Sonoma County, CA
drinking water quality
485,375 residents · 445 public water systems · 38 ZIP codes
EPA sampled 17 of 445 public water systems here, together serving about 408,014 people. The highest reading was PFOA at 6.2 ng/L at City of Sonoma. 3 of the sampled systems were above a federal limit. Last sampled 9/9/2024.
What was found
| Compound | Highest result | Federal limit | Ratio |
|---|---|---|---|
| PFOA | 6.2 ng/L | 4 ng/L | 1.6× |
Also detected, with no federal limit set: lithium (40000 ng/L), PFHxA (6.1 ng/L).
Health-based violations
Separately from PFAS, EPA records 107 health-based violations across 52 of the county’s water systems since 2020, the most recent in 2025. They involved contaminant 0200, total coliform, E. coli, trihalomethanes, haloacetic acids, arsenic.
What this means
PFAS are industrial chemicals that EPA describes as breaking down only very slowly in the environment. In April 2024 EPA set the first enforceable national limits: 4 ng/L for PFOA and PFOS, 10 ng/L for PFHxS, PFNA and GenX.
These figures are county-level and come from EPA’s sampling of public water systems. They describe the worst result found in the county, not what comes out of any particular tap — that depends on which system serves the address, and on the plumbing in the building. A private well is not covered by this programme at all.
ZIP codes in Sonoma County
94515 · 94922 · 94923 · 94928 · 94931 · 94951 · 94954 · 94972 · 95401 · 95403 · 95404 · 95405 · 95407 · 95409 · 95412 · 95419 · 95421 · 95425 · 95430 · 95431 · 95436 · 95439 · 95441 · 95442 · 95444 · 95446 · 95448 · 95450 · 95452 · 95462 · 95465 · 95471 · 95472 · 95476 · 95480 · 95486 · 95492 · 95497
Readings are county-level, so every ZIP here shares the figure above. Search your ZIP on the map to see it in context.