Public water system · CA4110008
ActiveCalifornia Water Service - San Mateo
At a glance
California Water Service - San Mateo (PWSID CA4110008) is an active community water system in San Mateo County, California. It reports serving 102,348 people through 25,440 service connections. Its primary water source is purchased surface water, and EPA lists it as privately owned. EPA's records list 2 violations for this system; all are health-based, and the most recent began in June 1993.
Dataset updated . Source: EPA SDWIS/ECHO public records.
Population served
102,348
Service connections
25,440
Primary source
SWP
Purchased surface water
System type
CWS
Community water system
System record
How does California Water Service - San Mateo compare with other water systems in San Mateo County?
By EPA's size categories, California Water Service - San Mateo is a very large system (more than 100,000 people). San Mateo County has 51 active public water systems, and California Water Service - San Mateo is the 2nd-largest of them by population served. Its population served is about 6.7 times the county average of 15,363 people per system, and accounts for 13% of the 783,522 people reported by systems filed under the county. Across California, the 7,479 active systems average 5,706 people each. Nationally, the average active community water system reports serving 6,703 people, so California Water Service - San Mateo is about 15 times that figure.
What does “community water system” mean?
A community water system serves at least 15 connections used by year-round residents, or regularly serves at least 25 year-round residents. Homes, apartments and mobile-home parks are served by these. California has 2,997 active community water systems, 40% of the state's 7,479 active systems. Purchased surface water means the system buys surface water from another public water system rather than drawing its own. 506 California systems (6.8%) list purchased surface water as their primary source.
Who owns and regulates California Water Service - San Mateo?
EPA lists California Water Service - San Mateo as privately owned. The CA state drinking water program is its primacy agency, the regulator with primary enforcement responsibility for the system under the Safe Drinking Water Act. 27 of the 51 active systems in San Mateo County (53%) are also privately owned.
Does California Water Service - San Mateo have drinking water violations?
EPA's records list 2 violations for this system; all are health-based, and the most recent began in June 1993. The violations listed here began between October 1992 and June 1993. By type: 1 required treatment not met and 1 contaminant level above the legal limit. The contaminants or rules involved are Surface Water Treatment Rule and Total coliform bacteria (TCR). EPA lists 2 as resolved. Resolved means at least one resolving action is on record: the system returned to compliance, the rule no longer applies, or no further action was needed.
| Began | Violation | Contaminant or rule | Health-based | Status |
|---|---|---|---|---|
| 1993-06-01 | Required treatment not metTreatment Technique (SWTR and GWR) · code 41 | Surface Water Treatment Rulecode 0200 | Yes | Resolved |
| 1992-10-01 | Contaminant level above the legal limitMaximum Contaminant Level Violation, Monthly (TCR) · code 22 | Total coliform bacteria (TCR)code 3100 | Yes | Resolved |
All 2 violations in EPA's records, newest first. Labels and codes come from EPA's SDWIS reference tables. A monitoring or reporting violation means a required test or report was missed or late; it does not by itself mean a contaminant was found. "Health-based" marks violations of a legal limit or required treatment.
California Water Service - San Mateo: frequently asked questions
Who runs California Water Service - San Mateo?
California Water Service - San Mateo (PWSID CA4110008) is a community water system privately owned. It is regulated by the CA state drinking water program.
How many people does California Water Service - San Mateo serve?
California Water Service - San Mateo reports serving 102,348 people through 25,440 service connections in San Mateo County, California.
Where does California Water Service - San Mateo get its water?
EPA SDWIS lists this system's primary water source as purchased surface water. Purchased surface water means the system buys surface water from another public water system rather than drawing its own.
Does California Water Service - San Mateo have any drinking water violations?
EPA's records list 2 violations for this system; all are health-based, and the most recent began in June 1993. A monitoring or reporting violation means a required test or report was missed or late; it does not by itself mean a contaminant was found.
Is California Water Service - San Mateo's water safe to drink?
AquaCensus shows EPA compliance records, not current water test results, so it cannot say whether the water is safe at a given moment. EPA's records list 2 violations for this system; all are health-based, and the most recent began in June 1993. For current results, consult the system's annual water quality report, EPA ECHO, or the CA state drinking water program.
Where can I find California Water Service - San Mateo's water quality report?
As a community water system, California Water Service - San Mateo must deliver an annual water quality report, called a Consumer Confidence Report, to its customers by July 1 each year, covering the previous calendar year. The system can provide a copy on request.
Other water systems in San Mateo County
| Water system | County | Population served | Type | Source |
|---|---|---|---|---|
| City of Daly City | San Mateo | 104,901 | Community | Purchased surface water |
| City of Redwood City | San Mateo | 92,819 | Community | Purchased surface water |
| California Water Service-S San Francisco | San Mateo | 64,146 | Community | Purchased surface water |
| California Water Service - Bear Gulch | San Mateo | 61,431 | Community | Surface water |
| City of San Bruno | San Mateo | 42,631 | Community | Purchased surface water |
| North Coast County Water Dist | San Mateo | 38,546 | Community | Purchased surface water |
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About this data
Source: U.S. EPA Safe Drinking Water Information System (SDWIS) and SDWA violation records, via EPA ECHO, as published October 1, 2026 — AquaCensus refreshes it quarterly. See the methodology for how records are matched, cleaned and scored.
Cite as: AquaCensus, “California Water Service - San Mateo, PWSID CA4110008, California”, https://aquacensus.com/water-systems/california/california-water-service-san-mateo-ca4110008 (data as of October 1, 2026).