Boston's water chemistry is adjusted partly to reduce the release of metals from plumbing. MWRA describes adding sodium carbonate and carbon dioxide at the John J. Carroll Water Treatment Plant to adjust pH and buffering capacity. That treatment can coexist with soft water: pH, alkalinity and hardness describe different properties, and none should be read as a complete result for a particular household tap. The 2025 treatment account.
Why MWRA adjusts pH
The Carroll plant treats the metropolitan Boston supply. Its facility description covers ozone disinfection and the later addition of ultraviolet treatment. Those processes answer questions about treatment at the plant; questions about a building's service line or indoor fittings need their own evidence. The Carroll plant description.
For an explicit operating reference, MWRA's August 2026 update gives a distribution-system pH target of 9.3 and an alkalinity target of 40 mg/L. It explains that adjusting those properties reduces corrosivity and helps minimize lead and copper leaching from service lines and home plumbing. The same report says the Chicopee Valley Aqueduct communities provide their own corrosion control. Use the Metro Boston section for Boston, not a value from that separate supply. The August 2026 update, page six.
The update was released on 20 September 2026. Keep that publication date separate from the August sampling month. An operating target also differs from a measured average: it describes the value the utility aims for, not an identical result expected at every outlet.
Soft water can have a high pH
MWRA's Water and Health page labels its summary tables as 2025 data. For Metro Boston it lists an average pH of 9.6, with a range of 9.3 to 9.7, and average hardness of 13.1 mg/L, with a range of 11 to 15.6 mg/L. The pH figure and the hardness figure cannot be compared as if they measured the same thing. One has no concentration unit; the other is reported in milligrams per litre. The dated secondary-parameter tables.
MWRA's FAQ explains hardness in terms of calcium and magnesium concentrations and calls its water soft. It says customers generally do not need a water softener. The mineral description leaves the condition of individual pipes and the claims of household filters unanswered. The hardness FAQ.
If you are comparing a pitcher with an installed softener, begin with the job you want the equipment to do. Our pitcher-filter and softener guide explains the differences and the documentation to check. A product offered for taste, scale or a named contaminant needs a matching claim; a regional pH number alone is not a product recommendation.
Read the local lead results separately
Boston's own section of the 2025 annual report records a lead 90th percentile of 22.5 parts per billion in September/October 2025, above the 15 ppb action level, in the sampled higher-risk homes with known lead service lines. This historical result is a reason to read the community's local section alongside the treatment description. It is not a measurement of every Boston home, and it does not establish the result at an untested tap today. The Boston letter and local results.
For newer monitoring, MWRA's monthly index separates source, treated and community-line samples. Choose the period and sample location that match your question rather than borrowing the most reassuring number from another table. The monthly-report guide. Our Boston guide gives the broader travel context; ask the property operator for its own service-line information and any current water instructions.
