Alkalinity (as CaCO3)

Alkalinity is not a contaminant and not a chemical — it is a property. USGS defines it as "the buffering capacity of a water body; a measure of the ability of the water body to neutralize acids and bases and thus maintain a fairly stable pH level", and notes it depends on the presence of bicarbonates, carbonates and hydroxides. It is reported as mg/L (or ppm) of calcium carbonate. Higher alkalinity means the water resists sudden pH swings. EPA sets neither a health limit (MCL/MCLG) nor a secondary standard for alkalinity, so there is no threshold at which a reading becomes unsafe to drink — the number is context for corrosivity, treatment and taste, not a safety verdict.

Source: USGS — Water Science School (retrieved 2026-07-27)

Water filters certified under the following NSF standards are effective at removing Alkalinity (as CaCO3):

Filter recommendations for this contaminant are being updated.

Alkalinity (as CaCO3) has been detected above EPA health guidelines in 22 cities serving 0.1M people across the US.

and 17 more cities