San Mateo County, California: drinking water report. San Mateo County occupies the peninsula between San Francisco and Silicon Valley, home to about
San Mateo County occupies the peninsula between San Francisco and Silicon Valley, home to about 740,000 residents. Water comes from the San Francisco Public Utilities Commission (Hetch Hetchy Reservoir in Yosemite National Park) for northern communities, the San Francisco Bay Area Water Supply and Conservation Agency for others, and the Coastside County Water District for the coast. The Hetch Hetchy system delivers some of the highest-quality municipal water in the state.
Hetch Hetchy water is snowmelt from the Sierra Nevada, delivered through a gravity-fed system that requires no pumping and minimal treatment. It is one of the few surface water systems in California that operates under an EPA Filtration Avoidance Determination. Communities on this supply enjoy water quality that most California residents can only envy.
However, not all San Mateo County communities receive Hetch Hetchy water. Coastside communities and some mid-peninsula areas use local groundwater or surface water that carries different quality profiles. UCMR5 data shows low-level PFAS detections in some county systems. According to the California SWRCB, three public water systems in the county reported PFAS above notification levels.
Determine whether your provider is on the Hetch Hetchy system or an independent source – the quality difference is significant.
Check your water for data specific to your provider. For Hetch Hetchy customers, filtration is optional. For other sources, activated carbon or reverse osmosis may be worthwhile. Our water filter guide covers both scenarios. Pull your detailed report, and visit our California page for statewide data.
Drinking water is one exposure route. These federal datasets also cover San Mateo County, CA.
The EPA places San Mateo County, CA in Zone 2 - a moderate-risk category, with a predicted average indoor radon level between 2 and 4 pCi/L. Zone maps predict an area's radon potential from geology and existing indoor readings; they say nothing about a specific house. Testing is the only way to know your own level.
74% of homes in San Mateo County, CA were built before 1980, and 19% before 1950. Residential lead paint was banned in 1978, so housing age is the strongest available proxy for lead-paint exposure. This describes the county's housing stock, not any one home.
Modelled annual fine-particle pollution (PM2.5) in San Mateo County, CA averages 7.4 µg/m³, within the EPA's annual standard of 9 µg/m³. These are CDC modelled county estimates rather than readings from a specific monitor.
How radon zones work · Air quality by county · Lead exposure guide
Across 41 public water systems serving San Mateo County, CA with EPA lead testing on record, 6 (14.6%) reported a 90th-percentile lead result above the EPA's 15 ppb action level, and the median is 5.9 ppb. 18 water systems serving San Mateo County, CA carry a health-based EPA Safe Drinking Water Act violation.
| Most-cited violation type | Systems affected |
|---|---|
| Surface Water Treatment Rule | 7 |
| Haloacetic Acids (HAA5) | 4 |
| Total Coliform Rule | 4 |
| Total Trihalomethanes (TTHM) | 3 |
| Revised Total Coliform Rule | 2 |
Source: EPA Lead and Copper Rule 90th-percentile tap sampling and EPA SDWIS Safe Drinking Water Act enforcement records, aggregated from the public water systems serving this county. Figures are counts of water systems, not a test of any individual home or tap.