EPA system CA3910012 · California

City of Stockton: PFOS at or above EPA's 4.0 ppt limit

System ID CA3910012; community water system; California; source: surface water; serves 191,302 people via 51,522 connections; status: active

In focus191,302people served, EPA registry

Public records. Clearer context.

City of Stockton is a local-government-owned community water system in California serving about 191,302 people, matched to 11 ZIP codes.

PFAS treatment declared to EPA: Ion exchange (1 of 6 sampling events) · PFAS found before UCMR 5: Yes, at 6 of 40 sampling events

PFAS and lithium: what EPA's national survey found

EPA tested this water for 30 substances and found 3. PFOS averaged 14.0 ppt where EPA's limit is 4.0 ppt. EPA's limit applies to a running yearly average and is enforced from 2029. That PFOS result is higher than 98% of the 10,307 water systems EPA tested for it.

Between February 2023 and December 2025, EPA's UCMR 5 program tested 1,140 samples from 17 sampling locations for 30 contaminants (29 PFAS plus lithium). 3 were detected above the reporting level in at least one sample. Averaged over a complete monitoring set, the highest sampling location for PFOA was 3.4 ppt, below the MCL of 4.0 ppt that applies from 2029 (highest single sample 6.7 ppt). Averaged over a complete monitoring set, the highest sampling location for PFOS was 14.0 ppt, at or above the MCL of 4.0 ppt that applies from 2029 (highest single sample 28.0 ppt).

Sampling-point average: mean of a location's two (ground water) or four (surface water) UCMR 5 results with non-detects counted as zero, EPA's method for comparing UCMR 5 data to the PFAS MCLs. EPA rounds to the reference value's significant digits before deciding whether an average is "greater than" an MCL (PFOA/PFOS averages count as above 4.0 ng/L only from 4.05 ng/L; lithium results count as above the 10 ppb reference only from 15 ppb). This site does not apply that rounding: "at or above" here means the measured value is numerically at or above the reference.

Utility-reported context (UCMR 5 questionnaire, aggregated across sampling events): PFAS previously found at this system: not known at 31 of 40 sampling events, yes at 6 of 40 sampling events, no at 3 of 40 sampling events; potential PFAS sources nearby: not known at 39 of 40 sampling events, no at 1 of 40 sampling events; PFAS treatment in place: no PFAS treatment at 5 of 6 sampling events, ion exchange at 1 of 6 sampling events.

Lead at customer taps

In the latest completed monitoring period, ending Dec 31, 2023, 9 in 10 sampled homes had lead at or below 0 ppb, below the 15 ppb action level that triggers utility action. None of the 23 periods on record reached it.

In the most recent completed lead monitoring period, ending Dec 31, 2023, the 90th-percentile lead result at customer taps was 0 ppb, below the action level of 15 ppb (23 monitoring periods on record, none at the action level). The 90th percentile is the value that 9 in 10 sampled homes were at or below. Values are in parts per billion (ppb).

Drinking water violations since Sep 3, 2016

EPA lists 4 violations since Sep 3, 2016. 2 are health-based, meaning a contaminant or treatment rule was not met. The other 2 concern missed monitoring, reporting or public notices.

EPA's SDWIS database lists 4 violations recorded since Sep 3, 2016: 2 health-based (a contaminant above its maximum contaminant level, a disinfectant above its maximum residual level, or a required treatment technique not met), 2 for monitoring or reporting.

Sanitary surveys and site visits

Most recent state inspection: Oct 30, 2025 (sanitary survey); 3 visits since Sep 3, 2021.

What you can do about these results

ZIP codes served

Sources

Data vintage: EPA UCMR 5 occurrence data, samples collected 2023-01-03 – 2026-06-17, final release, August 2026 (file dated Aug 28, 2026); EPA SDWIS via ECHO, 2026Q2 extract (file dated Jul 9, 2026); EPA service-area boundaries v3.0; Census 2020 ZCTAs. Page generated 2026-09-03.

• EPA ECHO detailed facility report
• EPA UCMR 5 Data Finder
• EPA service-area boundary: modeled, 35.8 sq mi