What Causes High Alkalinity in a Pool? (7 Likely Reasons)



High alkalinity in a pool isn’t the worst problem to have, but it certainly isn’t something you want to ignore, either.

When your alkalinity is too high, it can lead to rising pH, which eventually causes your water to turn cloudy, weakens your sanitizer, scales your equipment and surfaces, and makes pH much harder to adjust.

We’ll be giving a quick refresher on total alkalinity, the most likely reasons for rising alkalinity in a pool, how to prevent them, and how to effectively bring it back down.

A Quick Refresher on Alkalinity

Alkalinity is the measure of all alkaline substances in your pool water, which are mostly carbonates and bicarbonates.

This is why alkalinity is more appropriately referred to as total alkalinity.

The higher your total alkalinity (meaning the more carbonates and bicarbonates in the water) the better protected your pH level is against changes to the water, particularly negative changes.

Ideally, you want the total alkalinity reading in your pool to be somewhere between 80ppm and 120ppm.

7 Likely Causes for High Total Alkalinity in a Pool

Like pH, there are many causes for high alkalinity in a swimming pool, but each can be narrowed down.

These are the most common reasons to watch out for:

1. You’re Using a Hypochlorite Chlorine

If you recall from earlier, we said total alkalinity is mostly made up of carbonates and bicarbonates.

Well, hydroxides also contribute to total alkalinity.

This compound is usually found in very small amounts in pool water (which is why it’s rarely mentioned when talking about alkalinity), but certain types of chlorines do produce hydroxide as a byproduct.

Specifically, these are hypochlorite chlorines:

  • Liquid chlorine (or ‘sodium hypochlorite’)
  • Chlorine shock (or ‘calcium hypochlorite’)

If you happen to use either of the above chlorine types on a regular or semi-regular basis, you’re going to notice your alkalinity increase as more hydroxides are left behind in your water.

Prevention: Use slightly more muriatic acid than usual to account for the additional hydroxide in your pool water, effectively canceling it out.

2. You’re Using Soda Ash to Raise pH

Soda ash, or sodium carbonate, is one of the most commonly used chemicals for raising the pH level in a pool.

Since soda ash contains carbonate, using this product in your pool also raises alkalinity by an equally large amount.

On average, it only takes around 6 oz. of soda ash to raise pH by 0.1 and total alkalinity by 3.5ppm, and this additional carbonate quickly adds up with frequent adjustments.

In short, if you’re trying to lift your pH level, you might be using soda ash without realizing the impact it’s having on your total alkalinity.

Prevention: Use borax to raise your pH if you’re looking for the least impact on your total alkalinity, or aerate your water if you’re looking for zero impact on alkalinity.

3. You’re Using Borax to Raise the pH

Borax is primarily used in pools to raise the pH while having little effect on total alkalinity, but it still has some effect.

This is because borax adds borate to your water, and, you guessed it… borates also contribute to total alkalinity.

Unlike other compounds, though, borates only contribute a very small amount to your overall alkalinity. In fact, it would require excessive use of Borax to raise your alkalinity by any meaningful amount.

Basically, while this Borax may not be helping your high alkalinity problem, it’s extremely unlikely to be the sole driver of it.

Prevention: Similar to the last point. If you need to raise pH without having any impact on alkalinity, you’ll need to aerate your water.

4. You’re Using High Alkalinity Fill Water

Whether it be through splashing, backwashing, leakages, or evaporation, swimming pools eventually lose water.

Keeping your water level high enough is important for your filtration system to work effectively, so topping it off with more water can become a frequent part of your pool care routine.

However, if you’re using high alkalinity source water, even adding small amounts to your pool can slowly push up the total alkalinity over time.

A big part of this is due to evaporation.

When water evaporates from your pool, it leaves behind alkaline substances like carbonates and bicarbonates. Each time you add more water to replace it, you’re increasing the concentration of these substances.

Prevention: Test your fill water for alkalinity before using it in your pool. If it’s above 120ppm, consider switching to a different water source.

5. Your Pool Plaster is Curing

If you’ve recently plastered (or replastered) your pool, you’ll know it can take several weeks for the curing process to finish.

During this time, the plaster will “bleed” calcium hydroxide into your water.

As we’ve already established, hydroxide contributes to total alkalinity, so it’s hardly surprising that this process directly raises the alkalinity in the water as more calcium hydroxide is dissolved.

Prevention: You can mitigate the effects of this process by following a “new plaster startup procedure”. The exact steps depend on your water LSI and the type of pool finish you have, so you’ll need to look into this separately.

6. You Have High pH Water

It’s well established that pH and alkalinity have a close relationship.

Most pool owners are aware that making any changes to total alkalinity always, eventually, impacts pH… but it works both ways.

Here’s why:

As your pH level goes up, it causes carbonic acid (dissolved carbon dioxide) in the water to convert into bicarbonate, and we know bicarbonate is a big contributing factor to your total alkalinity.

And yes, as pH gets lower, bicarbonate converts back into carbonic acid.

Prevention: If your pH is too high (over 7.8), you’ll need to lower it back down using muriatic acid. Depending on your levels, you may then need to raise your total alkalinity in isolation using baking soda.

7. Your Test Results Are Inaccurate

Granted, this one isn’t exactly a cause of high alkalinity.

Even still, false readings aren’t as uncommon as you’d like to think, especially for pool owners who rely on testing strips as opposed to liquid test kits.

Liquid test kits use reagents to color your water, and this chemical reaction is considerably more reliable for gauging your levels, including pH, alkalinity, stabilizer, calcium hardness, and more.

Prevention: Testing strips are accurate enough for regular testing, but it’s better to use a liquid testing kit when troubleshooting issues.

Don’t Neglect to Correct

Even after identifying the cause of high alkalinity in your pool, you’ll still need to lower it back down into the ideal range. While this may seem like an impossible task, it’s very doable with the right technique.

If you need to lower alkalinity without lowering the pH, simply add muriatic acid to lower both the pH and total alkalinity, then aerate your water to raise the pH level back up to where it was.

Categories: Pool Care, Pool Chemistry