Stanislaus County, CA
drinking water quality
556,972 residents · 193 public water systems · 25 ZIP codes
EPA sampled 15 of 193 public water systems here, together serving about 477,878 people. The highest reading was PFOS at 26 ng/L at City of Modesto. 2 of the sampled systems were above a federal limit. Last sampled 9/9/2024.
What was found
| Compound | Highest result | Federal limit | Ratio |
|---|---|---|---|
| PFOS | 26 ng/L | 4 ng/L | 6.5× |
| PFOA | 8.6 ng/L | 4 ng/L | 2.1× |
| PFHxS | 9.3 ng/L | 10 ng/L | 0.9× |
Also detected, with no federal limit set: lithium (21000 ng/L), PFPeA (6.7 ng/L), PFHxA (6.1 ng/L), PFBS (4.3 ng/L), PFHpA (3.6 ng/L).
Health-based violations
Separately from PFAS, EPA records 393 health-based violations across 49 of the county’s water systems since 2020, the most recent in 2026. They involved E. coli, uranium, arsenic, nitrate, lead and copper, contaminant 1039.
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 Stanislaus County
95230 · 95307 · 95313 · 95316 · 95319 · 95323 · 95326 · 95328 · 95350 · 95351 · 95354 · 95355 · 95356 · 95357 · 95358 · 95360 · 95361 · 95363 · 95367 · 95368 · 95380 · 95382 · 95385 · 95386 · 95387
Readings are county-level, so every ZIP here shares the figure above. Search your ZIP on the map to see it in context.