EPA system CA4310022 · California

Great Oaks Wc INC: Lithium at or above EPA's 10 ppb health reference level

System ID CA4310022; community water system; California; source: groundwater; serves 110,088 people via 21,415 connections; status: active

In focus110,088people served, EPA registry

Public records. Clearer context.

Great Oaks Wc INC is a privately owned community water system in California serving about 110,088 people, matched to 9 ZIP codes.

PFAS treatment declared to EPA: None declared (40 of 40 sampling events) · PFAS found before UCMR 5: Yes, at 40 of 40 sampling events

PFAS and lithium: what EPA's national survey found

EPA tested this water for 30 substances and found 5. PFOS averaged 2.3 ppt where EPA's limit is 4.0 ppt. That PFOS result is higher than 89% of the 10,307 water systems EPA tested for it. Lithium reached 16 ppb; EPA's health reference level, which is guidance and not a limit, is 10 ppb.

Between June 2025 and December 2025, EPA's UCMR 5 program tested 1,140 samples from 19 sampling locations for 30 contaminants (29 PFAS plus lithium). 5 were detected above the reporting level in at least one sample. Averaged over a complete monitoring set, the highest sampling location for PFOS was 2.3 ppt, below the MCL of 4.0 ppt that applies from 2029 (highest single sample 4.6 ppt). Lithium reached 16 ppb in a single sample, at or above the health reference level of 10 ppb, which is non-regulatory.

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: yes at 40 of 40 sampling events; potential PFAS sources nearby: no at 40 of 40 sampling events; PFAS treatment in place: no PFAS treatment at 40 of 40 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. The action level has been reached in 3 of the 23 periods on record.

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; 3 of the 23 monitoring periods on record reached 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 no drinking-water violations for this utility since Sep 3, 2016.

EPA's SDWIS database lists no drinking water violations for this utility since Sep 3, 2016.

None on file.

Sanitary surveys and site visits

Most recent state inspection: Nov 6, 2025 (sanitary survey); 2 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, 26.6 sq mi