A84/100
CALIFORNIA WATER SERVICE - SAN MATEO — Water Quality Report 2026
Serving 102,348 people · California
Water Source
Purchased Surface Water
County
—
System ID
CA4110008
Violations
2
Contaminant Test Results
2 contaminants above EPA health goals
| Contaminant | Detected | Limit (MCL) | Status |
|---|---|---|---|
chlorate | 320.00 µg/L | — | Above Goal |
Lead (90th percentile) | 0.004 mg/L | 0.015 mg/L | Above Goal |
EPA Violation History
No violations in the past 5 years
Full EPA compliance since 1993.
Understanding violation severity
Tier 1Urgent health risk. Utility must notify all customers within 24 hours.Tier 2Important health or treatment issue. Customers notified within 30 days.Tier 3Administrative or monitoring issue. Reported in the annual water quality report.
Source: EPA Safe Drinking Water Information System (SDWIS) · Updated quarterly
Frequently Asked Questions About CALIFORNIA WATER SERVICE - SAN MATEO
Is CALIFORNIA WATER SERVICE - SAN MATEO water safe to drink?
CALIFORNIA WATER SERVICE - SAN MATEO water receives a grade of A (84/100), which is considered excellent. Out of 69 contaminants tested, none exceed EPA legal limits. 2 contaminant(s) exceed non-enforceable health goals. The water meets federal safety standards.
What contaminants has CALIFORNIA WATER SERVICE - SAN MATEO detected?
69 contaminants were tested in CALIFORNIA WATER SERVICE - SAN MATEO's water. Notable contaminants include Lead (90th percentile), chlorate. No contaminants exceed EPA legal limits. 2 exceed EPA health goals (MCLGs).
Does CALIFORNIA WATER SERVICE - SAN MATEO have any EPA violations?
Yes, CALIFORNIA WATER SERVICE - SAN MATEO has 2 EPA violation(s) on record, with the most recent in 1993. Violation types include TT, MCL. 2 are health-based violations.
How many people does CALIFORNIA WATER SERVICE - SAN MATEO serve?
CALIFORNIA WATER SERVICE - SAN MATEO serves approximately 102,348 people, California.
What type of water does CALIFORNIA WATER SERVICE - SAN MATEO provide?
CALIFORNIA WATER SERVICE - SAN MATEO sources its water from purchased surface water. Surface water comes from rivers, lakes, or reservoirs and typically requires more extensive treatment. The utility's system ID is CA4110008.
How does CALIFORNIA WATER SERVICE - SAN MATEO compare to other utilities in California?
CALIFORNIA WATER SERVICE - SAN MATEO scores 84/100 with a grade of A (excellent). This places it among the higher-performing utilities in the state. Visit our California state page for a full comparison of water systems.