This article discusses how to manage a pool when the source water is scale-forming (≥ +0.31 LSI). This could mean the water has high calcium hardness, high total alkalinity, high pH, or any combination of the three.
Covered in this article:
- Source water is the baseline for pool chemistry
- Calcium scale in pools
- The Orenda Startup™ with scale-forming tap water
- Tap water is usually low on the LSI
- Case Study 1: +0.51 LSI tap water
- Case Study 2: 240 ppm total alkalinity tap water
- How to reduce calcium hardness in a pool with hard tap water
- Conclusion
Source water is the baseline for pool chemistry
Source water chemistry will influence pool chemistry throughout the life of the swimming pool. This is especially important during the initial fill-up, called a startup. But the tap water impacts pool chemistry ongoing after that. Water evaporates and leaves everything behind. These factors can accumulate over time, and the rate of accumulation largely depends on the chemistry of the source water.
For instance, if your tap water is hard, every time the pool gets refilled to account for evaporation, the calcium hardness level will increase. Conversely, if the tap water is softened somewhere between the treatment plant and the hose, it may have low (or no) calcium hardness in it. This would dilute and reduce calcium levels in the pool...albeit slowly.1
It's much easier to deal with low-LSI, low-mineral tap water. And that sounds weird because we at Orenda consistently advocate for LSI balance and having enough calcium and alkalinity in the pool. And that's true. However, it's much easier to increase alkalinity and calcium with products than it is to drain and dilute repeatedly to reduce them.
Furthermore, if the tap water is high in a given parameter that you're trying to reduce, dilution may not even help. So what can be done? How should we deal with high-LSI, scale-forming tap water?
Calcium scale in pools
One of the most obvious challenges of high-LSI tap water is the formation of calcium scale in the pool and its equipment. While there are different types of scale, the most prevalent (by far) is calcium carbonate (CaCO3).
Any over-saturation of a calcium ion pair––meaning calcium bound to an anion like calcium carbonate, calcium sulfate, or calcium phosphate––will lead to that calcium compound precipitating out of solution and depositing somewhere within the pool or equipment. Since calcium carbonate scale is the most common form of scale found in swimming pools, let's use it as our example.
Calcium carbonate and the LSI
Calcium carbonate saturation is measured using the Langelier Saturation Index (LSI), which we can easily calculate using the free Orenda Calculator™ app on our website or your mobile device (the app is called "Orenda").
An over-saturation of calcium carbonate is quantified by an LSI value at or above +0.31. An undersaturation is an LSI value at or below -0.31. From -0.30 to +0.30, water is considered balanced, with 0.00 considered perfectly balanced. These values are color-coded in the Orenda Calculator™ for easy visual reference. Red is aggressive and etching, yellow is acceptable, green is ideal, and purple is scale-forming.
The LSI only applies to calcium carbonate saturation. It does not apply to calcium phosphate, calcium sulfate, calcium silicate, or any of the other potential scale types that can occur in pools. The LSI also does not apply to the solubility of calcium hydroxide, which is important when new cement is curing during a startup.
If the LSI is yellow or green on the Orenda Calculator (between -0.30 and +0.30), water is balanced enough that it will neither seek and dissolve calcium (aggressive, low LSI ≤ -0.31) nor get rid of some calcium in the form of scale (high LSI ≥ +0.31).
Preventing scale in pools is primarily a question of LSI balance. If the LSI is balanced, calcium carbonate scale should not occur in the first place, unless forced in a specific area due to a localized high-LSI violation. A great example of this is scale in a pool heater or a saltwater chlorine generator.
Need to remove calcium from pool tile?
Calcium scale can be difficult to remove from pool tile, especially if it's above the water line. This scale is what we call the wet/dry effect, and it's hard to prevent. Sure, we can chelate calcium to inhibit scale using SC-1000, but the wet/dry effect is the loss of water from evaporation, which leaves its minerals behind.
The LSI is the saturation of calcium carbonate in water. As water evaporates, the concentration of calcium hardness, total alkalinity, and everything else increases as the water volume decreases. This, in essence, creates a localized LSI violation on the waterline pool tile.
Calcium scale above the waterline is likely from evaporation. Calcium scale at or below the waterline is likely from a high LSI.
The best calcium remover for pool tiles is a combination of LSI-balanced pool water and SC-1000. You can follow this process of how to soften and remove scale.
If the calcium scale on the pool tile needs to be cleaned faster, there are options such as bead-blasting, dry ice removal, and physically cleaning it off with diluted acid and scrubbing equipment. Note, if you use diluted acid, use it with caution, always wear protective gear, and be sure to neutralize the acid going into the pool (with sodium bicarbonate) so it does not drop the pH too much.
The Orenda Startup™ with scale-forming tap water
Depending on the chemistry of the tap water, the required chemicals for pool startup may vary.
When doing the Orenda Startup™ procedure, the goal is to balance the water to an LSI value between +0.20 and +0.29 for the first few days. You have a slightly higher LSI (up to +0.49), but that may cause the calcium being added to not stay dissolved, and land on the floor. We call this condition snowing.
Notice that our recommended LSI target for startup is slightly positive, yet not scale-forming. This is because calcium hydroxide (Ca(OH)2) can still be dissolved by perfect 0.00 LSI-balanced water. Slightly positive LSI reduces the likelihood of the water dissolving calcium hydroxide from the cement.
Related: Protect new plaster with LSI balance from the beginning
As a refresher, traditional pool startups create symptoms like plaster dust and a constantly rising pH (above the pH ceiling). This is because they usually miss the LSI target, and instead focus on individual chemistry ranges. For example, aiming for a pH of 7.2 to 7.6, and an alkalinity of 80 ppm. These numbers make the desired LSI values virtually impossible to achieve.
Related: What is the Orenda Startup™?
Tap water is usually low on the LSI
The vast majority of water we have tested around the United States is low on the LSI (≤ -0.31). While this is a recipe for plaster dust, pH spikes and potential plaster issues, it is also easy to correct during the fill-up.
- Low calcium hardness? Add pre-dissolved and chelated calcium chloride.
- Low total alkalinity? Add sodium bicarbonate.
- Low pH? Aerate to release CO2.
On the contrary, if you have high-LSI tap water, things can be more difficult:
- High calcium hardness? Make sure it's chelated, and that pH and TA don't get too high. Draining and diluting will not help because the tap water is the cause of the issue.
- High total alkalinity? Reduce it no more than 20 ppm at a time using diluted acid and turbulence.
- High pH? Lower it with diluted acid, but not enough that the LSI goes red.
As you can see, it's simple to correct hungry water. It's more nuanced and complicated to correct scale-forming tap water. But no matter what comes out of the tap, we can find ways to adapt.
Case study 1: +0.51 LSI tap water
We have a customer who was struggling with startups because their tap water was a +0.51 LSI. Calcium dust kept forming, but it was not plaster dust.2 It looked like plaster dust, and it was indeed calcium carbonate. But it was snowing, meaning calcium carbonate coming out of solution from an oversaturation in the water (high LSI ≥ +0.31).
Here's a screenshot of their tap water on the left:
Notice the calcium hardness was only 140 ppm. We consider that low. Yet the TA was a high 140 ppm, causing the pH ceiling to be 8.58. So even though the pH at the moment was only 8.2, it was set to rise up to 8.58.3
A traditional startup procedure would say to first lower the pH to 7.2 to 7.6, then lower TA to 80 ppm. The industry-standard startup card (as of June 2024) still says to not exceed 150 ppm calcium hardness, and to add calcium slowly over several days to get there.
The issue with these targets is they force the LSI to go between low and high LSI violations, but never aim for a balanced LSI. Those parameters make it mathematically impossible to have a balanced LSI, which is what the water itself craves.
We, on the other hand, take an LSI-first approach to pool startups. We give water what it wants so it does not have to balance itself by dissolving cement from the plaster. It's that simple.
In this case study, we recommended decreasing TA by 20 ppm per day, feeding acid into the Orenda Startup Barrel™ so it feeds slowly and dilutes well. We also increased calcium hardness by pre-dissolving and chelating calcium chloride in the Startup Barrel™ to get calcium up to 300 ppm. Over the next four days, we instructed the customer to continue removing 20 ppm of TA with diluted acid until the TA was around 80 ppm.
Notice we did not aim for a specific pH. This is because the pH was going to be moved by the acid reducing the TA, and we know it will rise back up with aeration.
Case study 2: 240 ppm Total Alkalinity in the tap water
Another customer had decent calcium hardness (280 ppm) and an extremely high total alkalinity of 240 ppm. In this case, we also recommended working the TA down by 20 ppm at a time with diluted acid, but we instructed the customer not to increase calcium hardness until the TA was around 120 ppm. This patience allowed the TA to be worked down over time until the water's LSI was ready to receive calcium.
The Orenda Startup Barrel™ was used only for SC-1000 and muriatic acid. No calcium or bicarb was put in it. They filled the entire pool using the barrel and it was an effective means of diluting acid. By being patient and focusing on the LSI, the customer had a successful startup with zero plaster dust or pH spikes.
How to reduce calcium hardness in a pool with hard tap water
Since draining and diluting is ineffective with hard tap water, reducing calcium hardness requires removing calcium from the water itself. This can be accomplished with reverse osmosis (RO) filtration, or by forced precipitation using soda ash.
Reverse Osmosis (RO)
Reverse osmosis (RO) filtration also removes minerals and total hardness. Over time, however, the membranes will need to be replaced and cleaned. The process is cool, but can be expensive, depending on where you are located.
Forced calcium removal with soda ash
If you have ever used soda ash, you know it can cloud up in the water. That cloud is calcium carbonate coming out of solution due to a localized LSI violation. This process uses that LSI violation intentionally to force calcium out of solution so it can be vacuumed or filtered out of the pool to reduce calcium hardness.
Sometimes this is the most economical and practical way to reduce calcium hardness levels in water, though it can also be a mess. The process is explained in detail in this article from onBalance in the Journal of the Swimming Pool and Spa Industry. This process is also known as lime softening.
Similar to the ion exchange principles in water softeners, the sodium carbonate (Na2CO3), aka soda ash, will interact with the calcium bicarbonate dissolved in the water in the following reaction:
Ca(HCO3)2 + Na2CO3 → CaCO3 + 2NaHCO3
Calcium bicarbonate + Soda Ash → Calcium carbonate + 2 sodium bicarbonate
The article from onBalance (linked above) explains the dosing too:
Dosage
The potential amount of filterable precipitate that could be generated may be estimated by taking into consideration the following: The molecular weight of sodium carbonate is 105.98 and the molecular weight of calcium carbonate is 100.08. Therefore, one pound of added soda ash reacting with calcium bicarbonate in the pool water can potentially yield 0.944 lbs. of calcium carbonate precipitate. Or, 1.06 pounds of soda ash reacting with calcium bicarbonate can yield up to 1.0 lbs. of calcium carbonate. One pound of calcium carbonate precipitate translates to about 12 ppm of calcium hardness reduction in 10,000 gallons of water.
For easy math, if you want to precipitate and remove 240 ppm of calcium hardness in a 20,000-gallon pool, the dosing math is as follows:
Desired calcium removal = 240 ppm
Pool volume = 20,000 gal
1.06 lbs Soda Ash removes 1 pound of CaCO3, which = 12 ppm CH reduction in 10,000 gallons.
1.06 lbs Soda Ash removes 1 pound of CaCO3, which = 6 ppm CH reduction in 20,000 gallons.
And:
[(desired removal ppm) ÷ (ppm CH reduction per lb. of CaCO3 removed)] = lbs. of CaCO3 precipitation needed
(240 ppm) ÷ (6) = 40 lbs. of CaCO3 precipitation needed
So:[(40 lbs.) x (1.06 lbs of soda ash)] = lbs. of soda ash needed
42.4 lbs. of soda ash will remove 240 ppm CH in 20,000 gallons of water
Conclusion
While high-LSI tap water does not hurt cement, it can be a nuisance for maintenance and pool startup. It's much easier to add calcium chloride or sodium bicarbonate than to drain and dilute water. And if the tap water is hard or has high TA, dilution does not help in the first place.4
For startups, the name of the game is maintaining a slightly-positive LSI for the first few days, and LSI balance ongoing after that. For maintenance, combatting the accumulation of calcium may become necessary over time, especially in hotter, dry climates that do not get the benefit of regular rain dilution.
In any case, understanding the LSI allows you to adapt to whatever your tap water chemistry is. If you need help with your complicated tap water, contact us. We're happy to help.
1 The rate of dilution depends on too many factors to name in this article. Rain and snow dilution are effective because they contain zero calcium (distilled water). Calculating dilution rates requires knowing the exact pool volume, rate of dilution and the chemistry of the replacement water.
2 We know this because the pH was not spiking, indicating there was no dissolving of 12.6 pH calcium hydroxide. Had calcium hydroxide been stolen from the cement, the pH would spike well over its pH ceiling. In this customer's case, the pH was similar to the tap water, indicating the pH rise was slow and natural due to the off-gassing of CO2.
3 Assuming the water does not dissolve any cement from the plaster, the pH will only rise naturally until it reaches the pH ceiling. In this case, when the pH rises up to its ceiling of 8.58, the LSI will be even higher, forcing calcium carbonate back out of solution as scale or dust. In our customer's case, it was dust, or snowing.
4 Reducing calcium, TA, CYA or TDS with draining and diluting only works if the tap water has less concentration of those parameters. Diluting a pool with 400 ppm calcium doesn't work if the tap water also has 400 ppm calcium. It's a zero-sum change. High calcium is more of a challenge than the other factors because tap water rarely has high TDS, and should never have cyanuric acid (CYA). But tap water can absolutely have high calcium hardness, rendering draining and diluting pointless. In such cases, reverse osmosis (RO) filtration and forced precipitation with soda ash are the best methods we know of to reduce calcium hardness.