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Colonial Oak WC Inc

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
198 people
Connections
66
Water source
Groundwater
Based in
Salinas, CA
County
Monterey
Owner
Private
Hardness
Varies by source
The ZIP code it serves

93907

The short answer

1 result is above EPA’s legal limit. The worst: Nitrate, above the legal limit. 3 health-based violations since 2016.

What handles it

What was found

Nitrate, chromium (total), fluoride, arsenic and total trihalomethanes (TTHM): California’s own results, samples taken from Jan. 9, 2020 to June 3, 2026, with EPA’s 2012 to 2019 file for earlier years.

Above the legal limit1

Nitrate

Comes from the source

Above the legal limit

18.9ppm

Highest sample; found in 209 of 209

2026 average: 11 ppm

From 2021 to 2026, at least 25 samples were above EPA’s legal limit of 10 ppm, the highest 18.9 ppm in 2023. 25 have no follow-up sample on file within 30 days. In 2026, the latest year on file, its 20 samples averaged 11 ppm, above the limit. EPA recorded violations for nitrate at this utility in 2023 and 2024. Earlier, from 2012 to 2019, EPA’s file shows a highest sample of 12.7 ppm, in 2015.

What handles it

  • NSF/ANSI 58 look for “Nitrate/Nitrite Reduction”
  • NSF/ANSI 53 look for “Nitrate/Nitrite (30 ppm challenge)”

Below EPA’s limits2

Chromium (total)

Comes from the source

Below EPA’s limits

8.0ppb

Highest yearly average, 2021 to 2026

2026 average: 6.5 ppb

Averaged by year, the highest was 8.0 ppb, in 2021, below EPA’s legal limit of 100 ppb. In 2026, the latest year on file, its two samples averaged 6.5 ppb. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 34 ppb, in 2018.

What handles it

Fluoride

Forms or is added in treatment

Below EPA’s limits

0.05ppm

Highest yearly average, 2021 to 2026

2026: not found

Averaged by year, the highest was 0.05 ppm, in 2025, below EPA’s legal limit of 4.0 ppm. In 2026, the latest year on file, it wasn’t found in any of its two samples. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 0.14 ppm, in 2017.

What handles it

3 compounds tested and not foundBelow the reporting level in every sample. Not found is not the same as zero.
  • Arsenic
  • Lead
  • Total trihalomethanes (TTHM)

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

    Claim: Nitrate/Nitrite (30 ppm challenge)

    For: Nitrate

  • NSF/ANSI 58

    Claim: Nitrate/Nitrite Reduction

    For: Nitrate

    Other certifiers’ wording: Nitrate

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

Moderately hard to hard depending on the source, 92 to 152 mg/L (5.4 to 8.9 grains per gallon), across 4 sampling points, 2021 to 2026.

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, salt includedSoftener or salt-free conditioner?California water hardness, utility by utility

Violations since 2016

Contaminant or ruleTypePeriodMeasuredStatus
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleJan. 1, 2024 to March 31, 202411 MG/LArchived
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleJuly 1, 2023 to Sept. 30, 202311 MG/LArchived
NitrateA sample was above the legal limit, for a contaminant judged one sample at a time.EPA: Maximum Contaminant Level Violation, Single SampleApril 1, 2023 to June 30, 202311 MG/LArchived
Health-based violations only. Resolved and archived mean the utility returned to compliance or the state closed the record. 1 monitoring or reporting violation not shown.

What this report can’t tell you

  • Everything else. Nitrate, chromium (total), fluoride, arsenic and total trihalomethanes (TTHM) results here are California’s own from 2020 on, with EPA’s national file for 2012 to 2019. Colonial Oak WC Inc’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 Jan. 9, 2020 to June 3, 2026.

Testing your own tap

Colonial Oak WC Inc’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 Colonial Oak WC Inc serves, in a straight line. Call first: certification is test by test, and not every lab takes samples from homeowners.

  • Monterey County Consolidated Environmental Laboratory

    Salinas, CA · about 5 miles

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

    (831) 755-4516

  • Monterey Bay Analytical Services, Inc.

    Monterey, CA · about 15 miles

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

    (831) 375-6227

  • CM Analytical, Inc.

    Gilroy, CA · about 17 miles

    Certified for bacteria

    (408) 848-3619

All 145 certified labs for California

Sources

PWSID CA2700534. Rebuilt from EPA data on Oct. 18, 2026.

Towns it serves

Also in Monterey County

All California utilities

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