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Updated 2026-10-15
County water report

San Mateo County Water Quality: 13 Cities Ranked by the Records

San Mateo County, California: 13 city reports built from EPA and utility records. A county has many suppliers; each city shows the system that serves it.

Find my systemOpen on the mapPrivate wells in San Mateo County
Across the county's systems

Highest result for each contaminant

One row per contaminant: the highest result any system in the county reported, against its limit. Open a row for the systems behind it.

LithiumDetected in 3 systems of 13 tested10 µg/L EPA health reference level (screening, not a legal limit) · US EPAHighest40 µg/L
Lead (90th percentile)Detected in 7 systems of 13 tested15 µg/L federal action level · US EPAHighest6.3 µg/L
29 PFAS tested, none detected in any systemShow

Tested in these 13 systems: CALIFORNIA WATER SERVICE - BEAR GULCH, CALIFORNIA WATER SERVICE - SAN CARLOS, CALIFORNIA WATER SERVICE - SAN MATEO, CALIFORNIA WATER SERVICE-S SAN FRANCISCO, CITY OF BURLINGAME, CITY OF DALY CITY, CITY OF MILLBRAE, CITY OF REDWOOD CITY, CITY OF SAN BRUNO, COASTSIDE COUNTY WATER DISTRICT, MENLO PARK MUNICIPAL WATER, MID-PENINSULA WATER DISTRICT, NORTH COAST COUNTY WATER DIST

  • 11-Chloroeicosafluoro-3-oxaundecane-1-sulfonic acid (11Cl-PF3OUdS)
  • 1H, 1H, 2H, 2H-perfluorodecane sulfonic acid (8:2 FTS)
  • 1H, 1H, 2H, 2H-perfluorohexane sulfonic acid (4:2 FTS)
  • 1H, 1H, 2H, 2H-perfluorooctane sulfonic acid (6:2 FTS)
  • 4,8-Dioxa-3H-perfluorononanoic acid (ADONA)
  • 9-Chlorohexadecafluoro-3-oxanone-1-sulfonic acid (9Cl-PF3ONS)
  • Hexafluoropropylene oxide dimer acid (HFPO-DA) (GenX chemicals)
  • N-ethyl perfluorooctanesulfonamidoacetic acid (NEtFOSAA)
  • N-methyl perfluorooctanesulfonamidoacetic acid (NMeFOSAA)
  • Nonafluoro-3,6-dioxaheptanoic acid (NFDHA)
  • Perfluoro (2-ethoxyethane) sulfonic acid (PFEESA)
  • Perfluoro-3-methoxypropanoic acid (PFMPA)
  • Perfluoro-4-methoxybutanoic acid (PFMBA)
  • Perfluorobutanesulfonic acid (PFBS)
  • Perfluorobutanoic acid (PFBA)
  • Perfluorodecanoic acid (PFDA)
  • Perfluorododecanoic acid (PFDoA)
  • Perfluoroheptanesulfonic acid (PFHpS)
  • Perfluoroheptanoic acid (PFHpA)
  • Perfluorohexanesulfonic acid (PFHxS)
  • Perfluorohexanoic acid (PFHxA)
  • Perfluorononanoic acid (PFNA)
  • Perfluorooctanesulfonic acid (PFOS)
  • Perfluorooctanoic acid (PFOA)
  • Perfluoropentanesulfonic acid (PFPeS)
  • Perfluoropentanoic acid (PFPeA)
  • Perfluorotetradecanoic acid (PFTA)
  • Perfluorotridecanoic acid (PFTrDA)
  • Perfluoroundecanoic acid (PFUnA)
13 cities

Cities ranked by the records

Showing 6 of 13 cities · scores are system-level, not tap tests

Ranked by the Purity Score of each city's main water system, highest first. Scores belong to water systems; there is no county score. The score is computed only from contaminants measured in the published EPA record, so this ranks the record, not which city's water is safest. How the score works.

PFAS in San Mateo CountyPFAS detected in 0 of 13 cities; none had a sample above a 2024 EPA limit
4.0 ng/L federal MCL · PFOS, ng/LPFAS in California
Lead

0of 13 cities above the 15 µg/L action level

Based on each system's 90th-percentile result. Lead from household plumbing isn't in this record.

Lead in California
Who supplies the county

Largest water systems

  1. San MateoCALIFORNIA WATER SERVICE - SAN MATEO · CA4110008
    103,536 served
  2. Daly CityCITY OF DALY CITY · CA4110013
    102,593 served
  3. Redwood CityCITY OF REDWOOD CITY · CA4110022
    90,043 served
  4. South San FranciscoCALIFORNIA WATER SERVICE-S SAN FRANCISCO · CA4110009
    62,609 served
  5. North Fair OaksCALIFORNIA WATER SERVICE - BEAR GULCH · CA4110006
    60,382 served
  6. San BrunoCITY OF SAN BRUNO · CA4110023
    42,631 served
  7. PacificaNORTH COAST COUNTY WATER DIST · CA4110025
    38,546 served
  8. San CarlosCALIFORNIA WATER SERVICE - SAN CARLOS · CA4110007
    34,801 served
  9. BurlingameCITY OF BURLINGAME · CA4110003
    32,453 served
  10. BelmontMID-PENINSULA WATER DISTRICT · CA4110001
    31,059 served
Questions

Frequently asked

How does San Mateo County tap water compare across cities?

PurityMap ranks 13 cities in San Mateo County by the Purity Score of each city's main water system, computed only from contaminants measured in the published EPA record. Scores belong to water systems, not to cities. The highest published score is Daly City's main water system: CITY OF DALY CITY · PWSID CA4110013 · Purity Score 100 (Excellent) · Confidence: moderate · Assessed at the water-system level. The ranking compares the records, not which city's water is safest, and your own tap can differ with household plumbing.

How many San Mateo County cities had a PFAS sample above a 2024 EPA limit?

None of the 13 San Mateo County city reports had a PFAS sample above a 2024 EPA limit in EPA's UCMR 5 testing. PFAS were detected below the limit in 0.

Is there lead in San Mateo County tap water?

Utilities report lead as a 90th-percentile result from home taps. 7 of 13 San Mateo County cities reported a value above zero, and none was above the 15 µg/L federal action level. Lead usually comes from a building's own pipes and fixtures, so only a test at your tap tells you what you drink.

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