Alkalinity in well water: what it means
Alkalinity isn't a health concern. It measures how well your water resists changes in pH. Low alkalinity often goes with acidic, corrosive water. Ontario's operational guidance is 30 to 500 mg/L.
- US (EPA)
- No limit.
- Ontario
- 30 to 500 mg/L operational guideline.
What it is
Alkalinity comes mostly from dissolved limestone (bicarbonate and carbonate) and is reported as mg/L of calcium carbonate (CaCO3). It helps explain your pH and how corrosive your water is.
What your number means
- Worth a lookBelow 30 mg/L
Low buffering. Water is often acidic and corrosive to copper and lead plumbing.
- Typical30 to 500 mg/L
Normal for most wells.
- Worth a lookAbove 500 mg/L
Can cause scale and a bitter or soda taste.
What to do
- If alkalinity is low and pH is below 6.5, test for lead and copper at the kitchen tap.
- A neutralizing (calcite) filter raises both pH and alkalinity.
- High alkalinity with hard water usually just means scale. A softener can help.
How often to test
With a full panel every 3 to 5 years.
Common questions
Is alkalinity the same as pH?
No. pH is how acidic or basic the water is right now. Alkalinity is how much it can resist changing.
What does as CaCO3 mean?
It reports alkalinity as if it were all calcium carbonate, so results from different labs can be compared.
Sources: Ontario Drinking Water Quality Standards (O. Reg. 169/03); Ontario: testing and treating private wells. Limits checked November 2026.