San Mateo County Water Quality: 48 Violations (CA)
41 water systems · 35 ZIP codes · Grade B · Updated 2026-06-04
Data: EPA Safe Drinking Water Information System (SDWIS), EPA ECHO Last verified: 2026-06-04
Violation Trend
EPA violations across water systems in San Mateo County.
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Find the Right Filter →Water Quality Overview
San Mateo County has 41 community water systems serving approximately 3,647,497 people across 35 ZIP codes. These systems have accumulated 48 EPA violations, including 32 health-based violations.
Health-based violations account for 67% of all violations in San Mateo County. These violations indicate contaminant levels that may pose direct risks to human health.
Violation Breakdown
| Violation Type | Count | Percentage |
|---|---|---|
| Health-based | 32 | 66.7% |
| Monitoring/Reporting | 16 | 33.3% |
| Total | 48 | 100% |
Health-based violations indicate contaminant levels exceeding EPA Maximum Contaminant Levels (MCLs). Monitoring violations indicate failures to test or report.
Cities with Most Violations
| Rank | City | Violations | Health-Based | ZIP Codes | Population |
|---|---|---|---|---|---|
| 1 | San Mateo | 20 | 15 | 5 | 785 |
| 2 | Pescadero | 9 | 1 | 1 | 328 |
| 3 | San Francisco | 7 | 6 | 1 | 3,643 |
| 4 | Redwood City | 5 | 5 | 5 | 517,394 |
| 5 | La Honda | 5 | 4 | 1 | 1,418 |
| 6 | Millbrae | 2 | 1 | 1 | 64,172 |
Violation Trend
Year-by-year EPA violations across water systems in San Mateo County:
| Year | Total | Health-Based |
|---|---|---|
| 2021 | 4 | 4 |
| 2022 | 3 | 3 |
| 2023 | 5 | 5 |
| 2024 | 14 | 14 |
| 2025 | 6 | 6 |
Most Common Contaminants
| Contaminant | Category | Violations | ZIPs Affected | Health-Based |
|---|---|---|---|---|
| Stage 2 DBP Rule | Treatment Technique | 12 | 6 | Yes |
| Lead | Inorganic | 11 | 6 | Yes |
| Total Trihalomethanes (TTHM) | Disinfection Byproducts | 9 | 6 | Yes |
| Mercury | Inorganic | 4 | 1 | Yes |
| E. coli | Microbiological | 1 | 1 | No |
| Haloacetic Acids (HAA5) | Disinfection Byproducts | 1 | 1 | No |
| Surface Water Treatment Rule | Treatment Technique | 1 | 1 | No |
| Total Coliform | Microbiological | 1 | 1 | No |
Understanding Water Quality Scores
Each ZIP code in San Mateo County receives a safety score from 0-100 based on EPA compliance data. Scores factor in the number and severity of violations, with health-based violations weighted more heavily than monitoring failures. Resolved violations (where the system returned to compliance) receive reduced weight in the scoring.
The county-level grade (B) represents the average across all 35 ZIP codes. Individual ZIP codes may vary significantly — a county with a B grade may contain both A-rated and D-rated areas.
Water Systems in San Mateo County
San Mateo County is served by 41 community water systems, averaging 1.2 systems per ZIP code. Each system is independently regulated and tested by the EPA. Larger systems serving more people typically have more extensive monitoring requirements.
Water systems in San Mateo County source water from a mix of surface water (rivers, reservoirs) and groundwater (wells, aquifers). Source type affects which contaminants are most likely to be detected.
How to Check Your Water in San Mateo County
Enter your ZIP code on ZipCheckup for detailed water system data including contaminant levels, violation history, and safety scores
Request your CCR — every water utility must provide an annual Consumer Confidence Report
Get a home test — certified kits ($20-$50) or professional lab tests ($150-$400) can detect contaminants not covered by routine testing
Consider filtration — NSF-certified filters can address specific contaminants found in your area
Check for lead — homes built before 1986 should test for lead regardless of utility compliance
EPA Enforcement
The EPA and state regulators have taken 51 enforcement actions against water systems in San Mateo County. Enforcement actions range from compliance assistance and administrative orders to formal penalties. Systems under enforcement are required to remediate violations and may face ongoing monitoring requirements.
Data Sources
- Water quality: EPA Safe Drinking Water Information System (SDWIS)
- Enforcement: EPA ECHO
Updated daily.
Water Quality Map: San Mateo County, CA
Each dot represents a ZIP code. Color indicates water quality grade. Tap a dot for details.
Water Quality by ZIP Code in San Mateo County
Water quality varies across San Mateo County's 35 ZIP codes. Check your specific ZIP for detailed contaminant data.
| ZIP Code | City | Grade | Score | Violations | Health | |
|---|---|---|---|---|---|---|
| 94060 | Pescadero | C | 63 | 9 | 1 | View → |
| 94128 | San Francisco | D | 53 | 7 | 6 | View → |
| 94020 | La Honda | B | 74 | 5 | 4 | View → |
| 94497 | San Mateo | C | 68 | 4 | 3 | View → |
| 94401 | San Mateo | C | 66 | 4 | 3 | View → |
| 94402 | San Mateo | C | 66 | 4 | 3 | View → |
| 94403 | San Mateo | C | 66 | 4 | 3 | View → |
| 94404 | San Mateo | C | 66 | 4 | 3 | View → |
| 94030 | Millbrae | C | 68 | 2 | 1 | View → |
| 94064 | Redwood City | B | 82 | 1 | 1 | View → |
| 94065 | Redwood City | B | 82 | 1 | 1 | View → |
| 94061 | Redwood City | B | 77 | 1 | 1 | View → |
| 94062 | Redwood City | B | 77 | 1 | 1 | View → |
| 94063 | Redwood City | B | 72 | 1 | 1 | View → |
| 94005 | Brisbane | B | 83 | 0 | 0 | View → |
| 94021 | Loma Mar | B | 83 | 0 | 0 | View → |
| 94026 | Menlo Park | B | 83 | 0 | 0 | View → |
| 94083 | South San Francisco | B | 83 | 0 | 0 | View → |
| 94002 | Belmont | B | 78 | 0 | 0 | View → |
| 94019 | Half Moon Bay | B | 78 | 0 | 0 | View → |
What are EPA health violations?
Health-based violations mean a contaminant was detected above the EPA's Maximum Contaminant Level (MCL) — the legal limit designed to protect human health. San Mateo County has 32 such violations. Not all violations cause illness, but repeated exceedances increase risk, especially for children and immunocompromised individuals.
What is a water quality grade?
ZipCheckup grades water systems from A to F based on EPA violation history, contaminant levels, and system compliance. Grade A means few or no violations; Grade F indicates persistent or serious issues. San Mateo County's overall grade is B.
Not sure if your water needs a filter?
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Find the Right Filter →Frequently Asked Questions
Is the tap water safe in San Mateo County, California?
San Mateo County has 41 community water systems with 48 recorded EPA violations, including 32 health-based. Water quality varies by ZIP code — check your specific location for detailed results.
How many water violations does San Mateo County have?
San Mateo County has 48 total EPA violations across 35 ZIP codes. 32 are health-based violations indicating contaminant levels potentially harmful to health.
Which city in San Mateo County has the most water violations?
San Mateo has the most violations in San Mateo County with 20 recorded EPA violations across 5 ZIP codes.
What contaminants are found in San Mateo County water?
San Mateo County water systems are monitored for over 90 EPA-regulated contaminants including lead, copper, nitrates, disinfection byproducts, and bacteria. Check your ZIP code for contaminant-specific data.
How can I test my water in San Mateo County?
Contact your local water utility for free or low-cost testing. You can also use a certified home test kit ($20-$50) or hire a certified lab ($150-$400).