Public water system · CA4110021
ActiveEstero Municipal Improvement District
At a glance
Estero Municipal Improvement District (PWSID CA4110021) is an active community water system in San Mateo County, California. It reports serving 36,556 people through 8,035 service connections. Its primary water source is purchased surface water, and EPA lists it as owned by a local government. EPA's records list 5 violations for this system; 2 are health-based, and the most recent began in July 2022.
Dataset updated . Source: EPA SDWIS/ECHO public records.
Population served
36,556
Service connections
8,035
Primary source
SWP
Purchased surface water
System type
CWS
Community water system
System record
How does Estero Municipal Improvement District compare with other water systems in San Mateo County?
By EPA's size categories, Estero Municipal Improvement District is a large system (10,001 to 100,000 people). San Mateo County has 51 active public water systems. Its population served is about 138% above the county average of 15,363 people per system, and accounts for 4.7% 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 Estero Municipal Improvement District is about 5.5 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 Estero Municipal Improvement District?
EPA lists Estero Municipal Improvement District as owned by a local government. 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. 19 of the 51 active systems in San Mateo County (37%) are also owned by a local government.
Does Estero Municipal Improvement District have drinking water violations?
EPA's records list 5 violations for this system; 2 are health-based, and the most recent began in July 2022. The violations listed here began between August 1987 and July 2022. By type: 2 monitoring and reporting, 1 contaminant level above the legal limit, 1 required treatment not met and 1 public notice, recordkeeping or other. The contaminants or rules involved are Total trihalomethanes (TTHM), Total coliform bacteria (TCR), Lead and Copper Rule and Surface Water Treatment Rule. EPA lists 4 as resolved and 1 as archived. 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. Archived means the violation was not marked resolved but is more than five years past the end of the noncompliance period, so it no longer counts toward current compliance status.
| Began | Violation | Contaminant or rule | Health-based | Status |
|---|---|---|---|---|
| 2022-07-01 | Monitoring and reportingMonitoring and Reporting (DBP) · code 27 | Total trihalomethanes (TTHM)code 2950 | No | Resolved |
| 2007-10-01 | Contaminant level above the legal limitMaximum Contaminant Level Violation, Monthly (TCR) · code 22 | Total coliform bacteria (TCR)code 3100 | Yes | Resolved |
| 1993-06-30 | Monitoring and reportingInitial Tap Sampling for Pb and Cu · code 51 | Lead and Copper Rulecode 5000 | No | Resolved |
| 1992-04-01 | Required treatment not metTreatment Technique (SWTR and GWR) · code 41 | Surface Water Treatment Rulecode 0200 | Yes | Resolved |
| 1987-08-01 | Public notice, recordkeeping or otherNotification, Public · code 06 | Not reported | No | Archived |
All 5 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.
Estero Municipal Improvement District: frequently asked questions
Who runs Estero Municipal Improvement District?
Estero Municipal Improvement District (PWSID CA4110021) is a community water system owned by a local government. It is regulated by the CA state drinking water program.
How many people does Estero Municipal Improvement District serve?
Estero Municipal Improvement District reports serving 36,556 people through 8,035 service connections in San Mateo County, California.
Where does Estero Municipal Improvement District 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 Estero Municipal Improvement District have any drinking water violations?
EPA's records list 5 violations for this system; 2 are health-based, and the most recent began in July 2022. 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 Estero Municipal Improvement District'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 5 violations for this system; 2 are health-based, and the most recent began in July 2022. For current results, consult the system's annual water quality report, EPA ECHO, or the CA state drinking water program.
Where can I find Estero Municipal Improvement District's water quality report?
As a community water system, Estero Municipal Improvement District 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, “Estero Municipal Improvement District, PWSID CA4110021, California”, https://aquacensus.com/water-systems/california/estero-municipal-improvement-district-ca4110021 (data as of October 1, 2026).