Public water system · CA4110007
ActiveCalifornia Water Service - San Carlos
At a glance
California Water Service - San Carlos (PWSID CA4110007) is an active community water system in San Mateo County, California. It reports serving 35,344 people through 10,474 service connections. Its primary water source is purchased surface water, and EPA lists it as privately owned. EPA's records list 4 violations for this system; all are health-based, and the most recent began in March 2015.
Dataset updated . Source: EPA SDWIS/ECHO public records.
Population served
35,344
Service connections
10,474
Primary source
SWP
Purchased surface water
System type
CWS
Community water system
System record
How does California Water Service - San Carlos compare with other water systems in San Mateo County?
By EPA's size categories, California Water Service - San Carlos is a large system (10,001 to 100,000 people). San Mateo County has 51 active public water systems. Its population served is about 130% above the county average of 15,363 people per system, and accounts for 4.5% 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 Carlos is about 5.3 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 Carlos?
EPA lists California Water Service - San Carlos 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 Carlos have drinking water violations?
EPA's records list 4 violations for this system; all are health-based, and the most recent began in March 2015. The violations listed here began between June 1993 and March 2015. By type: 2 required treatment not met and 2 contaminant level above the legal limit. The contaminant or rule involved most often is Surface Water Treatment Rule (2 of them). EPA lists 4 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 |
|---|---|---|---|---|
| 2015-03-01 | Required treatment not metTreatment Technique (SWTR and GWR) · code 41 | Surface Water Treatment Rulecode 0200 | Yes | Resolved |
| 1996-06-01 | Contaminant level above the legal limitMaximum Contaminant Level Violation, Monthly (TCR) · code 22 | Total coliform bacteria (TCR)code 3100 | Yes | Resolved |
| 1995-12-01 | Contaminant level above the legal limitMaximum Contaminant Level Violation, Monthly (TCR) · code 22 | Total coliform bacteria (TCR)code 3100 | Yes | Resolved |
| 1993-06-01 | Required treatment not metTreatment Technique (SWTR and GWR) · code 41 | Surface Water Treatment Rulecode 0200 | Yes | Resolved |
All 4 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 Carlos: frequently asked questions
Who runs California Water Service - San Carlos?
California Water Service - San Carlos (PWSID CA4110007) 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 Carlos serve?
California Water Service - San Carlos reports serving 35,344 people through 10,474 service connections in San Mateo County, California.
Where does California Water Service - San Carlos 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 Carlos have any drinking water violations?
EPA's records list 4 violations for this system; all are health-based, and the most recent began in March 2015. 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 Carlos' 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 4 violations for this system; all are health-based, and the most recent began in March 2015. 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 Carlos' water quality report?
As a community water system, California Water Service - San Carlos 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 |
| California Water Service - San Mateo | San Mateo | 102,348 | 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 |
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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 Carlos, PWSID CA4110007, California”, https://aquacensus.com/water-systems/california/california-water-service-san-carlos-ca4110007 (data as of October 1, 2026).