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Clearlake Oaks County Water District

What EPA’s and California’s records show in this utility’s water: nitrate, arsenic and other regulated chemicals; lead and copper; and violations since 2016, with the certification that handles each result.

Serves
2,359 people
Connections
2,094
Water source
Surface water
Based in
Clearlake Oak, CA
County
Lake
Owner
Local government
Hardness
Hard
The ZIP code it serves

95423

The short answer

1 result is above EPA’s legal limit, and 1 more is above the health goal. The worst: Total trihalomethanes (TTHM), above the legal limit. 2 health-based violations since 2016.

What handles it

What was found

Copper: EPA’s file records a utility’s copper only when it exceeds the action level. The last here was 2.8 ppm, for the period ending Dec. 31, 1995. None since is on file, so it is not counted as a current finding.

Total trihalomethanes (TTHM), arsenic, haloacetic acids (HAA5), fluoride, chromium (total) and nitrate: California’s own results, samples taken from Feb. 5, 2020 to June 25, 2026, with EPA’s 2012 to 2019 file for earlier years.

Above the legal limit1

Total trihalomethanes (TTHM)

Forms or is added in treatment

Above the legal limit

81.2ppb

Highest yearly average, 2020 to 2026

2026 average: 68 ppb

Averaged by year, the highest was 81.2 ppb, in 2022, above EPA’s legal limit of 80 ppb. Its highest single sample was 134 ppb. In 2026, the latest year on file, its eight samples averaged 68 ppb. EPA recorded violations for total trihalomethanes (TTHM) at this utility in 2021 and 2023. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 70 ppb, in 2019.

What handles it

  • NSF/ANSI 53 look for “Trihalomethanes (TTHM) Reduction”
  • NSF/ANSI 58 look for “Trihalomethanes (TTHM) Reduction”

Above the health goal1

Arsenic

Comes from the source

Above the health goal

6.3ppb

Highest yearly average, 2020 to 2025

2025, one sample: 3.8 ppb

Averaged by year, the highest was 6.3 ppb, in 2022: below EPA’s legal limit of 10 ppb, but above its health goal of zero. In 2025, the latest year on file, its one sample was 3.8 ppb. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 3.9 ppb, in 2016.

What handles it

  • NSF/ANSI 58 look for “Arsenic (Pentavalent)<=50 ppb Reduction”
  • NSF/ANSI 53 look for “Arsenic (Pentavalent)<=50 ppb Reduction”

Below EPA’s limits2

Haloacetic acids (HAA5)

Forms or is added in treatment

Below EPA’s limits

33.4ppb

Highest yearly average, 2020 to 2026

2026 average: 30.8 ppb

Averaged by year, the highest was 33.4 ppb, in 2024, below EPA’s legal limit of 60 ppb. Its highest single sample was 94.4 ppb. In 2026, the latest year on file, its eight samples averaged 30.8 ppb. EPA recorded no violation for haloacetic acids (HAA5) at this utility since 2020. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 23.6 ppb, in 2019.

What handles it

No certification claim covers this compound yet.

Fluoride

Forms or is added in treatment

Below EPA’s limits

0.16ppm

Highest yearly average, 2020 to 2025

2025 average: 0.12 ppm

Averaged by year, the highest was 0.16 ppm, in 2022, below EPA’s legal limit of 4.0 ppm. In 2025, the latest year on file, its two samples averaged 0.12 ppm. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 0.18 ppm, in 2013.

What handles it

3 compounds tested and not foundBelow the reporting level in every sample. Not found is not the same as zero.
  • Chromium (total)
  • Lead
  • Nitrate

What handles it

A certification covers specific claims, not a whole standard. Look for these claims on the certifier’s listing, not the box. Wording varies a little: NSF writes “Lead Reduction,” WQA just “Lead.”

  • NSF/ANSI 53

    Claims: Trihalomethanes (TTHM) Reduction, Arsenic (Pentavalent)<=50 ppb Reduction

    For: Total trihalomethanes (TTHM), Arsenic

    Other certifiers’ wording: VOC Reduction; Arsenic (III) (50 ppb influent level)

  • NSF/ANSI 58

    Claims: Trihalomethanes (TTHM) Reduction, Arsenic (Pentavalent)<=50 ppb Reduction

    For: Total trihalomethanes (TTHM), Arsenic

    Other certifiers’ wording: Arsenic (Pentavalent)<=300 ppb Reduction; Arsenic (V) (300 ppb influent level)

At the tap, or for the whole house?

These come with the water itself, so a whole-house system certified for the same claims treats every tap and shower. A filter at the tap treats drinking water for much less. How whole-house systems compare.

Filters with those claims on a certifier’s listing

Disclosure: TapBuff earns a commission if you buy through some links on this page. It does not change which standard or filter we name. As an Amazon Associate, TapBuff earns from qualifying purchases. How TapBuff makes money

Compare five-year costs

The best reverse osmosis systems, ranked

Hardness

Hard, 145 mg/L (8.5 grains per gallon), at its one sampling point, 2019 to 2025.

Hardness is calcium and magnesium. EPA sets no limit for it: it shows up as scale on fixtures and in water heaters, and as soap that won’t lather. USGS calls water above 60 mg/L moderately hard, above 120 hard and above 180 very hard.

Softeners compared at 8.5 grains per gallon, salt includedSoftener or salt-free conditioner?California water hardness, utility by utility

Violations since 2016

Contaminant or ruleTypePeriodMeasuredStatus
TTHMThe running average was above the legal limit.EPA: Maximum Contaminant Level Violation, AverageOct. 1, 2023 to Dec. 31, 202383.5 UG/LResolved
TTHMThe running average was above the legal limit.EPA: Maximum Contaminant Level Violation, AverageApril 1, 2021 to June 30, 20210.087 MG/LResolved
Health-based violations only. Resolved and archived mean the utility returned to compliance or the state closed the record. 3 monitoring or reporting violations not shown.

What this report can’t tell you

  • Everything else. Total trihalomethanes (TTHM), arsenic, haloacetic acids (HAA5), fluoride, chromium (total) and nitrate results here are California’s own from 2020 on, with EPA’s national file for 2012 to 2019. Clearlake Oaks County Water District’s own annual report, which it must send you every year, has the contaminants this page does not show.

  • Your own plumbing. Lead results are the 90th percentile across homes the utility samples. Your home’s service line, solder and fixtures decide yours. A certified lab test is the only way to know.

  • What it means for your health. This page reports what the records show and EPA’s limits and goals; it is not medical advice. If you are worried about a health effect, talk to a doctor or your local health department.

  • Anything after the latest data. SDWIS data is from 2026 Q3; UCMR 5 didn’t sample this utility; California’s results run from Feb. 5, 2020 to June 25, 2026.

Testing your own tap

Clearlake Oaks County Water District’s results are for the water it sends out. What your own service line, solder and fixtures add shows up only in a test of your tap, by a lab California certifies.

The 3 labs California certifies closest to the area Clearlake Oaks County Water District serves, in a straight line. Call first: certification is test by test, and not every lab takes samples from homeowners.

  • Alpha Analytical Laboratories, Inc. - Ukiah

    Ukiah, CA · about 32 miles

    Certified for bacteria, nitrate, lead and copper, metals, minerals and other inorganics, organic chemicals, disinfection byproducts, radioactivity

    (707) 468-0401

  • K Prime, LLC.

    Santa Rosa, CA · about 40 miles

    Certified for bacteria, nitrate, lead and copper, metals, minerals and other inorganics, PFAS

    (707) 527-7574

  • Sonoma County Public Health Laboratory

    Santa Rosa, CA · about 41 miles

    Certified for bacteria

    (707) 565-4711

All 145 certified labs for California

Sources

PWSID CA1710001. Rebuilt from EPA data on Oct. 18, 2026.

Towns it serves

Also in Lake County

All California utilities

What to do next