What causes high total alkalinity in my Ottawa pool and how do I lower it?

High total alkalinity in an Ottawa pool — anything above 120 ppm — is most commonly caused by overuse of sodium bicarbonate (baking soda or alkalinity increaser), Ottawa's municipal water chemistry, or an accumulation of dissolved carbonates from shock treatments, and lowering it requires adding muriatic acid at a cost of roughly $10 to $18 per 3.78-litre jug, with most corrections needing 500 millilitres to 2 litres depending on how far above target your reading sits. The process is straightforward but requires patience and careful dosing to avoid crashing your pH in the process.

Total alkalinity measures the concentration of bicarbonate, carbonate, and hydroxide ions dissolved in your pool water, expressed in parts per million. These ions act as a chemical buffer that resists rapid pH changes — when something acidic enters the pool (like rain, body sweat, or chlorine byproducts), the alkalinity buffer absorbs the acid and prevents pH from dropping sharply. The ideal range for pool water is 80 to 120 ppm. When alkalinity climbs above 120 ppm, it over-buffers the water, making pH stubbornly resistant to downward adjustment and tending to lock pH at 7.8 to 8.2 — well above the ideal 7.2 to 7.6 range where chlorine is most effective.

Understanding why your Ottawa pool's alkalinity is high helps you prevent recurrence after you lower it. The most common cause is over-addition of sodium bicarbonate — sold at pool stores as "alkalinity increaser" or "alkalinity up" at $12 to $20 for a 3-kilogram bag. New pool owners frequently misread test results or follow generic dosing charts that assume a different pool volume, leading to overshooting the target. Ottawa's municipal water contributes modestly — city tap water typically runs 30 to 50 ppm alkalinity, which is actually below the pool target, so fill water alone rarely causes high alkalinity. However, calcium hypochlorite shock (the most popular granular shock product) has an alkaline pH and adds a small amount of alkalinity with each dose. Over a season of weekly or bi-weekly shock treatments, this accumulation can push alkalinity above 120 ppm even without any direct alkalinity increaser addition.

Sodium bicarbonate from non-pool sources is a surprising contributor in some Ottawa pools. Homeowners who use baking soda from the grocery store as a budget alkalinity increaser (chemically identical to pool-branded alkalinity up, at a fraction of the price) sometimes overshoot because they do not account for the difference in granule density between the grocery product and the pool product. A cup of grocery-store baking soda weighs differently than a cup of pool-branded sodium bicarbonate despite being the same chemical, and eyeballing the dose rather than weighing it on a kitchen scale leads to over-addition. Always weigh sodium bicarbonate additions rather than measuring by volume.

Lowering alkalinity requires muriatic acid, also called hydrochloric acid, which is available at every Ottawa pool store for $10 to $18 per 3.78-litre jug. The standard procedure for lowering alkalinity while minimizing pH impact involves a technique called "acid column" or "slug dosing." Turn off the pool pump and any water features. Measure the required dose of muriatic acid based on your pool volume and current alkalinity reading — approximately 750 millilitres of full-strength muriatic acid lowers alkalinity by roughly 10 ppm in a 60,000-litre pool. Pour the acid slowly into the deep end in one concentrated area, allowing it to sink and form an acid column that reacts intensely with the bicarbonate alkalinity in that zone. Wait 30 to 60 minutes, then turn the pump back on to circulate the treated water throughout the pool. Retest alkalinity after 6 hours of circulation.

The alternative to muriatic acid is dry acid (sodium bisulphate), which costs $15 to $25 for a 3.5-kilogram bag and is preferred by some Ottawa pool owners because it does not produce the strong hydrochloric acid fumes that muriatic acid generates. Dry acid works through the same chemistry — adding hydrogen ions that consume bicarbonate alkalinity — but it dissolves as a powder rather than a liquid, making it harder to perform the concentrated "acid column" technique. For modest alkalinity corrections of 10 to 20 ppm, dry acid works well. For larger corrections exceeding 30 ppm, muriatic acid is more practical because you can dose more precisely and the liquid form settles to the deep end more reliably.

A critical point about lowering alkalinity: it always lowers pH simultaneously, and you need to manage both. After an acid addition that successfully drops alkalinity from 150 ppm to 100 ppm, your pH will also have dropped — potentially below 7.0, which is corrosive to equipment and uncomfortable for swimmers. Once alkalinity is in the target range, you then raise pH back to 7.4 by adding soda ash (sodium carbonate) at $10 to $15 per 2-kilogram bag. Soda ash raises pH with a relatively small impact on alkalinity, so you can fine-tune pH without undoing your alkalinity correction. This two-step process — acid to lower alkalinity, then soda ash to raise pH — is the standard approach that every Ottawa pool service technician uses.

Preventing high alkalinity from recurring requires understanding your pool's chemical inputs. If you use calcium hypochlorite shock weekly, consider alternating with liquid chlorine shock (sodium hypochlorite), which has a high pH but does not contribute to alkalinity the same way. If you use an automatic chemical feeder that dispenses alkalinity increaser, verify the feed rate is calibrated to your actual pool volume rather than a generic default. If your pool has a water feature — waterfall, fountain, or spillover spa — the aeration from the falling water naturally raises both pH and alkalinity over time by driving off dissolved carbon dioxide. Reducing water feature run time or operating features only when the pool is in use can slow the alkalinity creep.

For Ottawa pools with persistently high alkalinity that resists correction, consider whether your test method is giving accurate readings. Test strips are notoriously inaccurate for alkalinity, sometimes reading 40 ppm high or low. Liquid drop test kits using the acid demand test are more accurate but require careful counting of drops — one drop missed changes the reading by 10 ppm. A digital photometer provides the most reliable home reading, or take a sample to an Ottawa pool store for a free computerized analysis that confirms your home test before you start adding acid.

When to Call a Professional

If your alkalinity is above 180 ppm or you have tried acid additions without success, the issue may be more complex than a simple chemical correction. Some Ottawa pools built with concrete or plaster surfaces leach calcium and carbonate compounds into the water from deteriorating surfaces, creating a continuous source of alkalinity that chemical treatment alone cannot overcome. A pool professional can assess whether the surface condition is contributing to persistent alkalinity problems and recommend repair options ranging from acid washing at $500 to $1,000 to full resurfacing at $8,000 to $15,000 depending on pool size and finish material.

Need help getting your Ottawa pool's alkalinity under control? Ottawa Pool Installation connects you with local water chemistry specialists who can diagnose the root cause and implement a correction plan tailored to your pool's specific conditions.