We read 15 results from California Water Service - San Mateo’s 2025 water quality report. No results over a legal limit; 1 above a health goal or guideline.
Serves about 102,348 people · Water system ID CA4110008 · Read the district’s 2025 report (opens in a new tab)
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Results from the 2025 report
Each number is copied from the district’s report and links to the page it’s printed on. Where the report lists several plants, wells or purchased water, the highest is shown.
| Contaminant | Result | Health goal | Legal limit | What it means |
|---|---|---|---|---|
| Chromium-6average · range ND to 0.1 · SFPUC water · 2025 report, page 13 | 0.1 ppb | 0.02 ppb | 10 ppb | Above the health goal5x the health goal |
| Chloramineaverage · range 0.96 to 3.6 · Distribution system · 2025 report, page 13 | 2.7 ppm | None set | 4 ppm | Below the legal limit |
| Chlorideaverage · range ND to 19 · SFPUC water · 2025 report, page 14 | 9.8 ppm | — | 250 ppm guideline | Within the guideline |
| Copper90th percentile of 50 homes tested, 0 over the action level · Customer taps · 2025 report, page 13 | Not detected | 0.3 ppm | 1.3 ppm (action level) | Not detected |
| Disinfection byproducts (HAA5)highest running annual average · range 10 to 50 · Distribution system · 2025 report, page 13 | 39 ppb | None set | 60 ppb | Below the legal limit |
| Disinfection byproducts (TTHM)highest running annual average · range 16 to 54 · Distribution system · 2025 report, page 13 | 43 ppb | None set | 80 ppb | Below the legal limit |
| E. colias reported · Distribution system · 2025 report, page 13 | Not detected | 0 positive samples | 0 positive samples | Not detected |
| Fluorideaverage · range 0.5 to 0.8 · SFPUC water · 2025 report, page 13 | 0.7 ppm | 1 ppm | 2 ppm | Below the health goal |
| Lead90th percentile of 50 homes tested, 1 over the action level · Customer taps · 2025 report, page 13 | Not detected | 0.2 ppb | 15 ppb (action level) | Not detected |
| Sulfateaverage · range 1 to 45 · SFPUC water · 2025 report, page 14 | 21 ppm | — | 250 ppm guideline | Within the guideline |
| Total dissolved solidsaverage · range 24 to 197 · SFPUC water · 2025 report, page 14 | 117 ppm | — | 500 ppm guideline | Within the guideline |
| Turbidity (cloudiness)highest measured · SFPUC water · 2025 report, page 14 | 0.3 NTU | None set | 1 NTU | Below the legal limit |
| Conductivityaverage · range 32 to 346 · SFPUC water · 2025 report, page 14 | 207 µS/cm | None set | None | No health limit |
| Hardnessaverage · range 0.47 to 6.55 · SFPUC water · 2025 report, page 15 | 3.63 gpg (62 ppm) | — | — | Moderately hard |
| Sodiumaverage · range 3.1 to 29 · SFPUC water · 2025 report, page 15 | 18 ppm | None set | None | No health limit |
Utility results describe the whole system, measured where water enters the pipes, not your tap. Your home’s plumbing can add lead or copper.
What you can do
Nothing here is an emergency notice: if the district has a violation, it must tell customers directly. If you’d like to lower these at your tap, these are the treatments that remove them, and the catalog shows the models certified for each. Ranking is by certified removal; commissions never change the order.
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