Public EPA test results
What EPA records show for California Water Service - San Mateo
EPA system ID CA4110008 · serves San Mateo County, CA · about 103,536 people (EPA records)
Here's what EPA has on file for this water system: its testing for newer contaminants like PFAS (UCMR 5), lead testing and violations. It's not a test of your tap, and there's no Water Score here.
Public EPA test results
California Water Service - San Mateo (EPA CA4110008)
What EPA records show for California Water Service - San Mateo, as of October 14, 2026
We found no health-standard violations in EPA's records for this system (CA4110008) since July 1, 2021, as of October 14, 2026.
Source: EPA ECHO record for California Water Service - San Mateo (SDWIS). Data as of October 14, 2026.
Lead: 9 in 10 homes tested came in at or below 4.4 ppb, at or below EPA's 15 ppb action level (latest EPA result for a testing period that ran on or after July 1, 2021: Jan 1, 2023–Dec 31, 2025). This is one number for the whole water system, from homes the utility tested. It isn't a result for your home.
EPA's UCMR 5 testing looked for 30 contaminants here and found none.
PFOA and PFOS were not detected at or above EPA's reporting level in EPA's UCMR 5 testing.
Source: EPA records (SDWIS, UCMR 5). SDWIS: data as of Oct 14, 2026. EPA UCMR 5 data, samples through June 2026.
Your water bill names your utility.
Does California Water Service - San Mateo use chloramine?
We didn't find chloramine in California Water Service - San Mateo's EPA treatment records. That's not a complete list; your utility's annual water quality report says which disinfectant it uses.
What EPA records show for this system
Reported results in EPA records (data as of Oct 14–15, 2026)