Why Pool Alkalinity Gets High: 7 Common Causes Explained

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Quick Answer

Pool alkalinity becomes elevated through several interconnected processes. The most common causes include filling with high-alkalinity source water, adding alkalinity-raising chemicals, excessive water agitation from features like fountains, and biological processes. Understanding these causes helps you maintain proper alkalinity levels between 80-120 ppm for traditional pools or 60-80 ppm for salt water generators.

Step-by-Step Breakdown of Alkalinity Increases

1. Test your source water first - Before diagnosing pool issues, test your tap or well water with a reliable test kit like the Taylor K-2006C. Many municipal water supplies maintain alkalinity between 80-150 ppm, while well water can exceed 200 ppm. If your source water has high alkalinity, every gallon added to your pool introduces more alkaline compounds.
2. Identify recent chemical additions - Review what chemicals you've added in the past week. Sodium bicarbonate (baking soda) directly raises total alkalinity by approximately 10 ppm per 1.5 lbs per 10,000 gallons. Sodium carbonate (soda ash) primarily raises pH but also increases alkalinity by about 8 ppm per pound per 10,000 gallons. Even some calcium hypochlorite shock products contain alkaline compounds.
3. Examine water movement and aeration - Excessive splashing, fountains, waterfalls, and spa jets introduce carbon dioxide exchange with the atmosphere. This process, called outgassing, removes CO2 from water and drives pH up, which can appear as increased alkalinity on standard titration tests. Pool parties with heavy bather loads also increase agitation.
4. Check for algae or biological activity - Green water or visible algae growth consumes carbon dioxide during photosynthesis, similar to aeration effects. This biological process raises pH and can contribute to alkalinity increases. Dead algae decomposition can also release alkaline compounds back into the water.
5. Assess recent weather patterns - Heavy rainfall introduces whatever alkalinity exists in your local precipitation. While rainwater itself is typically low in alkalinity, runoff from concrete decking, limestone aggregate, or alkaline soil can carry high-alkalinity water into your pool during storms.
6. Evaluate equipment and surfaces - New plaster surfaces leach calcium carbonate for several months, raising both calcium hardness and alkalinity. Calcium hypochlorite feeders, if over-dosed, contribute alkaline compounds. Even some pool salt brands contain trace alkaline minerals that accumulate over time.
7. Consider evaporation concentration effects - While evaporation doesn't create alkalinity, it concentrates existing alkaline compounds. If you're regularly topping off with high-alkalinity water without testing, the alkalinity gradually climbs even if your original pool water was balanced.

Chemical Interactions That Raise Alkalinity

Understanding the relationship between pH and alkalinity helps explain sudden increases. When pH rises above 7.8, your standard alkalinity test (measuring to pH 4.5 endpoint) captures more carbonate species, showing higher total alkalinity readings even if you haven't added alkaline chemicals.

Calcium hypochlorite shock (cal-hypo) deserves special attention because many pool owners don't realize it raises pH and alkalinity significantly. Each pound of 68% cal-hypo per 10,000 gallons increases alkalinity by approximately 8-12 ppm while adding substantial calcium. Using liquid chlorine (sodium hypochlorite) instead prevents this alkalinity increase.

Source Water Complications

Municipal water treatment plants often maintain higher alkalinity as a corrosion control measure for pipe systems. This means every time you add water for evaporation replacement, you're introducing alkaline compounds. Well water passing through limestone formations frequently shows alkalinity levels exceeding 200 ppm.

Safety Warning: Never attempt to lower alkalinity by adding large amounts of muriatic acid at once. The acid-alkalinity reaction generates carbon dioxide gas, and rapid additions can cause dangerous off-gassing.

Seasonal and Environmental Factors

Summer months typically show higher alkalinity increases due to increased evaporation rates, more frequent chemical additions, and higher bather loads. Pool equipment running longer hours increases water circulation and aeration. Automatic pool cleaners and in-floor cleaning systems also contribute to water agitation.

Geographic location matters significantly. Pools in areas with limestone geology, high-alkalinity municipal water, or frequent dust storms containing alkaline minerals experience more alkalinity drift than pools in areas with naturally soft water.

Prevention and Monitoring

Test alkalinity weekly using a quality test kit, not just test strips which can be inaccurate for alkalinity readings. Track your source water alkalinity seasonally, as municipal supplies often change treatment protocols. Consider using liquid chlorine instead of cal-hypo shock to minimize alkalinity increases.

Maintain proper water balance using the LSI (Langelier Saturation Index) to understand how alkalinity interacts with pH, calcium hardness, and temperature. This prevents both alkalinity problems and related issues like scale formation or surface etching.