Water Chemistry /
What Causes Alkalinity to Rise in a Pool?
Pool alkalinity usually rises from fill water, chemical additions, or repeated top-offs. Testing and good circulation help you spot the cause before pH becomes harder to control.
Archive date: this article was written for PoolSkimmer on Oct 24, 2026 and given the earlier archive date of Nov 23, 2025 so the blog reads as an organized archive. It was not published on that date.
The source of rising alkalinity in a pool
It is not uncommon for pool alkalinity to climb, particularly with the regular introduction of water or chemicals that carry it. The usual suspects are fill water and top-offs to make up for evaporation, as well as chemical work done by the homeowner to put bicarbonate or other alkaline compounds into the system. Even an alteration in your chlorine or pH regimen can, over time, put upward pressure on total alkalinity in some pools.
First steps when alkalinity won’t stay down
Before jumping to the conclusion there is a major problem with the pool, run through a few basics:
1. Confirm the numbers. Retest to be sure the reading holds up. 2. Go over what has been added lately. Make sure you have been in line with the equipment manual and product label. 3. Examine the water supply. If you have refilled after a drain or used an autofill, the source could be at fault. 4. Consider evaporation. Replacing lost water will gradually alter the chemistry. 5. Assess maintenance and circulation. Inadequate mixing can give the impression of instability in your pH and alkalinity from one test to another.
Hayward’s own pool care literature would have you focus on filter upkeep, circulation and routine testing to keep the water in balance.
Routine matters
There are a number of everyday decisions that will see alkalinity go up:
- Correcting water balance or pH with alkaline products.
- Putting in baking soda or sodium bicarbonate specifically for the purpose of raising alkalinity.
- Topping off with hard, mineral-rich water that has more buffer capacity to begin with.
- Overcorrecting based on a test and putting in more than was called for.
More often than not, high alkalinity is the result of these cumulative maintenance actions rather than any sudden defect in the pool.
Is pH involved?
They are related but distinct. Total alkalinity is the buffer for pH; the higher it is, the more the water will resist change and the harder you will find it to move the pH. You may well end up with both a higher pH and rising alkalinity.
A few things to bear in mind:
- An increase in pH is not a guarantee of an equal rise in alkalinity.
- Uneven circulation or low alkalinity can cause pH to swing.
- One reading should not dictate a reaction; look for stability in the results.
Water or chemistry? How to tell
Follow this approach to narrow it down:
1. Put old and new test results side by side. A steady incline is usually indicative of fill water or repeated dosing. 2. Note if any water has been put in of late. Top-offs are among the most frequent causes. 3. Look at any recent sanitizer or pH corrections; certain treatments will have an effect. 4. Do a safe inspection of the circulation gear. Turn the pump off before you open the skimmer basket and never put your hands in operating suction. 5. Wait for the pool to circulate before retesting so you get a true sample of the water.
Time to bring in a pro?
If you have ceased topping off and the alkalinity continues its way up, or if you are finding it impossible to put the pH and alkalinity to rest, put in a call to a qualified professional. The same applies if you have reason to believe there is a filtration or dosing issue beyond your ability to diagnose safely.
Leave electrical repairs, structural changes and live suction work to the experts. Wear the proper PPE, heed the labels on your chemicals and manuals, and under no circumstances should one swim if the water is unsafe or there is any hint of an electrical problem with the equipment.
Sources and further reading
- [Hayward: Weekly pool maintenance](https://www.hayward.com/maintenance/weekly-pool-maintenance)