt cell damage, cause less chlorine to be made, and require longer run times. None of these conditions are noticeable weekly but if these water conditions are checked every two months, the small changes are multiplied by 8 or 9 (weeks in 2 months). So, a loss of 2 inches per week is 16-18 inches in 2 months.

Water Loss Goes Unnoticed but Can Be Huge

If the average depth of the pool is 4 feet 6 inches or 54” (shallow end of 3 feet and a deep end of 6 feet equals an average depth of 4.5 feet), then

the water loss is 18/54 or 33.3% water volume loss! Because the pool is automatically refilled, it may not be clear that this is happening. But service techs may wonder why this pool uses more chlorine than others, why the pH and alkalinity are always changing, why the salt level has gone down by 800 ppm, why borate is half of the 50 ppm that was added, why CYA is half of the 50 ppm it was adjusted to, why calcium hardness has dropped by 100 ppm and why TDS is going down. These unexplained events are indicating that the pool is losing water. Testing the water every two months will show a difference. What has happened to the pool chemistry? Look for a leak, or wait another two months?

Pool Chemistry Changes