South Bay Water Hardness by City (2025 Water Quality Reports)

Water hardness in the South Bay depends less on your city than on where your water comes from. Hetch Hetchy water bought from the San Francisco Public Utilities Commission (SFPUC) averages about 60 ppm (soft), Valley Water’s treated surface water about 97 to 101 ppm (moderately hard), and local groundwater roughly 230 to 400 ppm (very hard).

Within San Jose alone, the 2025 reports range from 60 ppm in North San José to an average of 407 ppm from Muni Water’s Evergreen wells. The table below gathers the figures from each water supplier’s latest Consumer Confidence Report, the annual water quality report every supplier must publish, for San Jose and the South Bay and Peninsula cities we serve.

Water Hardness by City and Supplier (2025 Reports, Checked October 2026)

Hardness is shown as calcium carbonate, in parts per million (ppm, the same as mg/L), and in grains per gallon (ppm ÷ 17.1). “—” means the figure isn’t one we have from that report; check the report itself. Suppliers blend sources, so your tap can sit anywhere between a city’s low and high rows.

* Muni Water’s 2025 report lists its well figures from 2023 sampling; suppliers test some minerals less than once a year because they change slowly.

How to Read a Hardness Number

• ppm and mg/L are the same unit. Reports state hardness “as calcium carbonate” (CaCO₃), the minerals that form scale.
• Grains per gallon is the unit most softener and water heater manuals use. Alameda County Water District and San José Municipal Water both say to divide ppm by 17.1.
• The scale: the U.S. Geological Survey calls 0 to 60 ppm soft, 61 to 120 moderately hard, 121 to 180 hard, and more than 180 very hard. Suppliers use their own words too: Cal Water calls anything under 75 ppm soft, which is why its 62 ppm Bear Gulch water reads as soft in its own report.
• Valley Water’s figure differs a little by report (97 to 101 ppm) because each supplier samples where the water enters its own system.

Why Does One City Have Several Numbers?

Most South Bay suppliers blend sources and send different blends to different neighborhoods:

• San Jose has three water retailers. San José Municipal Water serves North San José, Alviso, Evergreen, Edenvale and Coyote Valley; Great Oaks Water serves Blossom Valley, Santa Teresa, Edenvale, Coyote Valley and Almaden Valley; San Jose Water serves most of the rest, along with Campbell, Los Gatos, Saratoga and parts of Cupertino. Your water bill, or the City’s utility services lookup, tells you which one serves you.
• Groundwater fills gaps. Muni Water added well water to Valley Water’s supply in Evergreen during 2025, and Sunnyvale runs its wells in summer peaks and emergencies, so hardness can rise for part of the year.
• Maps tell you the usual source. San Jose Water’s report includes a map of which source mainly serves each area, and Santa Clara, Milpitas, Sunnyvale and Fremont describe their zones by street or neighborhood.

What Does Hard Water Do in a South Bay Home?

Hardness isn’t a health risk; the U.S. Geological Survey and San José Municipal Water both say so. It’s a plumbing problem, and it shows up in three places.

Water heaters

When hard water is heated, calcium carbonate drops out as scale. The USGS notes that scale can shorten an appliance’s life, raise heating costs and lower an electric water heater’s efficiency. In a tank it settles as sediment that rumbles as the burner fires; in a tankless unit it coats the heat exchanger until the unit loses output or shows error codes. On groundwater above 250 ppm, flushing a tank and descaling a tankless heater on the maker’s schedule matter far more than on Hetch Hetchy water. See water heater services, tankless water heaters and our water heater maintenance tips.

Faucets, aerators and showerheads

Alameda County Water District identifies the white crust around faucets as calcium carbonate. The same mineral collects in aerator screens and showerheads, cutting flow, and on cartridges and seals, which stiffens handles or starts a drip. Soaking an aerator in white vinegar often restores flow; a dripping faucet usually needs a new cartridge. See faucet repair and installation.

Pipes

Santa Clara’s Water & Sewer Utilities explains that carbonate scale builds up inside household pipes and flakes off, clogging aerators and washer screens, and that there’s no practical way to remove it from inside the pipe; in severe cases it suggests replumbing. Older galvanized steel pipe also corrodes and narrows from the inside, which is where pinhole leaks tend to start. See repiping and leak detection.

Do You Need a Water Softener in San Jose?

For safety, no. Whether one is worth it depends on your source:

• On Hetch Hetchy water (about 60 ppm): scale builds slowly. Mountain View, which gets about 90% of its water from the SFPUC, calls its water soft to moderately hard and says a softener generally isn’t needed.
• On Valley Water (about 100 ppm): some scale over the years; routine water heater flushing and aerator cleaning usually keep up.
• On groundwater (roughly 230 to 400 ppm): scale comes fast, in heaters, fixtures and pipes alike.

San José Municipal Water’s report weighs it both ways. It lists less soap, longer water heater life and less scale in pipes as benefits of softening, and added sodium (depending on the type of softener), regular maintenance and possible harm to plants and landscaping as drawbacks. If you have a sodium concern, ask your doctor; if you’re weighing a softener, start from your own supplier’s number in the table above.

Protect Your Plumbing from Hard Water

Whatever your number, the same habits help: flush a tank water heater and test its relief valve on schedule, descale a tankless unit as its maker directs, clean aerators and showerheads, and have older galvanized pipe checked before it starts to leak. CPI Service maintains, repairs and replaces water heaters, faucets and piping throughout San Jose and the South Bay, with same-day service and every job quoted as a fixed price before we start. Current specials include $250 off water heater installation and $50 off any plumbing service. Request a free estimate or call (408) 978-3093.

Sources

San Jose: San Jose Water: 2025 Annual Water Quality Report · San José Municipal Water System: 2025 Water Quality Report (data table, PDF) · Great Oaks Water Co.: 2025 Consumer Confidence Report · City of San José: Utility Services Lookup.

Other suppliers’ 2025 reports: Cal Water, Los Altos Suburban District · Cal Water, Bear Gulch District · Purissima Hills Water District · City of Milpitas · City of Santa Clara (water quality table) · City of Sunnyvale · City of Mountain View · City of Palo Alto Utilities · Stanford University · City of Menlo Park · Alameda County Water District · City of Morgan Hill.

Background: U.S. Geological Survey: Hardness of Water · City of Mountain View: Water and Sewer FAQs · City of Santa Clara: Troubleshooting Water Quality. Figures for each city were first researched for our city pages linked in the table, each of which lists its sources.

Checked October 2026.