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Morro Rock Mutual Water Co

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
2,181 people
Connections
659
Water source
Surface water bought from another system
Based in
Cayucos, CA
County
San Luis Obispo
Owner
Private
Hardness
Very hard
The ZIP code it serves

93430

The short answer

2 results are above the health goal. The worst: Arsenic, above the health goal. No health-based violations on file since 2016.

What handles it

What was found

Lead and copper: EPA SDWIS monitoring periods ending 2004 to 2025.

Arsenic, total trihalomethanes (TTHM), haloacetic acids (HAA5), fluoride, nitrate, chromium (total) and uranium: California’s own results, samples taken from Jan. 14, 2020 to May 12, 2026, with EPA’s 2012 to 2019 file for earlier years.

Above the health goal2

Arsenic

Comes from the source

Above the health goal

1.8ppb

Highest yearly average, 2021 to 2025

Averaged by year, the highest was 1.8 ppb, in 2025: below EPA’s legal limit of 10 ppb, but above its health goal of zero. Its highest single sample was 2.1 ppb. That was also the latest year on file.

What handles it

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

Lead

Comes from pipes, often your own

Above the health goal

0.75ppb

90th percentile, 2023 to 2025

In the latest period, 9 in 10 homes sampled were at or below 0.75 ppb, below EPA’s action level of 15 ppb. The action level drops to 10 ppb on Nov. 1, 2027. EPA’s health goal for lead is zero.

What handles it

Below EPA’s limits5

Total trihalomethanes (TTHM)

Forms or is added in treatment

Below EPA’s limits

47.7ppb

Highest yearly average, 2020 to 2026

2026 average: 33.9 ppb

Averaged by year, the highest was 47.7 ppb, in 2024, below EPA’s legal limit of 80 ppb. Its highest single sample was 87.5 ppb. In 2026, the latest year on file, its four samples averaged 33.9 ppb. EPA recorded no violation for total trihalomethanes (TTHM) at this utility since 2020. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 55.4 ppb, in 2014.

What handles it

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

Haloacetic acids (HAA5)

Forms or is added in treatment

Below EPA’s limits

13.6ppb

Highest yearly average, 2020 to 2026

2026 average: 8.5 ppb

Averaged by year, the highest was 13.6 ppb, in 2024, below EPA’s legal limit of 60 ppb. Its highest single sample was 23.2 ppb. In 2026, the latest year on file, its four samples averaged 8.5 ppb. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 12.6 ppb, in 2014.

What handles it

No certification claim covers this compound yet.

Fluoride

Forms or is added in treatment

Below EPA’s limits

0.28ppm

Highest yearly average, 2021 to 2025

Averaged by year, the highest was 0.28 ppm, in 2025, below EPA’s legal limit of 4.0 ppm. That was also the latest year on file.

What handles it

Nitrate

Comes from the source

Below EPA’s limits

0.18ppm

Highest sample; found in 1 of 16

2025: not found

The highest sample, in 2021, was 0.18 ppm, below EPA’s legal limit of 10 ppm. In 2025, the latest year on file, it wasn’t found in any of its three samples.

What handles it

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

Chromium (total)

Comes from the source

Below EPA’s limits

0.9ppb

Highest yearly average, 2021 to 2025

Averaged by year, the highest was 0.9 ppb, in 2025, below EPA’s legal limit of 100 ppb. That was also the latest year on file.

What handles it

One compound tested and not foundBelow the reporting level in every sample. Not found is not the same as zero.
  • Uranium

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: Arsenic (Pentavalent)<=50 ppb Reduction, Lead Reduction

    For: Arsenic, Lead

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

  • NSF/ANSI 58

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

    For: Arsenic, Lead

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

At the tap, or for the whole house?

Lead and copper usually come from pipes and fixtures inside or near your home. A whole-house system sits where water enters, so it cannot catch what your own plumbing adds after it. For those, the filter that counts is the one at the tap. 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

The best water filters for lead, ranked

Hardness

Very hard, typically 377 mg/L (22 grains per gallon), across 2 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 at 22 grains per gallon, salt includedSoftener or salt-free conditioner?California water hardness, utility by utility

Violations since 2016

No health-based violations on file with EPA since 2016.

What this report can’t tell you

  • Everything else. Arsenic, total trihalomethanes (TTHM), haloacetic acids (HAA5), fluoride, nitrate, chromium (total) and uranium results here are California’s own from 2020 on, with EPA’s national file for 2012 to 2019. Morro Rock Mutual Water Co’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. 14, 2020 to May 12, 2026.

Testing your own tap

Morro Rock Mutual Water Co’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 Morro Rock Mutual Water Co serves, in a straight line. Call first: certification is test by test, and not every lab takes samples from homeowners.

  • County of San Luis Obispo Public Health Laboratory

    San Luis Obispo, CA · about 22 miles

    Certified for bacteria

    (805) 781-5507

  • Fruit Growers Laboratory, Inc.

    San Luis Obispo, CA · about 22 miles

    Certified for bacteria

    (805) 783-2940

  • Abalone Coast Analytical, Inc.

    San Luis Obispo, CA · about 23 miles

    Certified for bacteria

    (805) 595-1080

All 145 certified labs for California

Sources

PWSID CA4010015. Rebuilt from EPA data on Oct. 18, 2026.

Towns it serves

Also in San Luis Obispo County

All California utilities

What to do next